Method for driving an electro-optic display

Abstract

A method of driving an electro-optic display having a plurality of display pixels is described. The method includes determining a level of stress for a display pixel of the electro-optic display based on at least one previous update to the optical state of the display pixel, and receiving a request to update the optical state of the display pixel. The method also includes applying a drive waveform from a first or second update scheme to the display pixel in response to comparing the level of stress with an update scheme used for the previous update of the display pixel and two levels of stress thresholds.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Application No. 63 / 492,217, filed March 24, 2023, the entire contents of which are incorporated herein by reference.

[0002] This application is a division of U.S. Patent Nos. 5,930,026, 6,445,489, 6,504,524, 6,512,354, 6,531,997, 6,753,999, 6,825,970, 6,900,851, 6,995,550, 7,012,600, 7,023,420, 7,034,783, 7,116,466, 7,119,772, 7,193,625, 7,202,847, 7,259,744, 7,304,787, and 7,313,626. No. 12,794, No. 7,327,511, No. 7,453,445, No. 7,492,339, No. 7,528,822, No. 7,545,358, No. 7,583,251, No. 7,602,374, No. 7,612,760, No. 7,679,599 No. 7,688,297, No. 7,729,039, No. 7,733,311, No. 7,733,335, No. 7,787,169, No. 7,952,557, No. 7,956,841, No. 7,999,787, No. 8,077,141, No. 10,6 72,350, 11,145,261, and 11,462,183, and U.S. Patent Application Publication Nos. 2003 / 0102858, 2005 / 0122284, 2005 / 0179642, 2005 / 0253777, 2006 / 0139308, 2007 / 0013683, 2007 / 0091418, 2007 / 0103427, 2007 / 0200874, 2008 / 0024429, 2008 / 0024482, 2008 / 0048969, 2009 / 00602858, 2009 / 00702858, 2010 / 0102858, 2011 / 0102858, 2012 / 0102858, 2013 / 0102858, 2014 / 0102858, 2015 / 0102858, 2016 / 0102858, 2017 / 0102858, 2018 / 0102858, 2019 ... Nos. 08 / 0129667, 2008 / 0136774, 2008 / 0150888, 2008 / 0291129, 2009 / 0174651, 2009 / 0179923, 2009 / 0195568, 2009 / 0256799, 2009 / 0322721, 2010 / 0045592, 2010 / 0220121, 2010 / 0220122, 2010 / 0265561, 2011 / 0285754, and 2022 / 0415268.

[0003] The aforementioned patents and applications may hereinafter, for convenience, be collectively referred to as the "MEDEOD" (Method of Driving an Electro-Optic Display) Application. The entire contents of these patents and co-pending applications, and all other U.S. patents and published and co-pending applications, or other published work described below, are incorporated herein by reference.

[0004] (Technical field)

[0005] The present invention relates to a method of driving an electro-optic display, in particular a bistable electro-optic display, and to an apparatus for use in such a method. More particularly, the invention relates to a driving method that may enable reduced "afterglow" and edge effects and reduced flashing in such displays. The invention is particularly, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of electrically charged particles are present in a fluid and are caused to move through the fluid under the influence of an electric field, changing the appearance of the display. [Background technology]

[0006] The term "electro-optic," as applied to a material or display, is used herein in its conventional sense in the imaging arts to refer to a material having first and second display states that differ in at least one optical property, where the material is changed from its first display state to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye, but may be another optical property such as optical transmittance, reflectance, luminescence, or, in the case of displays intended for machine-reading, pseudocolor in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

[0007] The term "gray state" is used herein in its conventional sense in the imaging arts to refer to a state intermediate between two extreme optical states of a pixel, and does not necessarily imply a black-to-white transition between these two extreme states. For example, some of the E INK patents and published applications referenced below describe electrophoretic displays whose extreme states are white and dark blue, such that an intermediate "gray state" is actually light blue. In fact, as already noted, a change in optical state may not be a change in color at all. The terms "black" and "white" may be used hereinafter to refer to the two extreme optical states of a display and should generally be understood to include extreme optical states that are not strictly black and white, such as the aforementioned white and dark blue states. The term "monochrome" may be used hereinafter to refer to a drive scheme that drives pixels to only those two extreme optical states, with no intervening gray states.

[0008] The terms "bistable" and "bistable" are used herein in their conventional sense in the art to refer to a display comprising display elements having first and second display states that differ in at least one optical property, such that after any given element is driven with a finite-duration address pulse to exhibit either its first or second display state, that state will persist for at least several times, e.g., at least four times, the minimum duration of the address pulse required to change the state of the display element after the address pulse has terminated. U.S. Pat. No. 7,170,670 shows that some grayscale-capable particle-based electrophoretic displays are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true for several other types of electro-optic displays. Displays of this type are appropriately referred to as "multistable" rather than "bistable," although for convenience the term "bistable" may be used herein to include both bistable and multistable displays.

[0009] The term "impulse" is used herein in its conventional sense of the integral of voltage with respect to time. However, some bistable electro-optic media act as charge transducers, and when using such media, an alternative definition of impulse may be used: the integral of current over time (equal to the total charge applied). The appropriate definition of impulse should be used depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.

[0010] Much of the following discussion will focus on methods for driving one or more pixels of an electro-optic display through a transition from an initial gray level to a final gray level (which may or may not be different from the initial gray level). The term "waveform" will be used to refer to the overall voltage versus time curve used to effect a transition from a particular initial gray level to a particular final gray level. Typically, such a waveform will comprise multiple waveform elements. That is, if these elements are essentially rectangular (i.e., if a given element comprises the application of a constant voltage over a period of time), the element may be referred to as a "pulse" or "drive pulse." The term "drive scheme" refers to a set of waveforms sufficient to effect all possible transitions between gray levels for a particular display. A display may utilize two or more drive schemes. For example, the aforementioned U.S. Patent No. 7,012,600 teaches that a drive scheme may need to be modified depending on parameters such as the temperature of the display or the time it has operated during its lifetime, and thus a display may be provided with multiple different drive schemes for use at different temperatures, etc. A set of drive schemes used in this manner may be referred to as a "set of related drive schemes." As described in some of the aforementioned MEDEOD applications, it is also possible to use two or more drive schemes simultaneously in different areas of the same display, and a set of drive schemes used in this manner may be referred to as a "set of simultaneous drive schemes."

[0011] Several types of electro-optic displays are known. One type of electro-optic display is the rotating dichroic member type, as described in, for example, U.S. Patent Nos. 5,808,783, 5,777,782, 5,760,761, 6,054,0716,055,091, 6,097,531, 6,128,124, 6,137,467, and 6,147,791. (This type of display is often referred to as a "rotating dichroic ball" display, but in some of the above-mentioned patents, the rotating member is not spherical, so the term "rotating dichroic member" is preferred as it is more accurate.) Such displays use a large number of small bodies (typically spherical or cylindrical) having two or more segments with different optical properties and an internal dipole. These bodies are suspended within liquid-filled vacuoles in a matrix, and the vacuoles are filled with liquid so that the bodies are free to rotate. The appearance of the display can be changed by applying an electric field to it, thus rotating the bodies to various positions and varying the position of segments of the bodies as seen through the viewing surface. This type of electro-optic medium is typically bistable.

[0012] Another type of electro-optic display uses an electrochromic medium, e.g., in the form of a nanochromic film, consisting of electrodes formed at least in part from a semiconducting metal oxide and a plurality of dye molecules attached to the electrodes that are capable of reversing their color change. See, for example, O'Regan, B., et al., Nature 1991, 353, 737 and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Patent Nos. 6,301,038, 6,870,657, and 6,950,220. This type of medium is also typically bistable.

[0013] Another type of electro-optic display is the electrowetting display developed by Philips and described in Hayes, R.A., et al., "Video-Speed ​​Electronic Paper Based on Electrowetting," Nature, 425, 383-385 (2003). U.S. Patent No. 7,420,549 shows that such electrowetting displays can be made bistable.

[0014] Another type of electro-optic display that has been the subject of intensive research and development for many years is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared to liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in an inadequate usable lifespan for these displays.

[0015] As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can also be produced using gaseous fluids (see, e.g., Kitamura, T., et al., "Electrical toner movement for electronic paper-like display," IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., "Toner display using insulative particles charged triboelectrically," IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media are believed to be susceptible to the same types of problems as liquid-based electrophoretic media due to particle settling when used in an orientation that allows such settling, for example, in a sign where the media is positioned in a vertical plane. In fact, particle settling is believed to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media because the lower viscosity of the gaseous suspending fluid compared to the viscosity of a liquid allows for faster settling of the electrophoretic particles.

[0016] Numerous patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise a multiplicity of small capsules, each of which itself comprises an internal phase containing electrophoretically movable particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held within a polymer binder to form a coherent layer positioned between two electrodes. Techniques described in these patents and applications include the following: (a) electrophoretic particles, fluids, and fluid additives (see, e.g., U.S. Patent Nos. 7,002,728 and 7,679,814); (b) capsules, binders, and encapsulation processes (see, e.g., U.S. Patent Nos. 6,922,276 and 7,411,719); (c) films and subassemblies containing electro-optical materials (see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564); (d) backplanes, adhesive layers, other auxiliary layers, and methods used in displays (see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624); (e) color formation and color control (see, e.g., U.S. Pat. No. 7,075,502 and U.S. Patent Application Publication No. 2007 / 0109219); (f) a method for driving a display (see the aforementioned MEDEOD application); (g) display applications (see, e.g., U.S. Patent No. 7,312,784 and U.S. Patent Application Publication No. 2006 / 0279527); (h) Non-electrophoretic displays such as those described in U.S. Patent Nos. 6,241,921, 6,950,220, and 7,420,549, and U.S. Patent Application Publication No. 2009 / 0046082.

[0017] Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing so-called "polymer-dispersed electrophoretic displays," in which the electrophoretic medium consists of a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymer material, and that the discrete droplets of electrophoretic fluid in such polymer-dispersed electrophoretic displays may be considered capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet. See, for example, the aforementioned U.S. Patent No. 6,866,760. Therefore, for purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0018] A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, the charged particles and fluid are not encapsulated in microcapsules, but instead are held within a plurality of cavities formed in a carrier medium, typically a polymer film. See, for example, U.S. Patent Nos. 6,672,921 and 6,788,449, both of which are assigned to SiPix Imaging, Inc.

[0019] Although electrophoretic media are often opaque (e.g., because in many electrophoretic media the particles substantially block the transmission of visible light through the display) and can operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "obscured mode," in which one display state is substantially opaque and one is light transmissive. See, e.g., U.S. Patent Nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode. See U.S. Patent No. 4,418,346. Other types of electro-optic displays may also be capable of operating in a obscured mode. Electro-optic media operating in the shielding mode can be useful in multilayer structures for full-color displays, in which at least one layer adjacent to the viewing surface of the display operates in the shielding mode, exposing or obscuring a second layer more remote from the viewing surface.

[0020] Encapsulated electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices and offer additional advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (The use of the term "printing" is intended to include, but is not limited to, all forms of printing and coating, including pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coatings such as knife-over-roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, silk screen printing processes, electrostatic printing processes, thermal printing processes, inkjet printing processes, electrophoretic deposition (see U.S. Pat. No. 7,339,715), and other similar techniques.) Thus, the resulting display can be flexible. Furthermore, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

[0021] Other types of electro-optic media may also be used in displays of the present invention.

[0022] The bistable or multistable behavior of particle-based electrophoretic displays, and other electro-optical displays that exhibit similar behavior (such displays may hereafter be referred to as "impulse-driven displays" for convenience), contrasts markedly with that of conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bistable or multistable, but rather function as voltage transducers, whereby applying a given electric field to a pixel of such a display produces a specific gray level at that pixel, regardless of the pre-existing gray level at that pixel. Furthermore, LC displays can only be driven in one direction (from non-transmissive or "dark" to transmissive or "bright"), and the reverse transition from a brighter state to a darker state is achieved by reducing or eliminating the electric field. Finally, the gray level of an LC display pixel is not sensitive to the polarity of the electric field, only its magnitude; in practice, for technical reasons, commercial LC displays typically frequently reverse the polarity of the driving field. In contrast, bistable electro-optic displays, to a first approximation, act as impulse transducers, whereby the final state of a pixel depends not only on the applied electric field and the time for which this field is applied, but also on the state of the pixel prior to the application of the electric field.

[0023] Whether the electro-optic medium used is bistable or not, to obtain a high-resolution display, each pixel of the display must be addressable without interference from neighboring pixels. One way to achieve this goal is to provide an array of nonlinear elements, such as transistors or diodes, with at least one nonlinear element associated with each pixel, producing an "active matrix" display. The address or pixel electrode that addresses a pixel is connected to an appropriate voltage source through the associated nonlinear element. Typically, when the nonlinear elements are transistors, the pixel electrode is connected to the drain of the transistor; although this arrangement will be assumed in the following discussion, it is essentially arbitrary; the pixel electrode may also be connected to the source of the transistor. Conventionally, in a high-resolution array, pixels may be arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of one defined row and one defined column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode. Again, the assignment of sources to rows and gates to columns, while conventional, is essentially arbitrary and can be reversed as desired. The row electrodes are connected to row drivers, which essentially ensure that only one row is selected at any given moment; i.e., a voltage is applied to the selected row electrode to ensure that all transistors in the selected row are conductive, while a voltage is applied to every other row to ensure that all transistors in those unselected rows remain non-conductive. The column electrodes are connected to column drivers, which apply voltages to the various column electrodes selected to drive the pixels in the selected row to their desired optical states (these voltages are conventionally relative to a common front electrode provided opposite the nonlinear array of electro-optic medium and spanning the entire display).After a preselected interval known as the "line address time," the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed so that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row fashion.

[0024] At first, it might be thought that the ideal way to address such an impulse-driven electro-optic display would be the so-called "general grayscale image flow," in which the controller arranges each write of the image so that each pixel makes a direct transition from its initial gray level to its final gray level. However, there are inevitably some errors in writing an image on an impulse-driven display. Some such errors encountered in practice include: (a) Prior state dependence: for at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends not only on the current and desired optical states, but also on the prior optical state of the pixel. (b) Dwell-time dependence: for at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends on the time the pixel has spent in its various optical states. The precise nature of this dependence is not well understood, but in general, the longer the pixel has been in its current optical state, the more impulse is required. (c) Temperature dependence: the impulse required to switch a pixel to a new optical state is highly dependent on temperature. (d) Humidity Dependence: The impulse required to switch a pixel to a new optical state depends on the ambient humidity, at least for some types of electro-optic media. (e) Mechanical uniformity: the impulse required to switch a pixel to a new optical state can be affected by mechanical variations in the display, for example, variations in the thickness of the electro-optic medium or associated lamination adhesive. Other types of mechanical non-uniformity can result from inevitable variations between different manufacturing batches of the medium, manufacturing tolerances, and material variations. (f) Voltage Errors: The actual impulse applied to a pixel will inevitably differ slightly from the theoretically applied impulse due to unavoidable minor errors in the voltage delivered by the driver.

[0025] General grayscale image flows suffer from the "accumulation of errors" phenomenon. For example, suppose the temperature dependence results in 0.2L* (where L* has the usual CIE definition: L*=116(R / R0) 1 / 3 -16 where R is the reflectance and R0 is the standard reflectance value), the error for each transition is in the positive direction. After 50 transitions, this error will accumulate to 10L*. Perhaps more realistically, the average error for each transition, expressed in terms of the difference between the theoretical and actual reflectance of the display, is assumed to be ±0.2L*. After 100 consecutive transitions, the pixel will display an average deviation from its expected state of 2L*. Such deviation is apparent to the average observer of an image of some type.

[0026] This error accumulation phenomenon applies not only to temperature-induced errors, but also to all of the types of errors listed above. As explained in the aforementioned U.S. Patent No. 7,012,600, it is possible to compensate for such errors, but only to a certain degree of accuracy. For example, temperature errors can be compensated for using a temperature sensor and a lookup table, but the temperature sensor has limited resolution and may read a temperature that is slightly different from the temperature of the electro-optic medium. Similarly, prior state dependence can be compensated for by storing prior states and using a multidimensional transition matrix, but the controller's memory limits the number of states that can be recorded and the size of the transition matrix that can be stored, limiting the accuracy of this type of compensation.

[0027] Thus, general grayscale image flow requires very precise control of the applied impulses to give good results, and experience has shown that, in the current state of electro-optic display technology, general grayscale image flow is not feasible in commercial displays.

[0028] In some situations, it may be desirable for a single display to utilize multiple drive schemes. For example, a display capable of three or more gray levels may utilize a grayscale drive scheme ("GSDS") that can effect transitions between all possible gray levels and a monochrome drive scheme ("MDS") that only effect transitions between two gray levels, with the MDS providing faster display refresh than the GSDS. The MDS is used when all pixels being changed during a display refresh effect only the transitions between the two gray levels used by the MDS. For example, the aforementioned U.S. Patent No. 7,119,772 describes a display in the form of an e-book or similar device that can display grayscale images and also display a monochrome dialog box that allows the user to enter text related to the displayed image. When the user is entering text, a high-speed MDS is used for rapid updating of the dialog box, thus providing the user with rapid confirmation of the text being entered. On the other hand, when the entire grayscale image shown on the display is being changed, a slow GSDS is used.

[0029] Alternatively, the display may utilize GSDS in conjunction with a "Direct Update" drive scheme ("DUDS"). Although DUDSs may have two or more gray levels, typically fewer than GSDSs, the most important feature of DUDSs is that transitions are handled by simple one-way driving from an initial gray level to a final gray level, as opposed to "indirect" transitions. In GSDSs, at least some transitions involve driving a pixel from an initial gray level to one extreme optical state and then back to the final gray level; in some cases, a transition may be achieved by driving from an initial gray level to one extreme optical state, then to the opposite extreme optical state, and then to the final extreme optical state. See, for example, the driving scheme illustrated in Figures 11A and 11B of the aforementioned U.S. Patent No. 7,012,600. Thus, this electrophoretic display may have an update time in grayscale mode of about 2-3 times the saturation pulse length ("saturation pulse length" is defined as the period at a particular voltage that is sufficient to drive a pixel of the display from one extreme optical state to the other), or about 700-900 milliseconds, while DUDS have a maximum update time equal to the saturation pulse length, or about 200-300 milliseconds.

[0030] However, variations in the drive schemes are not limited to differences in the number of gray levels used. For example, drive schemes can be divided into global drive schemes, in which a drive voltage is applied to every pixel in the area (which may be the entire display or some defined portion thereof) to which the global update drive scheme (more precisely referred to as a "global full" or "GC" drive scheme) is applied, and partial update drive schemes, in which a drive voltage is applied only to pixels undergoing a non-zero transition (i.e., a transition in which the initial and final gray levels are different from each other), but in which no drive voltage is applied during a zero transition (in which the initial and final gray levels are the same). An intermediate form of drive scheme (designated a "global limited" or "GL" drive scheme) is similar to the GC drive scheme, except that no drive voltage is applied to pixels undergoing a zero white-to-white transition. For example, in a display used as an e-book reader that displays black text on a white background, there are many white pixels, especially in the margins and between the lines of the text, that remain unchanged from one page of text to the next, and therefore not rewriting these white pixels substantially reduces the apparent "flashiness" of display rewriting. However, certain problems remain with this type of GL driving scheme.

[0031] First, as discussed in detail in some of the aforementioned MEDEOD applications, bistable electro-optic media are typically not perfectly bistable, and pixels placed in one extreme optical state will gradually drift toward intermediate gray levels over a period of minutes to hours. In particular, pixels that are driven white will slowly drift toward a light gray color. Thus, in a GL drive scheme, if a white pixel is allowed to remain undriven through several page turns, during which time other white pixels (e.g., pixels forming part of a text character) are driven, the newly updated white pixel will be slightly brighter than the undriven white pixel, and eventually the difference will be apparent even to an untrained user.

[0032] These drifts can also be caused by "remnant voltage," a term used to describe a persistent or decaying voltage (which may also be referred to as the open-circuit electrical potential and is typically measured in volts or millivolts) that may remain within an electro-optic display after an address pulse (a voltage pulse used to change the optical state of an electro-optic medium) has been terminated. Remnant voltage can cause drift in the optical state because the optical state of a pixel may depend on the pixel's remnant voltage, and the pixel's remnant voltage may decay over time.

[0033] Second, such residual voltages can lead to undesirable effects on images displayed on electro-optic displays, including but not limited to the so-called "persistence" phenomenon, in which traces of the previous image are still visible after the display has been rewritten. Residual voltages can also cause edge effects, such as "edge persistence," a type of persistence in which the contours (edges) of parts of the previous image remain visible.

[0034] Furthermore, in both monochrome and color systems, the electric field generated by a pixel electrode tends to affect an area of ​​the electro-optic medium larger than that of the pixel electrode itself, thereby, in effect, causing the optical state of one pixel to extend into a portion of the area of ​​an adjacent pixel. For example, when an undriven pixel resides adjacent to a pixel being updated, driving the driven pixel causes a change in optical state over an area slightly larger than the area of ​​the driven pixel, which encroaches into the area of ​​the adjacent pixel, a phenomenon known as "blooming." Such blooming manifests as an edge effect along the edge where the undriven pixel resides adjacent to the driven pixel. In some cases, driving adjacent pixels results in a final optical state within the area between the pixels that differs from the optical state reached by either of the adjacent pixels themselves. This final optical state within the area between adjacent pixels is caused by the electric field experienced within the inter-pixel region, which is the average of the electric fields applied to the adjacent pixels.

[0035] Similar edge effects occur when using regional updates (where only a specific region of the display is updated, e.g., to show an image), except that in the case of regional updates, the edge effect occurs at the boundaries of the region being updated. Over time, such edge effects become visually distracting and must be cleared.

[0036] Until now, such edge effects (and the effect of color drift in undriven white pixels) have typically been eliminated by using a single GC update at intervals. Unfortunately, using such occasional GC updates can reintroduce the problem of "flashy" updates, and in fact the flashy effect of the updates can be enhanced by the fact that the flashy updates only occur at long intervals. The present invention is directed to reducing or eliminating the problems discussed above, while still avoiding flashy updates whenever possible.

[0037] It has been found that edge persistence can be mitigated by driving electro-optic displays with DC-unbalanced waveforms. However, as discussed in many of the aforementioned MEDEOD applications, if the drive scheme used is not substantially DC-balanced (i.e., the algebraic sum of the impulses applied to a pixel during any series of transitions beginning and ending at the same gray level is not near zero), the electro-optical performance and the display's useful life can be adversely affected. See, in particular, the aforementioned U.S. Pat. No. 7,453,445, which discusses the issue of DC balance in so-called "non-homogeneous loops" with transitions performed using two or more drive schemes. A DC-balanced drive scheme ensures that the total net impulse bias at any given time is bounded (for a finite number of gray states). In a DC-balanced drive scheme, each optical state of the display is assigned an impulse potential (IP), and individual transitions between optical states are defined so that the net impulse of the transition is equal to the difference in impulse potential between the initial and final states of the transition. A DC-balanced drive scheme requires that any round-trip net impulse be substantially zero.

[0038] DC unbalanced waveforms can also produce remnant voltages. For example, U.S. Pat. No. 7,012,600 describes how DC unbalanced waveforms can result in remnant voltages, which can be confirmed by measuring the open-circuit electrochemical potential of display pixels. In addition, the use of DC unbalanced waveforms can result in polarization kickback, a change in the optical state of an electro-optic medium that occurs immediately after driving of the medium is stopped. This "kickback" or "self-erase" is a phenomenon observed in some electro-optic displays. See, for example, Ota, I., et al., "Developments in Electrophoretic Displays," Proceedings of the SID, 18, 243 (1977). In that paper, self-erase is reported in a non-encapsulated electrophoretic display, in which the electro-optic medium at least partially reverses its optical state shortly after a voltage applied across the electro-optic medium is no longer applied. For example, a pixel that is driven black may revert to dark gray within a short period of time after the pixel is deselected. In some cases, it can be observed that a reverse voltage, which may be greater than the operating voltage, develops across the electrodes, often leading to electrode damage.

[0039] Conventionally, post-drive discharge ("PDD") techniques are used to reduce or eliminate remnant voltage by draining charge from display pixels after a display update is completed. For example, U.S. Patent No. 10,475,396 describes PDD techniques for draining remnant voltage, such as activating a transistor in a display pixel and setting the voltages on the front and back electrodes of the display pixel to approximately the same voltage for a defined period of time and / or until the remnant voltage is reduced to an amount below a threshold. PDD techniques are particularly important for use in conjunction with intentional DC unbalanced waveform driving schemes, which can cause remnant voltage to build up more quickly.

[0040] However, one disadvantage of conventional PDD techniques is that such techniques require time to discharge residual voltage after an update, and the display device essentially “freezes” and is unresponsive to user input while the PDD routine is running. Typical use cases for display devices involve substantially constant interaction and input from a user, which can often require the display to perform several consecutive updates using a DC imbalance waveform, which is capable of performing faster display updates than the waveform used in a standard update mode. Thus, rendering the display device inoperable to allow time for the residual voltage discharge routine can result in an unsatisfactory user experience.

[0041] To reduce or eliminate periods of unresponsiveness to user interaction, conventional drive methods can be configured to suspend execution of a PDD routine and subsequently allow the required updates to be performed. However, conventional drive methods operate under the assumption that use cases requiring suspension of PDD routine execution or having short dwell times between display updates are rare, and that the suspended PDD routine will be followed shortly thereafter by a fully executed PDD routine to discharge the display. Given the typical display device use case described above, the PDD routine may be suspended several times before the fully executed PDD routine can sufficiently discharge the display. This may therefore allow residual charge to build up sufficiently to cause display degradation or display artifacts (e.g., afterimages), which also adversely affect the user experience. The present invention is also directed to reducing or eliminating the problems related to residual charge buildup discussed above, while still avoiding, to the greatest extent possible, long periods of device unresponsiveness that can result in an unsatisfactory user experience. [Prior art documents] [Patent documents]

[0042] [Patent Document 1] U.S. Patent No. 5,808,783 [Patent Document 2] U.S. Patent No. 6,301,038 [Patent Document 3] U.S. Patent No. 7,420,549 Summary of the Invention [Means for solving the problem]

[0043] Thus, in one aspect, the invention provides a (first) method of driving an electro-optic display having a plurality of pixels using a first drive scheme in which all pixels are driven at each transition and a second drive scheme in which pixels undergoing some transitions are not driven. In the first method of the invention, the first drive scheme is applied to a non-zero few pixels during a first update of the display, while the second drive scheme is applied to the remaining pixels during the first update. During a second update following the first update, the first drive scheme is applied to a different non-zero few pixels, while the second drive scheme is applied to the remaining pixels during the second update.

[0044] This first driving method of the present invention may hereinafter for convenience be referred to as the "Selective General Update" or "SGU" method of the present invention.

[0045] The present invention provides a (second) method of driving an electro-optic display having a plurality of pixels, each of which can be driven using either a first or a second drive scheme. When a global full update is required, the pixels are divided into two (or more) groups and a different drive scheme is used for each group, the drive schemes being different from each other so that, for at least one transition, pixels in different groups with the same transition between optical states will not experience the same waveform. This second drive method of the present invention may hereinafter, for convenience, be referred to as the "global full multiplexed drive scheme" or "GCMDS" method of the present invention.

[0046] The SGU and GCMDS methods discussed above reduce the perceived flashing effect of image updates. However, the present invention also provides multiple methods for reducing or eliminating edge artifacts when driving a bistable electro-optic display. One such edge artifact reduction method, hereafter referred to as the third method of the present invention, requires the application of one or more balanced pulse pairs (a balanced pulse pair or "BPP" is a pair of drive pulses of opposite polarity, such that the net impulse of the balanced pulse pair is substantially zero) during a white-to-white transition at pixels that can be identified as likely to produce edge artifacts and that are in a spatiotemporal configuration such that the balanced pulse pair would be effective in eliminating or reducing the edge artifacts. Desirably, the pixels to which the BPPs are applied are selected so that the BPPs are masked by other update activity. Note that the application of one or more BPPs does not affect the desired DC balance of the drive scheme, as each BPP has essentially zero net impulse and therefore does not alter the DC balance of the drive scheme. This third driving method of the present invention may hereinafter be conveniently referred to as the "Balanced Pulse Pair White / White Transition Driving Scheme" or "BPPWWTDS" method of the present invention.

[0047] In a related fourth method of the present invention for reducing or eliminating edge artifacts, a "top-off" pulse is applied during a white-to-white transition at pixels that can be identified as likely to cause edge artifacts and that are in a spatiotemporal configuration such that the top-off pulse will be effective in eliminating or reducing the edge artifacts. This fourth drive method of the present invention may hereinafter be referred to for convenience as the "white / white top-off pulse drive scheme" or "WWTOPDS" method of the present invention.

[0048] A fifth method of the present invention also seeks to reduce or eliminate edge artifacts. This fifth method seeks to eliminate such artifacts that, in the absence of special adjustments, occur along straight edges between what would otherwise be driven pixels and what would otherwise be undriven pixels. In the fifth method, a two-stage drive scheme is used, where in a first stage, some "extra" pixels on the "undriven" side of the straight edge are actually driven to the same color as the pixels on the "driven" side of the edge. In a second stage, both the pixels on the driven side of the edge and the extra pixels on the undriven side of the edge are driven to their final optical states. Thus, the present invention provides a method of driving an electro-optic display having a plurality of pixels, wherein a plurality of pixels in a first area of ​​the display are driven to change their optical state, and a plurality of pixels in a second area of ​​the display are not required to change their optical state, and when the first and second areas are contiguous along a straight line, a two-stage drive scheme is used, where in a first stage, some pixels in the second area adjacent to the line are actually driven to the same color as the pixels in the first area adjacent to the line, while in a second stage, both the pixels in the first area and the same number of pixels in the second area are driven to their final optical state. Driving a limited number of additional pixels in this manner has been found to significantly reduce the visibility of edge artifacts, as any edge artifacts that occur along the serpentine edge defined by the additional pixels are much less noticeable than would be the corresponding edge artifacts along the original straight edge. This fifth drive method of the present invention may hereinafter for convenience be referred to as the "straight-edge additional pixel drive scheme" or "SEEPDS" method of the present invention.

[0049] A sixth method of the present invention allows a pixel to temporarily deviate from DC balance. Many situations arise in which it would be beneficial to allow a pixel to temporarily deviate from DC balance. For example, a pixel may require a special pulse toward white because it is predicted to contain dark artifacts, or fast display switching may be required such that the full impulse required for balance cannot be applied. The transition may be interrupted by an unexpected event. In such situations, it is necessary, or at least desirable, to have a way to allow and rectify impulse deviations, especially on short-time scales.

[0050] In a sixth method of the present invention, the display maintains an "impulse bank register" containing one value for each pixel of the display. When a pixel is required to deviate from the normal DC-balanced drive scheme, the impulse bank register for the associated pixel is adjusted to indicate the deviation. When the register value for any pixel is non-zero (i.e., when the pixel deviates from the normal DC-balanced drive scheme), at least one subsequent transition of the pixel is made using a waveform that differs from the corresponding waveform of the normal DC-balanced drive scheme and reduces the absolute value of the register value. The absolute value of the register value for any pixel is not allowed to exceed a predetermined amount. This sixth drive method of the present invention may hereinafter be referred to for convenience as the "Impulse Bank Drive Scheme" or "IBDS" method of the present invention.

[0051] In a seventh method of the present invention, a state machine algorithm monitors display updates that are prone to remnant voltage buildup and dwell times between updates. The algorithm then calculates a numerical quantity indicative of the level of stress experienced by the display based on the number of DC imbalance updates that do not last long enough to discharge the remnant voltage through natural decay or to fully execute the PDD routine. When the calculated quantity increases to a first threshold indicating the limit of an acceptable level of stress for the display device, the algorithm overrides the next requested DC imbalance update and instead uses an update mode that is less stressful to the display. For example, the algorithm may override the requested DC imbalance update with a DC balance update scheme and / or a scheme in which display updates are longer in duration to reduce accumulated remnant charge. This seventh driving method of the present invention may hereinafter be referred to for convenience as the “mode update request override driving scheme” or “MURODS” method of the present invention.

[0052] A seventh method of the present invention is a method of driving an electro-optic display having a plurality of display pixels. The method includes determining a level of stress for a display pixel of the electro-optic display based on at least one previous update to the optical state of the display pixel. The method also includes receiving a request to update the optical state of the display pixel. The method also includes applying a drive waveform from a first update scheme to the display pixel when (i) a drive waveform from a first update scheme was used for the previous update of the display pixel and the level of stress is not greater than a first stress threshold level, or (ii) a drive waveform from a second update scheme was used for the previous update of the display pixel and the level of stress is less than a second stress threshold level. The method also includes applying a drive waveform from a second update scheme to the display pixel when (i) a drive waveform from the first update scheme was used for the previous update of the display pixel and the level of stress is greater than the first stress threshold level, or (ii) a drive waveform from the second update scheme was used for the previous update of the display pixel and the level of stress is not greater than a second stress threshold level.

[0053] In some embodiments, the request to update the optical states of the display pixels of the electro-optic display is triggered by a user interaction with the electro-optic display. In some embodiments, the user interaction includes swiping a screen surface of the electro-optic display. In some embodiments, the user interaction includes presenting an animation on the electro-optic display.

[0054] In some embodiments, the first update scheme includes a drive waveform that is DC unbalanced. In some embodiments, the second update scheme includes a drive waveform that is DC balanced.

[0055] In some embodiments, the second update scheme comprises a drive waveform that is longer in duration than the drive waveform of the first update scheme. In some embodiments, the drive waveform of the second update scheme is approximately 350 ms to 500 ms longer in duration than the drive waveform of the first update scheme. In some embodiments, the drive waveform of the second update scheme is approximately 30% to 55% longer in duration than the drive waveform of the first update scheme. In some embodiments, the drive waveform of the second update scheme is approximately 50% to 70% longer in duration than the drive waveform of the first update scheme.

[0056] In some embodiments, the level of the stress amount is a numerical quantity that includes an approximation of the actual amount of remnant voltage accumulated on the display pixel, hi some embodiments, the level of the stress amount is a scalar quantity that includes an index that indicates the growth or decay in the amount of remnant voltage accumulated on the display pixel.

[0057] In some embodiments, determining the level of stress for the display pixel includes calculating the following formula:

number

[0058] where x(n) indicates the level of stress amount for update n, UT(n) indicates the duration in milliseconds of the drive waveform used for the previous update of the display pixel, DT(n) indicates the number of milliseconds of the post-drive discharge routine applied after the previous update of the display pixel, TAU represents the time constant of the decay of the stress amount level x(n), x(n-1) is the stress amount level calculated based on the previous update of the display pixel, and B has a value that varies depending on the drive mode used for update n.

[0059] In some embodiments, B is set to a non-zero positive value after applying a drive waveform from a first update scheme to a display pixel, and in some embodiments, the value of B is set to zero after applying a drive waveform from a second update scheme to a display pixel.

[0060] In some embodiments, the method further includes interrupting the post-drive-discharge routine after applying the drive waveform from the first update scheme. In some embodiments, the method further includes substantially not implementing a dwell time after applying the drive waveform from the first update scheme.

[0061] In some embodiments, the method further includes interrupting the post-drive-discharge routine after applying the drive waveform from the second update scheme. In some embodiments, the method further includes not substantially implementing a dwell time after applying the drive waveform from the second update scheme.

[0062] In some embodiments, the first stress threshold level indicates a limit on the level of acceptable stress on the electro-optic display.

[0063] The present invention also provides a novel display controller configured to perform the method of the present invention. In one such novel display controller, a standard image or one of a series of standard images is flashed on the display at an intermediate stage in the transition from a first arbitrary image to a second arbitrary image. To display such a standard image, it is necessary to vary the waveform used to transition from the first image to the second image for any given pixel depending on the state of that pixel in the standard image being displayed. For example, if the standard image is monochrome, two possible waveforms would be required for each transition between specific gray levels in the first and second images, depending on whether the particular pixel is black or white in the standard image. On the other hand, if the standard image has 16 gray levels, 16 possible waveforms would be required for each transition. This type of controller may hereinafter be conveniently referred to as the "intermediate standard image" or "ISI" controller of the present invention.

[0064] Furthermore, in some of the methods of the present invention (e.g., the SEEDPS method), it is necessary or desirable to use a controller that is capable of updating arbitrary regions of the display, and the present invention provides such a controller, which may hereinafter for convenience be referred to as the "arbitrary region allocation" or "ARA" controller of the present invention.

[0065] In all methods of the present invention, the display may utilize any of the types of electro-optic media discussed above. Thus, for example, the electro-optic display may include a rotating dichroic member or an electrochromic material. Alternatively, the electro-optic display may include an electrophoretic material comprising a plurality of charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. The charged particles and fluid may be confined within a plurality of capsules or microcells. Alternatively, the charged particles and fluid may exist as a plurality of separate droplets surrounded by a continuous phase comprising a polymeric material. The fluid may be a liquid or a gas. [Brief explanation of the drawings]

[0066] [Figure 1A] 1A and 1B of the accompanying drawings show voltage versus time curves for two balanced pair waveforms that may be used in the GCMDS method of the present invention. [Figure 1B] 1A and 1B of the accompanying drawings show voltage versus time curves for two balanced pair waveforms that may be used in the GCMDS method of the present invention.

[0067] [Figure 1C] FIG. 1C shows a graph of reflectance versus time for a display in which an equal number of pixels are driven using the waveforms shown in FIGS. 1A and 1B.

[0068] [Figure 2] 2, 3, 4, and 5 diagrammatically illustrate the GCMDS method of the present invention proceeding through intermediate images. [Figure 3] 2, 3, 4, and 5 diagrammatically illustrate the GCMDS method of the present invention proceeding through intermediate images. [Figure 4] 2, 3, 4, and 5 diagrammatically illustrate the GCMDS method of the present invention proceeding through intermediate images. [Figure 5] 2, 3, 4, and 5 diagrammatically illustrate the GCMDS method of the present invention proceeding through intermediate images.

[0069] [Figure 6] 6A and 6B illustrate the difference in L* values ​​for various gray levels achieved using the BPPWWTDS of the present invention and the prior art global constrained driving scheme, respectively.

[0070] [Figure 7] 7A and 7B are graphs similar to those of FIGS. 6A and 6B, respectively, but illustrating overcorrection that can occur in certain BPPWWTDS of the present invention.

[0071] [Figure 8-1] 8A-8D are graphs similar to that of FIG. 7A but showing the effect of using 1, 2, 3 and 4 balanced pulse pairs, respectively, in a BPPWWTDS of the present invention. [Figure 8-2] 8A-8D are graphs similar to that of FIG. 7A but showing the effect of using 1, 2, 3 and 4 balanced pulse pairs, respectively, in a BPPWWTDS of the present invention.

[0072] [Figure 9] FIG. 9 shows diagrammatically the various transitions that occur in the combined WWTOPDS / IBDS of the present invention.

[0073] [Figure 10] 10A and 10B are graphs similar to those of FIGS. 6A and 6B, respectively, but showing the error in gray level achieved using the combined WWTOPDS / IBDS of the present invention illustrated in FIG.

[0074] [Figure 11] 11A and 11B are graphs similar to those of FIGS. 10A and 10B, respectively, showing the error in gray level achieved using the WWTOPDS method of the present invention, in which the top-off pulse is applied regardless of DC imbalance.

[0075] [Figure 12] 12A and 12B illustrate, in somewhat schematic fashion, the transitions that occur in a prior art driving method and the SEEPDS driving scheme of the present invention that result in the same overall change in the display.

[0076] [Figure 13] FIG. 13 illustrates diagrammatically the controller architecture required for a SEEPDS that allows regions of arbitrary shape and size to be updated, compared to prior art controllers that only allow selection of rectangular areas.

[0077] [Figure 14] FIG. 14 shows a flow chart of an exemplary driving method incorporating a residual voltage management state machine algorithm.

[0078] [Figure 15] FIG. 15 shows a flow chart of an exemplary driving method incorporating a residual voltage management state machine algorithm. DETAILED DESCRIPTION OF THE INVENTION

[0079] From the foregoing, it will be apparent that the present invention provides a number of separate inventions relating to apparatus for driving electro-optic displays and for use in such methods. These various inventions will be described separately below, but it will be understood that a single display may incorporate two or more of these inventions. For example, it will be readily apparent that a single display may utilize the selective general update and straight edge addition pixel drive scheme methods of the present invention and use the arbitrary region allocation controller of the present invention.

[0080] (Part A: Selective General Update Method of the Present Invention)

[0081] As described above, the selective general update (SGU) method of the present invention is intended for use in an electro-optic display having a plurality of pixels. The method utilizes a first drive scheme in which all pixels are driven at each transition and a second drive scheme in which pixels undergoing some transitions are not driven. In the SGU method, the first drive scheme is applied to a non-zero minority of pixels during a first update of the display, while the second drive scheme is applied to the remaining pixels during the first update. During a second update following the first update, the first drive scheme is applied to a different non-zero minority of pixels, while the second drive scheme is applied to the remaining pixels during the second update.

[0082] In a preferred form of the SGU method, the first drive scheme is a GC drive scheme and the second drive scheme is a GL drive scheme. In this case, the SGU method essentially replaces prior art methods in which a small number of pixels use the GC drive scheme and a large number of pixels use the GL drive scheme in each update; in the prior art, most updates are performed using the (relatively non-flashing) GL drive scheme and occasional updates are performed using the (relatively flashy) GC drive scheme. By careful selection of the distribution of pixels using the GC drive scheme, each update using the SGU method of the present invention can be achieved in a manner that is perceived (to a non-expert user) as significantly less flashy than a mere GL update, and in which the rare flashy and distracting mere GC update is avoided.

[0083] For example, suppose a particular display is found to require the use of a GC drive scheme in every fourth update. To implement the SGU method of the present invention, the display can be divided into 2x2 groups of pixels. During the first update, one pixel in each group (e.g., the top-left pixel) is driven using the GC drive scheme, while the three remaining pixels are driven using the GL drive scheme. During the second update, a different pixel in each group (e.g., the top-right pixel) is driven using the GC drive scheme, while the three remaining pixels are driven using the GL drive scheme. The pixels driven using the GC drive scheme alternate with each update. In theory, each update is about one-quarter as flashy as a simple GC update, but the increase in flashing effect is not particularly noticeable, and the distracting simple GC update on every fourth update in prior art methods is avoided.

[0084] The decision as to which pixels receive the GC drive scheme in each update can be determined systematically using some tessellated pattern, such as in the 2x2 grouping arrangement discussed above, or statistically, with an appropriate percentage of pixels randomly selected in each update (e.g., 25 percent of the pixels can be selected in each update). It will be readily apparent to those skilled in the art of visual psychology that some "noise patterns" (i.e., distributions of selected pixels) may perform better than others. For example, if one pixel from each adjacent 3x3 group is selected to use the GC drive scheme in each update, it may be advantageous not to set the corresponding pixel in each group in each update, because this would produce a regular array of "flashing" pixels, which may be more noticeable than the pseudo-random array of "flashing" pixels caused by selecting a different pixel in each group.

[0085] In at least some cases, it may be desirable to arrange various groups of pixels that use a GC driving scheme in each update on a parallelogram or pseudo-hexagonal grid. An example square or rectangular "tile" of pixels, repeated in both directions, would then provide a parallelogram or pseudo-hexagonal grid as follows (the numbers refer to the number of updates the GC driving scheme is applied to the pixel): 125463 631254 546312 and 12678345 34512678 67834512 51267834 83451267 26783451 45126783 78345126

[0086] Two or more patterns of selected pixels can be used to account for different usage models. Two or more patterns of different strengths can be used to lightly watermark a page during an update (e.g., a 2x2 block with one pixel using a GC drive scheme compared to a 3x3 block with one pixel using a GC drive scheme). This watermark can change on the fly. Patterns can be shifted relative to each other to generate other desirable watermark patterns.

[0087] The SGU method of the present invention is, of course, not limited to the combination of GC and GL drive schemes, but can be used with other drive schemes as long as one drive scheme is less flashy than the other, while the second yields better performance. A similar effect can also be produced by using two or more drive schemes and varying which pixels experience partial and full updates.

[0088] The SGU method of the present invention can be usefully used in combination with the BPPWWTDS or WWTOPDS methods of the present invention, which are described in detail below. Implementing the SGU method does not require extensive development of modified drive schemes (as the method can use a combination of prior art drive schemes), and allows for a substantial reduction in the apparent flashing effect of the display.

[0089] (Part B: Global Fully Multiple Drive Scheme Method of the Present Invention)

[0090] As explained above, the global full multiplexed drive scheme or GCMDS method of the present invention is a second method of driving an electro-optic display having a plurality of pixels, each of which can be driven using either a first or a second drive scheme. When a global full update is required, the pixels are divided into two (or more) groups and a different drive scheme is used for each group, the drive schemes being different from each other so that, for at least one transition, pixels in different groups with the same transition between optical states will not experience the same waveform.

[0091] Part of the reason for the flashing effect of prior art global complete (GC) updates is that in such updates, a large number of pixels are typically exposed to the same waveform simultaneously. For reasons explained above, this is often a white-to-white waveform, but in other cases (e.g., when white text is displayed on a black background), a black-to-black waveform may be responsible for most of the flashing effect. In the GCMDS method, instead of simultaneously driving (and thus flashing) all pixels of a display undergoing the same transition with the same waveform, pixels are assigned group values ​​such that, for at least some transitions, different waveforms are applied to different groups of pixels undergoing the same transition. Thus, pixels undergoing the same image state transition will not (necessarily) experience the same waveform and therefore will not flash at the same time. Furthermore, the pixel groups and / or waveforms used may be adjusted between image updates.

[0092] Using the GCMDS method, it is possible to achieve a substantial reduction in the perceived flashing effect of global full updates. For example, assume that pixels are divided on a checkerboard grid, with pixels of one parity assigned to class A and pixels of the other parity assigned to class B. The white-to-white waveforms of the two classes can then be chosen so that they are offset in time so that the two classes are never simultaneously in the black state. One arrangement method for such waveforms is to use conventional balanced pulse-pair waveforms (i.e., waveforms comprising two rectangular voltage pulses of equal impulse but opposite polarity) for both waveforms, but delay one waveform by the duration of a single pulse. A pair of waveforms of this type is illustrated in Figures 1A and 1B of the accompanying drawings. Figure 1C shows the reflectance versus time for a display in which half of the pixels are driven using the Figure 1A waveform and the other half are driven using the Figure 1B waveform. From Figure 1C, it can be seen that the reflectance of the display never approaches black, as would occur, for example, if only the Figure 1A waveform were used.

[0093] Other waveform pairs (or larger populations—more than two classes of pixels can be used) can provide similar benefits. For example, for a mid-gray / mid-gray transition, two “single rail bounce” waveforms could be used, one of which would drive from a mid-gray level to white and back to mid-gray, while the other would drive from a mid-gray level to black and then back to mid-gray. Other spatial arrangements of pixel classes are also possible, such as horizontal or vertical stripes or random white noise.

[0094] In a second form of the GCMDS method, the division of pixels into classes is arranged so that one or more transient monochrome images are displayed during the update. This reduces the apparent flashing effect of the display by drawing the user's attention to the intermediate images rather than to any flashing light that occurs during the update, in the same way that a magician distracts the audience from an elephant entering from stage right. Examples of intermediate images that may be employed include monochrome checkerboards, company logos, stripes, clocks, page numbers, or Escher prints. For example, Figure 2 of the accompanying drawings illustrates a GCMDS method in which two transient horizontal stripe images are displayed during the transition; Figure 3 illustrates a GCMDS method in which two transient checkerboard images are displayed during the transition; Figure 4 illustrates a GCMDS method in which two transient random noise patterns are displayed during the transition; and Figure 5 illustrates a GCMDS method in which two transient Escher images are displayed during the transition.

[0095] The two ideas discussed above (using multiple waveforms and using transient intermediate images) can be used simultaneously to both reduce the flashing effect of transitions and to distract the user by drawing them into the interesting image.

[0096] It will be appreciated that implementation of the GCMDS method typically requires a controller that can maintain a map of pixel classes; such a map may be hardwired to the controller or loaded via software, the latter having the advantage that the pixel map can be changed at will. To derive the waveform required for each transition, the controller retrieves the pixel class of the associated pixel from the map and uses it as an additional pointer to a lookup table, which defines the various possible waveforms (see the aforementioned MEDEOD application, in particular U.S. Pat. No. 7,012,600). Alternatively, a simpler structure may be used where the waveforms for the various pixel classes are simply delayed versions of a single basic waveform; for example, a single waveform lookup table may be referenced to update two separate classes of pixels, with the two pixel classes starting to update with a time shift that may be equal to a multiple of the basic drive pulse length. It will be appreciated that in some divisions of pixels into classes, a map may be unnecessary, as the class of any pixel can be calculated simply from its row and column number. For example, in the stripe pattern flash shown in FIG. 2, a pixel can be assigned to its class based on whether its row number is even or odd, while in the checkerboard pattern shown in FIG. 3, a pixel can be assigned to its class based on whether the sum of its row number and column number is odd or even.

[0097] The GCMDS method of the present invention provides a relatively simple mechanism for reducing the visual effects of flashing during updates of a bi-stable display. The use of the GCMDS method with time delay waveforms for different pixel classes greatly simplifies the implementation of the GCMDS method, at the expense of some sacrifice in overall update time.

[0098] Part C: Balanced Pulse vs. White / White Transition Driving Scheme of the Present Invention

[0099] As explained above, the balanced pulse pair white / white transition drive scheme (BPPWWTDS) of the present invention is intended to reduce or eliminate edge artifacts when driving a bistable electro-optic display. The BPPWWTDS requires the application of one or more balanced pulse pairs (a balanced pulse pair or "BPP" is a pair of drive pulses of opposite polarity such that the net impulse of the balanced pulse pair is substantially zero) during a white-to-white transition, where the white-to-white transition can be identified as likely to produce an edge artifact, and the one or more balanced pulse pairs are in a spatiotemporal configuration such that the balanced pulse pair will be effective in eliminating or reducing the edge artifact.

[0100] BPPWWTDS attempts to reduce the visibility of accumulated error during the transition in a manner that does not have a distracting appearance and has limited DC imbalance. This is achieved by applying one or more balanced pulse pairs to a portion of the display's pixels, where the proportion of pixels in that portion is small enough that application of the balanced pulse pairs is not visually distracting. The visual distraction caused by application of BPP can be reduced by selecting pixels to which BPP is applied adjacent to other pixels undergoing easily visible transitions. For example, in one form of BPPWWTDS, BPP is applied to any pixel undergoing a white-to-white transition and having at least one of its eight neighboring pixels undergoing a non-white-to-white transition. A non-white-to-white transition is likely to induce a visible edge between the pixel to which it is applied and an adjacent pixel undergoing a white-to-white transition, and this visible edge can be reduced or eliminated by application of BPP. While this scheme for selecting pixels to which BPP should be applied has the advantage of being simpler, other, particularly more conservative, pixel selection schemes can be used. Conservative schemes (i.e., those that ensure that only a small number of pixels have BPP applied during any one transition) are desirable because such schemes have the least effect on the overall appearance of the transition.

[0101] As previously indicated, the BPP used in the BPPWWTDS of the present invention can comprise one or more balanced pulse pairs. Each half of a balanced pulse pair can consist of a single or multiple drive pulses, as long as each half of the pair has the same amplitude. The voltage of the BPP can vary, as long as the two halves of the BPP have the same amplitude but opposite signs. A period of zero voltage can occur between the two halves of the BPP or between consecutive BPPs. For example, in one experiment, the results of which are described below, the balanced BPP comprised a train of six pulses: +15V, −15V, +15V, −15V, +15V, −15V, each pulse lasting 11.8 milliseconds. Experimentally, it has been found that the longer the train of BPPs, the more edge cancellation is obtained. When a BPP is applied to a pixel adjacent to a pixel undergoing a non-white-to-white transition, shifting the BPP in time relative to the (non-white) / white waveform has also been found to affect the degree of edge reduction obtained. Currently, there is no complete theoretical explanation for these findings.

[0102] In the experiments referenced in the previous paragraph, BPPWWTDS was found to be effective in reducing the visibility of accumulated edges compared to a prior art global confinement (GL) drive scheme. Figure 6 of the accompanying drawings shows the difference in L* values ​​at various gray levels for the two drive schemes, and it can be seen that the L* difference for BPPWWTDS is much closer to zero (ideal) than for the GL drive scheme. Microscopic examination of the edge region after application of BPPWWTDS reveals two types of response that can account for the improvement. In some cases, the actual edge appears to be eroded by application of BPPWWTDS. In other cases, the edge is not significantly eroded, but a separate bright edge appears to be formed adjacent to the dark edge. This edge pair cancels out when viewed from a normal user distance.

[0103] It has been found that in some cases, application of BPPs (WWTDS) can actually overcorrect for edge effects (indicated by negative L* differences in plots such as those in Figure 6). See Figure 7, which shows such overcorrection in an experiment using a train of four BPPs. When such overcorrection occurs, it has been found that it can be reduced or eliminated by reducing the number of BPPs employed or by adjusting the temporal position of the BPPs relative to the non-white-to-white transition. For example, Figure 8 shows the results of an experiment using one to four BPPs to correct for edge effects. For the particular medium being tested, two BPPs appear to provide the best edge correction. The number of BPPs and / or the temporal position of the BPPs relative to the non-white-to-white transition can be adjusted in a time-varying manner (i.e., on the fly) to provide optimal correction of the predicted edge visibility.

[0104] As already discussed, drive schemes used for bistable electro-optic media should typically be DC balanced, i.e., the nominal DC imbalance of the drive scheme should be limited. While BPP is inherently DC balanced and therefore should not affect the overall DC balance of the drive scheme, a sudden reversal of voltage on a pixel capacitor (typically present in a backplane used to drive a bistable electro-optic medium) (see, e.g., U.S. Pat. No. 7,176,880) may result in incomplete charging of the capacitor during the latter half of the BPP, and incomplete charging may induce some DC imbalance in practice. BPP applied to a pixel where none of its neighboring pixels are undergoing a non-zero transition may lead to whitening or other variations in the optical state of the pixel, and BPP applied to a pixel with neighboring pixels undergoing a transition other than white may result in some darkening of the pixel. Therefore, careful consideration should be given to selecting the rules by which pixels are selected to receive BPP.

[0105] In one form of the BPPWWTDS of the present invention, a logical function is applied to the initial and final images (i.e., the images before and after the transition) to determine whether a particular pixel should have one or more BPPs applied during the transition. For example, various forms of the BPPWWTDS may specify that a pixel undergoing a white-to-white transition will have a BPP applied if all four primary direction neighboring pixels (i.e., pixels that share a common edge, not just a corner, with the pixel) have a final white state and at least one primary direction neighbor has an initial non-white state. If this condition is not true, a null transition is applied to the pixel, i.e., the pixel is not driven during the transition. Other logical selection rules can, of course, be used.

[0106] Another variation of the BPPWWTDS effectively combines the BPPWWTDS of the present invention with an SGU drive scheme by applying a global full drive scheme to certain selected pixels undergoing a white-to-white transition, further increasing edge cancellation. As noted above, in the discussion of the SGU drive scheme, the GC waveform for a white-to-white transition is typically very flashy, and therefore it is important to apply this waveform to only a small number of pixels during any one transition. For example, a logical rule may apply that the GC white-to-white waveform is only applied to a pixel when three of its primary directional neighboring pixels undergo a non-zero transition during the associated transition; in such a case, the flashing effect of the GC waveform is hidden within the activity of the three transitioning primary directional neighboring pixels. Furthermore, if a fourth primary directional neighbor undergoes a zero transition, the GC white-to-white waveform applied to the associated pixel may accentuate edges within the fourth primary directional neighborhood, and therefore it may be desirable to apply BPP to this fourth primary directional neighborhood.

[0107] Another variant of BPPWWTDS involves the application of a GC white-to-white (hereinafter "GCWW") transition to select areas of the background, i.e., areas in which both the initial and final states are white. This is done so that all pixels are processed once every predetermined number of updates, thereby clearing the display of edge and drift artifacts over time. The key difference from the variant discussed in the previous paragraph is that the decision as to whether a pixel should receive a GC update is based on spatial location and update number, rather than on the activity of neighboring pixels.

[0108] In one such variation, the GCWW transition is applied to a dithered subset of background pixels on a per-update basis. As discussed in Section A above, this produces only a minor flash or dimming of the background white state during the update, but can reduce the effects of image drift because after a certain number of updates, all background pixels are updated. However, the method may produce its own edge artifacts around the updated pixels, which persist until the surrounding pixels are themselves updated. According to the BPPWWTDS, edge-reduction BPP can be applied to pixels neighboring pixels of pixels undergoing the GCWW transition so that the background pixels can be updated without introducing significant edge artifacts.

[0109] In a further variation, the subset of pixels driven with the GCWW waveform is further separated into sub-subsets. At least some of the resulting sub-subsets receive a time-delayed version of the GCWW waveform so that only a portion of them are in the dark state at any given time during the transition. This further reduces the already attenuated effect of flashing during updates. A time-delayed version of the BPP signal is also applied to neighboring pixels of these sub-subsets. In this way, apparent background flashing can be reduced for a fixed reduction in exposure to image drift. The number of sub-subsets is limited by the increase in update time (caused by the use of the delayed signal) that is deemed acceptable. Typically, two sub-subsets are used, which nominally increases the update time by one basic drive pulse width (typically about 240 ms at 25°C). Having overly sparse sub-subsets also makes individual updated background pixels more psychovisually apparent, which adds a different type of distraction that may be undesirable.

[0110] Modification of a display controller (such as that described in the aforementioned U.S. Pat. No. 7,012,600) to implement various forms of the BPPWWTDS of the present invention is straightforward. One or more buffers store grayscale data representing the initial and final images for the transition. From this data and other information, such as temperature and drive scheme, the controller selects the correct waveform to apply to each pixel from a lookup table. To implement the BPPWWTDS, a mechanism must be provided to choose among several different transitions for the same initial and final gray state (particularly a state representing white) depending on the transitions undergone by neighboring pixels, the subgroup to which each pixel belongs, and the number of updates (when different subgroups of pixels are updated in different updates). For this purpose, the controller may store additional "substates" (as if they were additional gray levels). For example, if a display uses 16 gray tones (numbered 0-15 in the lookup table), states 16, 17, and 18 could be used to represent the type of white transition required. These sub-state values ​​can be generated at various different levels in the system, for example at the host level, at the time of rendering to the display buffer, or at an even lower level in the controller when generating LUT addresses.

[0111] Several variations of the BPPWWTDS of the present invention can be envisioned. For example, any short DC-balanced or even DC-unbalanced sequence of drive pulses can be used instead of the balanced pulse pair. The balanced pulse pair can be replaced by a top-off pulse (see Part D below), or BPP and top-off pulses can be used in combination.

[0112] Although the BPPWWTDS of the present invention has been described above primarily in relation to white state edge reduction, it may also be applicable to dark state edge reduction, which can be easily achieved simply by reducing the polarity of the drive pulses used in the BPPWWTDS.

[0113] The BPPWWTDS of the present invention can provide a "flash-free" drive scheme, which does not require periodic global full updates, which are considered undesirable by many users.

[0114] (Part D: White / White Top-Off Pulse Driving Scheme Method of the Present Invention)

[0115] As described above, the fourth method of the present invention for reducing or eliminating edge artifacts is similar to the BPPWWTDS described above in that a "special pulse" is applied during a white-to-white transition at pixels that can be identified as likely to cause edge artifacts and that are in a spatiotemporal configuration such that the special pulse will be effective in eliminating or reducing the edge artifacts. However, this fourth method differs from the third method in that the special pulse is not a balanced pulse pair, but rather a "top-off" or "refresh" pulse. The terms "top-off" or "refresh" pulse are used herein in the same manner as in the aforementioned U.S. Patent No. 7,193,625 to refer to a pulse applied to a pixel that is at or near one extreme optical state (usually white or black) and tends to drive the pixel toward that extreme optical state. In this case, the term "top-off" or "refresh" pulse refers to the application to a white or near-white pixel of a drive pulse having a polarity that drives the pixel toward its extreme white state. This fourth driving method of the present invention may hereinafter for convenience be referred to as the "white / white top-off pulse driving scheme" or "WWTOPDS" method of the present invention.

[0116] In the WWTOPDS method of the present invention, the criteria for selecting pixels to which a top-off pulse is applied are similar to those for pixel selection in the BPPWWTDS method described above. Thus, the percentage of pixels to which a top-off pulse is applied during any one transition should be small enough that the application of the top-off pulse is not visually distracting. The visual distraction caused by the application of a top-off pulse can be reduced by selecting pixels to which a top-off pulse is applied adjacent to other pixels undergoing easily visible transitions. For example, in one form of WWTOPDS, a top-off pulse is applied to any pixel undergoing a white-to-white transition and having at least one of its eight neighboring pixels undergoing a non-white-to-white transition. A non-white-to-white transition is likely to induce a visible edge between the pixel to which the non-white-to-white transition is applied and the adjacent pixel undergoing a white-to-white transition, and this visible edge can be reduced or eliminated by the application of a top-off pulse. While this scheme for selecting pixels to which a top-off pulse should be applied has the advantage of being simpler, other, particularly more conservative, pixel selection schemes can be used. Conservative schemes (i.e., those that ensure that a top-off pulse is applied to only a small number of pixels during any one transition) are desirable because such schemes have minimal impact on the overall appearance of the transition. For example, a typical black-to-white waveform is unlikely to induce an edge in a neighboring pixel, and therefore, if there are no other expected edge buildups at a pixel, there is no need to apply a top-off pulse to this neighboring pixel. For example, consider two neighboring pixels (designated P1 and P2) that display the following sequence: P1: W→W→B→W→W and P2:W→B→B→B→W Although P2 is likely to induce an edge in P1 during its white-to-black transition, this edge is subsequently erased during the P1 black-to-white transition, and therefore the final P2 black-to-white transition should not trigger the application of a top-off pulse in P1. Many more complex and conservative schemes can be developed. For example, edge induction can be predicted on a neighborhood-by-neighborhood basis. Furthermore, it may be desirable to leave some minority edges alone if they are below a certain threshold. Alternatively, edge cleaning may not be necessary unless the pixel is surrounded only by white pixels, as edge effects tend not to be easily visible when they are adjacent to an edge between two pixels with very different gray levels.

[0117] It has been found experimentally that when the application of a top-off pulse to a pixel is associated with at least one of its eight neighboring pixels undergoing a transition from non-white to white, the timing of the top-off pulse relative to the transition in the adjacent pixel substantially affects the degree of edge reduction achieved, with best results being obtained when the top-off pulse coincides with the end of the waveform applied to the adjacent pixel. The reasons for this experimental finding are not currently fully understood.

[0118] In one form of the WWTOPDS method of the present invention, a top-off pulse is applied in conjunction with an impulse bank drive scheme (see section F below regarding that). In such a combined WWTOPDS / IBDS, in addition to the application of a top-off pulse, a clear slideshow waveform (i.e., a waveform that repeatedly drives the pixel to its extreme optical state) is applied to the pixel as appropriate when DC balance is to be restored. This type of drive scheme is illustrated in FIG. 9 of the accompanying drawings. Both the top-off and clear (slideshow) waveforms are applied only when pixel selection conditions are met; in all other cases, a null transition is used. Such a slideshow waveform will remove edge artifacts from the pixel, but is a visible transition. The results of one drive scheme of this type are shown in FIG. 10 of the accompanying drawings; these results can be compared to those in FIG. 6, although it should be noted that the vertical scale is different in the two sets of graphs. Due to the periodic application of the clear pulse, the sequence is not monotonic. The application of the slideshow waveform can be controlled to occur only infrequently and only adjacent to other visible activity, so that it is rarely noticeable. The slideshow waveform essentially has the advantage of completely clearing a pixel, but the disadvantage of inducing edge artifacts in adjacent pixels that require cleaning. These adjacent pixels can be flagged as likely to contain edge artifacts and therefore require cleaning at the next available opportunity, but it should be understood that the resulting drive scheme can lead to complex developments in the edge artifacts.

[0119] In another form of the WWTOPDS method of the present invention, the top-off pulse is applied regardless of DC imbalance. While this presents some risk of long-term damage to the display, potentially such a small DC imbalance spread over a long time frame would not be very significant; indeed, commercial displays already experience DC imbalances of the same magnitude due to unequal reservoir capacitor charging on the TFTs in the positive and negative voltage directions. The results of one drive scheme of this type are shown in Figure 11 of the accompanying drawings; these results can be compared to those in Figure 6, although it should be noted that the vertical scale is different in the two sets of graphs.

[0120] The WWTOPDS method of the present invention can be applied so that the top-off pulse is statistically DC balanced without the DC imbalance being mathematically bounded. For example, a "payback" transition can be applied to balance the "top-off" transition in a manner that would be balanced on average for a typical electro-optic medium, but where the aggregate net impulse would not be tracked for individual pixels. Top-off pulses applied in a spatiotemporal context that reduces edge visibility are useful, regardless of the precise mechanism by which they operate; in some cases, edges appear to be significantly erased, while in other cases, pixel centers appear to be brightened to an extent that locally compensates for the darkness of the edge artifact.

[0121] The top-off pulse may comprise one or more drive pulses and may use a single drive voltage or a series of different voltages in the different drive pulses.

[0122] The WWTOPDS method of the present invention can provide a "flash-free" driving scheme that does not require periodic global full updates, which are considered undesirable by many users.

[0123] Part E: Linear Edge Addition Pixel Driving Scheme Method of the Present Invention

[0124] As previously mentioned, the "straight edge additional pixel driving scheme" or "SEEPDS" method of the present invention seeks to reduce or eliminate edge artifacts that occur along straight edges between driven and undriven pixels. The human eye is particularly sensitive to linear edge artifacts, especially those that extend along rows or columns of a display. In the SEEPDS method, some pixels adjacent to the straight edge between driven and undriven areas are, in fact, driven such that any edge effects caused by the transition are not only along the straight edge, but also include edges perpendicular to this straight edge. Driving a limited number of additional pixels in this manner has been found to significantly reduce the visibility of edge artifacts.

[0125] The basic principle of the SEEPDS method is illustrated in Figures 12A and 12B of the accompanying drawings. Figure 12A illustrates a prior art method in which a regional or partial update is used to transition from a first image, the top half of which is black and the bottom half is white, to a second image that is all white. Because a regional or partial driving scheme is used for the update and only the black top half of the first image is rewritten, edge artifacts are very likely to occur along the boundary between the original black and white areas. Such long-term horizontal edge artifacts tend to be easily visible and annoying to a viewer of the display. According to the SEEPDS method, the update is divided into two separate steps, as illustrated in Figure 12B. The first step of the update is to change certain white pixels on the nominally "undriven" side of the original black / white boundary (i.e., the side where the pixels are the same color, i.e., white, in both the initial and final images) to black, and the white pixels, and therefore the driven black, are arranged in a series of substantially triangular areas adjacent to the original boundary, such that the boundary between the black and white areas becomes serpentine, with the originally straight boundary comprising multiple segments extending perpendicular to the original boundary. The second step changes all black pixels, including the "extra" pixels driven black in the first step, to white. Even if this second step leaves edge artifacts along the boundary between the white and black areas present after the first step, these edge artifacts will be distributed along the serpentine boundary shown in FIG. 12B and will be much less visible to a viewer than similar artifacts that would extend along the straight boundary shown in FIG. 12A. Edge artifacts may in some cases be further reduced because some electro-optic media have at least a majority of black pixels adjacent to the serpentine boundary established after the first step, and therefore exhibit fewer visible edge artifacts when they remain in one optical state for only a short period of time.

[0126] When selecting a pattern to be implemented in the SEEPDS method, care should be taken to ensure that the frequency of the serpentine boundary shown in FIG. 12B is not too high. A frequency that is too high, comparable to that of the pixel spacing, causes edges perpendicular to the original boundary to bleed and have a darker appearance, accentuating the edge artifact rather than reducing it. In such cases, the boundary frequency should be reduced. However, a frequency that is too low can also make the artifact very visible.

[0127] In the SEEPDS method, the update scheme may follow the pattern as follows: -Region update → Standard image [any amount of time] -Region update (extended slightly to capture new edges) → Image with modified edges →Region update → Next image or -Partial update → Standard image [any amount of time] -Partial update → Image with modified edges →Partial update → Next image Alternatively, if a full update is used in a particular region, the pattern could be: -Full region update → Standard image [any amount of time] -Region update (extended slightly to capture new edges) → Next image

[0128] Provided there is no unacceptable interference with the electro-optical properties of the display, the display may utilize the SEEPDS method at all times according to the following pattern: -Partial update → Standard image with fixed edges [any amount of time] -Partial update → Next image

[0129] To reduce edge artifacts over multiple updates, the SEEPDS method can be arranged to vary the location of the curve of the serpentine boundary, such as that shown in FIG. 12B, to reduce repeated edge gain in repeated updates.

[0130] The SEEPDS method can substantially reduce visible edge artifacts in displays that utilize regional and / or partial updates. The method does not require changes to the overall drive scheme used, and some forms of the SEEPDS method can be implemented without requiring changes to the display controller. The method can be implemented either via hardware or software.

[0131] Part F: Impulse Bank Drive Scheme Method of the Present Invention

[0132] As previously mentioned, in the impulse bank driving scheme (IBDS) method of the present invention, pixels are "enabled" to borrow or return impulse units from a "bank" that tracks their impulse "debt." Generally, pixels borrow impulses (either positive or negative) from the bank when needed to achieve a certain goal, and return impulses when they can reach the next desired optical state using a smaller impulse than that required for a fully DC balanced driving scheme. In practice, the impulse payback waveform may include zero net impulse adjustment elements, such as balanced pulse pairs and periods of zero voltage, to achieve the desired optical state with reduced impulse.

[0133] Notably, the IBDS method requires the display to maintain an "impulse bank register" containing one value for each pixel of the display. When a pixel is required to deviate from the normal DC-balanced drive scheme, the impulse bank register for the associated pixel is adjusted to indicate the deviation. When the register value for any pixel is non-zero (i.e., the pixel deviates from the normal DC-balanced drive scheme), at least one subsequent transition of the pixel is performed using a reduced impulse waveform that differs from the corresponding waveform in the normal DC-balanced drive scheme and reduces the absolute value of the register value. The maximum amount of impulse that any one pixel can borrow should be limited to a predetermined value, since excessive DC imbalance is likely to adversely affect the pixel's performance. Application-specific methods should be developed to address situations where the predetermined impulse limit is reached.

[0134] A simple form of the IBDS method is shown in Figure 9 of the accompanying drawings. The method uses a commercial electrophoretic display controller, which is designed to control a 16-gray level display. To implement the IBDS method, the 16 controller states normally assigned to 16 gray levels are reassigned to four gray levels and four impulse liability levels. It should be understood that commercial implementations of the IBDS controller will allow for additional storage to allow the full number of gray levels to be used with several impulse liability levels (see Part H below).

[0135] In the IBDS method illustrated in FIG. 9, a single unit of impulse (a −15V drive pulse) is borrowed to implement a top-off pulse during a white-to-white transition under predetermined conditions (a zero transition should normally have zero net impulse). The impulse is paid back by implementing a black-to-white transition that lacks one drive pulse toward white. Without any corrective action, the omission of one drive pulse tends to make the resulting white state slightly darker than a white state using the full number of drive pulses. However, there are several known “adjustment” methods, such as a pre-pulse balancing pulse pair or an intermediate period of zero voltage, that can achieve a satisfactory white state.

[0136] When the maximum impulse borrow (3 units) is reached, a clear transition is applied; the clear transition is 3 impulse units short of a full white-to-white slideshow transition; the waveform used for this transition must, of course, be adjusted to eliminate the visual effect of impulse starvation. Such a clear transition is undesirable due to its greater visibility; therefore, it is important to design rules for the IBDS to be conservative in impulse borrow and rapid in impulse payback.

[0137] Other forms of IBDS methods may utilize additional transitions for impulse payback, thereby reducing the number of forced clear transitions required. Still other forms of IBDS methods may utilize impulse banks in which the impulse deficit or surplus decays over time, such that DC balance is maintained only over short time scales, and there is some experimental evidence that at least some types of electro-optic media only require such short-term DC balance. Clearly, decaying the impulse deficit or surplus over time reduces the number of opportunities for reaching the impulse limit and, therefore, the number of opportunities for a clear transition to be required.

[0138] The IBDS method of the present invention can reduce or eliminate some practical problems in bi-stable displays, such as edge persistence in non-photic drive schemes, while still providing target-dependent adaptation of the drive scheme down to the individual pixel level while maintaining bounds on DC imbalance.

[0139] Part G: Mode Update Request Override Driving Scheme of the Present Invention

[0140] As discussed above, the Mode Update Request Override Driving Scheme (MURODS) method employs a state machine algorithm that calculates a numerical quantity indicative of the level of stress experienced by the display device and switches between update schemes as needed to manage remnant voltage buildup. Figure 14 shows a flowchart of an exemplary driving method 1400 incorporating the remnant voltage management state machine algorithm.

[0141] Method 1400 begins at step 1405, where when an update to the display is required, the display controller used to control method 1400 selects drive waveforms to apply to the display pixels.

[0142] There are many events that can trigger an update request. For example, an update request can be triggered by a user interaction with the display, such as swiping the screen, advancing pages, or scrolling the screen. As another example, if the display device presents moving images, each successively displayed image will cause an update request.

[0143] In step 1410, a fast mode update scheme is used to apply drive waveforms to the display pixels. The fast mode update scheme prioritizes update time over color accuracy. For example, the update time on a display device including a full-color electronic ink system with four color pigments can be reduced to approximately 500-650 ms using the fast mode update scheme in conjunction with pausing the PDD routine after every update and providing no or substantially no dwell time after each update. This makes the fast mode update scheme well suited to handle use cases in which a user interacts frequently with the display device. However, the fast mode update scheme typically includes DC-unbalanced drive waveforms, which can lead to a buildup of residual voltage and cause display artifacts such as image retention.

[0144] In step 1415, a numerical quantity indicative of the level of stress experienced by the display device is calculated. For example, a display controller used to control method 1400 may calculate the level of stress quantity based on several variables and parameters of the system that are tracked for each update. For each update n, a level of stress or "LOS" quantity x(n) is calculated. An exemplary formula for calculating x(n) is illustrated here:

number

[0145] In some embodiments, x(n) is an approximation of the remnant voltage expected to have accumulated on a display pixel of an electro-optic display based on update n and all previous updates. In some embodiments, x(n) is a fraction of the remnant voltage or an order of magnitude greater than the remnant voltage. In some embodiments, x(n) is a scalar quantity that can be used as an index or indicator of the rise or decay of the remnant voltage, but is not an approximation of the actual remnant voltage expected to have accumulated based on update n and all previous updates.

[0146] In the first term of the LOS quantification, UT(n) denotes the update time in milliseconds of the update mode used for the most recently completed update, and DT(n) denotes the number of milliseconds of PDD applied after the most recently completed update.

[0147] TAU represents the time constant for the decay of the stress level x(n). TAU is determined before updates begin to be applied to the display and remains static throughout all updates. In some embodiments, TAU directly corresponds to the RC time constant for the decay of remnant voltage in a particular display system or production batch of electrophoretic material. For example, a live display system can be characterized by measuring not only the amount of remnant voltage generated by several different update sequences, but also the decay time for the remnant voltage. A value for TAU can then be calculated based on the curve resulting from the remnant voltage measurements over time.

[0148] In some embodiments, the ink system can be represented as a parameterized mathematical model, and the TAU can be approximated based on one or more simulations of the updates and measurements described above. In some embodiments, the TAU is not specific to a particular display system or production batch of electrophoretic material. For example, the time constant of decay of the level of stress amount x(n) can be calculated based on several different display systems and / or production batches of electrophoretic material, and a value can be chosen for the TAU that roughly approximates the time constant of decay of the level of stress amount x(n) across several ink platforms and display systems.

[0149] Finally, x(n-1) is the level of stress calculated after update n-1 is completed. Those skilled in the art will understand that some initial conditions are set so that x(n) can be calculated after the first update.

[0150] Turning to the second term of the LOS quantification, DT(n) and TAU represent the same quantities as in the first term, and B is a quantity that varies depending on the drive mode used for update n. For example, if a fast mode update was used for update n, B may be a non-zero positive value or amount that increases the value of x(n) because a fast mode update is likely to increase the residual voltage. Alternatively, if a standard mode update was used for update n, B would be set to zero and the second term would be zero.

[0151] The non-zero value of B used after a fast mode update remains unchanged throughout all updates, and the value of B is always set to zero after a standard mode update. In some embodiments, when a fast mode update is used for update n, the value of B used is a coefficient representing the worst-case residual voltage rise based on an evaluation of the update sequence that has the most DC imbalance and causes the most residual voltage rise. In some embodiments, when a fast mode update is used for update n, the value of B used is a coefficient representing the average amount of residual voltage rise based on an evaluation of the update sequence that has the most DC imbalance and causes the most residual voltage rise.

[0152] It should be understood from the stress level equation that increasing the update time UT(n) has the effect of reducing the rise in x(n) after each update. Similarly, increasing the post-drive discharge time DT(n) has the effect of reducing the rise in x(n) after each update. Conversely, it should be understood that updating in high-speed mode and suspending all PDD routines has the effect of increasing the rise in x(n) after each update.

[0153] Once x(n) is calculated, method 1400 proceeds to step 1417, which includes receiving a subsequent update request. For example, one or more of the events described above in connection with step 1405 may cause a subsequent request to be received to update the optical states of display pixels of the electro-optic display.

[0154] Next, method 1400 proceeds to step 1420 where it is determined whether update n was a fast mode update or a standard mode update. If update n was a fast mode update, method 1400 proceeds to step 1425 where it is determined whether x(n) is greater than a first threshold. If update n was a standard mode update, method 1400 proceeds to step 1435 where it is determined whether x(n) is less than a second threshold.

[0155] The first threshold indicates a limit on the level of tolerable stress on the display device. If x(n) does not exceed the first threshold, as shown in step 1425, the algorithm returns to step 1410 and another fast mode update is applied. However, if x(n) does exceed the first threshold, the algorithm overrides the next requested fast mode update and instead proceeds to step 1430 where a standard mode update, which is less stressful on the display, is used.

[0156] The standard mode update scheme prioritizes both update time and color accuracy. For example, the update time on a display device, including a full-color electronic ink system with four color pigments, can be 1,000 ms in duration using the standard mode update scheme, in conjunction with suspending the PDD routine after every update and providing no or substantially no dwell time after each update. However, the waveforms used for standard mode updates are DC-balanced and have been observed to reduce the amount of residual voltage even when the PDD routine is immediately suspended. This only somewhat impacts the user experience, as updates can take longer than 350-500 ms in standard mode, making the standard mode update scheme highly suitable for reducing the level of stress on the display device. In some embodiments, the drive waveforms for the standard mode update scheme are approximately 30%-55% longer in duration than the drive waveforms for the fast update scheme. In some embodiments, the drive waveforms for the standard mode update scheme are approximately 50%-70% longer in duration than the drive waveforms for the fast update scheme.

[0157] If the algorithm overrides the fast mode update and update n becomes a standard mode update, method 1400 proceeds from step 1420 to step 1435 where it is determined whether x(n) is less than a second threshold.

[0158] This second lower threshold is incorporated into the algorithm to allow hysteresis so that the display can only be activated in a less stressful mode until the level of stress calculated and tracked by the algorithm is reduced to a level indicative of an acceptable level of stress experienced by the display device. The first and second thresholds can be set to allow the average level of stress due to residual charge build-up to be reduced, while also reducing the frequency at which the display device switches between operating modes under certain use cases.

[0159] If x(n) is still not less than the second threshold, as shown in step 1435, the algorithm returns to step 1430 and another standard mode update is applied. However, if x(n) is less than the second threshold, the algorithm returns to step 1410 and again applies the fast mode update.

[0160] 15 shows a flowchart of another exemplary driving method 1500 incorporating a residual voltage management state machine algorithm. The operations underlying the steps of method 1500 are substantially similar to the steps of method 1400. Therefore, steps of method 1500 are numbered consistently with similar steps of method 1400, whenever possible.

[0161] Method 1500 begins by determining 1505 a level of stress amount for a display pixel of an electro-optic display based on at least one previous update to the optical state of the display pixel. For example, an algorithm using the example formula described in connection with step 1415 of method 1400 can be used to calculate the level of stress amount based on the previous update (e.g., n, n-1). As discussed above, initial conditions can be set such that x(n) can be calculated when there have been no previous updates to the optical state of the display pixel (and thus x(n) is undefined and / or cannot be determined).

[0162] Method 1500 then includes receiving 1518 a request to update the optical states of the display pixels. For example, one or more of the events described above in connection with step 1405 of method 1400 may cause a request to update the optical states of the display pixels of the electro-optic display to be received.

[0163] Next, in step 1520, method 1500 determines whether the drive waveform used for the previous update of the display pixel was from a first update scheme (e.g., a fast mode update scheme) or a second update scheme (e.g., a standard mode update scheme). If the drive waveform used for the previous update was from the first update scheme, method 1500 proceeds to step 1525, where it is determined whether the level of the stress amount x(n) is greater than a first stress threshold level. If the drive waveform used for the previous update was from a second update scheme, method 1500 proceeds to step 1535, where it is determined whether the level of the stress amount x(n) is less than a second stress threshold level.

[0164] Proceeding from step 1520 to step 1525, method 1500 includes applying a drive waveform from the first update scheme to the display pixel when the drive waveform from the first update scheme was used for a previous update of the display pixel and the level of the amount of stress is not greater than the first stress threshold level. Alternatively, method 1500 includes applying a drive waveform from the second update scheme to the display pixel when the drive waveform from the first update scheme was used for a previous update of the display pixel and the level of the amount of stress is greater than the first stress threshold level.

[0165] For example, similar to step 1425 of method 1400, if the level of stress does not exceed the first stress threshold level, the algorithm proceeds to step 1510, where drive waveforms from a first update scheme (e.g., a fast mode update) are applied to the display pixel. However, if the level of stress does exceed the first stress threshold level, the algorithm overrides the next requested fast mode update and instead proceeds to step 1530, where drive waveforms from a second update scheme (e.g., a standard mode update) that is less stressful to the display are applied. After updating the optical state of the display pixel by applying drive waveforms from the first update scheme (1510) or the second update scheme (1530), method 1500 returns to step 1515, where the level of stress is determined based on the just-completed update in preparation for any subsequent update requests (e.g., step 1518).

[0166] If a drive waveform from the second update scheme was used for the previous update of the display pixel, method 1500 proceeds from step 1520 to step 1535, where it is determined whether the level of the stress amount is less than the second stress threshold level. Proceeding from step 1520 to step 1535, method 1500 includes applying a drive waveform from the first update scheme to the display pixel when a drive waveform from the second update scheme was used for the previous update of the display pixel and the level of the stress amount is less than the second stress threshold level. Alternatively, method 1500 includes applying a drive waveform from the second update scheme to the display pixel when a drive waveform from the second update scheme was used for the previous update of the display pixel and the level of the stress amount is not greater than the second stress threshold level.

[0167] For example, similar to step 1435 of method 1400, if the level of the amount of stress is not yet below the second stress threshold level, the algorithm proceeds to step 1530, where a drive waveform from a second update scheme (e.g., a standard mode update) is applied to the display pixel. However, if the level of the amount of stress is below the second stress threshold level, the algorithm proceeds to step 1510, where a drive waveform from a first update scheme (e.g., a fast mode update) is applied to the display pixel. After updating the optical state of the display pixel by applying a drive waveform from either the first update scheme (1510) or the second update scheme (1530), method 1500 returns to step 1515, where a level of the amount of stress is determined based on the just-completed update in preparation for any subsequent update requests (e.g., step 1518).

[0168] The overall application of the methods described above employing stress management state machines allows the device to remain responsive to the user under constant use. While applying longer display updates may be less desirable to the user, this is preferable to having the device stall and unresponsive for a period of time (even if the device provides a notification to the user such as a "Please wait, processing..." pop-up window).

[0169] Applying a longer and / or DC balance refresh in this alternate state also has the benefit of accelerating the remnant voltage reduction, helping to lower the stress induced on the display.

[0170] While the application of DC unbalanced drive schemes has several advantages (such as improved performance), display remnant voltage control becomes more important. The state machine approach described above enables the implementation of these classes of drive schemes in a device in such a way that it monitors and manages stress on the display while maintaining the device's responsiveness to the user.

[0171] (Part H: Display Controller)

[0172] As will be readily apparent from the foregoing description, many of the methods of the present invention require or render desirable modifications in prior art display controllers. For example, the form of the GCMDS method described in Section B above, in which an intermediate image is flashed on the display between two desired images (this variation is hereinafter referred to as the “intermediate image GCMDS” or “II-GCMDS” method), may require pixels undergoing the same overall transition (i.e., having the same initial and final gray levels) to experience two or more different waveforms depending on the gray levels of the pixel in the intermediate image. For example, in the II-GCMDS method illustrated in FIG. 5, pixels that are white in both the initial and final images will experience two different waveforms depending on whether they are white in the first intermediate image and black in the second intermediate image, or black in the first intermediate image and white in the second intermediate image. Thus, a display controller used to control such a method must typically map each pixel to one of the available transitions according to an image map associated with the transition images. Clearly, more than two transitions can be associated with the same initial and final states. For example, in the II-GCMDS method illustrated in Figure 4, a pixel can be black in both intermediate images, or white in both intermediate images, or black in one intermediate image and white in the other intermediate image, so that a white-to-white transition between the initial and final images can be associated with four different waveforms.

[0173] Various modifications of the display controller can be used to enable storage of transition information. For example, an image data table that normally stores the gray level of each pixel in the final image can be modified to store one or more additional bits that specify the classification to which each pixel belongs. For example, an image data table that previously stored four bits for each pixel to indicate which of 16 gray levels the pixel will exhibit in the final image can be modified to store five bits for each pixel, with the most significant bit for each pixel defining which of two states (black or white) the pixel will exhibit in a monochrome intermediate image. Obviously, if the intermediate image is not monochrome or if more than one intermediate image is used, two or more additional bits may need to be stored for each pixel.

[0174] Alternatively, different image transitions can be encoded into different waveform modes based on a transition state map, for example, waveform mode A would take pixels through transitions with a white state in the intermediate image, while waveform mode B would take pixels through transitions with a black state in the intermediate image.

[0175] Clearly, it would be desirable for both waveform modes to begin updating simultaneously so that the intermediate images appear smoothly; to this end, modifications to the display controller's architecture would be necessary. The host processor (i.e., the device providing the image to the display controller) must indicate to the display controller whether the pixels loaded into the image buffer are associated with either waveform mode A or waveform mode B. This capability does not exist in prior art controllers. However, a reasonable approximation would be to take advantage of the regional update feature of current controllers (i.e., the feature that allows the controller to use different drive schemes in different areas of the display) and start the two modes offset by one scan frame. To allow the intermediate images to appear properly, waveform modes A and B must be constructed with this single scan frame offset in mind. In addition, the host processor would be required to load two images into the image buffer and command two regional updates. Image 1, loaded into the image buffer, should be a composite of the initial and final images, with only pixels corresponding to waveform mode A regions being changed. Once the composite image is loaded, the host must command the controller to begin regional updates using waveform mode A. The next step is to load Image 2 into the image buffer and command a global update using waveform mode B. Since the pixels commanded in the first region update command are already fixed for update, only pixels in the dark regions of the intermediate image assigned to waveform mode B will undergo a global update. In modern controller architectures, only controllers with a per-pixel pipeline architecture and / or controllers with no restrictions on rectangular region size will be able to perform the above procedure.

[0176] The same result can be achieved using a single waveform, since each individual transition in waveform mode A and waveform mode B is the same, but simply delayed by the length of their respective first pulse. Here, the second update (the global update in the previous paragraph) is delayed by the length of the first waveform pulse. Image 2 is then loaded into the image buffer and commanded with a global update using the same waveform. The same degrees of freedom are required for rectangular regions.

[0177] Another modification of the display controller is required by the BPPWWTG method of the present invention, described in Part C above. As already explained, the BPPWWTG method requires the application of a balance pulse pair to a given pixel according to rules that take into account the transitions undergone by neighboring pixels of the pixel to which the balance pulse pair may be applied. To accomplish this, at least two additional transitions (not transitions between gray levels) are needed; however, current 4-bit waveforms cannot accommodate the additional states, so a new approach is required. Three options are discussed below.

[0178] The first option is to store at least one additional bit for each pixel in the same manner as described above with reference to the GCMDS method. For such a system to function, the calculation of the following state information must be performed for every pixel upstream of the display controller itself. The host processor must evaluate the initial and final image states for every pixel, as well as those of its nearest neighbors, to determine the appropriate waveform for that pixel. An algorithm for such a method is proposed above.

[0179] A second option for implementing the BPPWWTG method is again similar to that for implementing the GCMDS method, i.e., encoding additional pixel states (beyond the usual 16 states representing gray levels) into two separate waveform modes. An example would be waveform Mode A, a traditional 16-state waveform encoding transitions between optical gray levels, and waveform Mode B, a new waveform mode encoding two states (States 16 and 17) and the transition between them and State 15. However, this raises a potential problem in that the impulse potentials of the special states in Mode B would not be the same as those in Mode A. One solution would be to have as many modes as there are white-to-white transitions, and in each mode, use only that transition, thus producing Modes A, B, and C, but this would be very inefficient. Alternatively, one could first send a null waveform that maps pixels transitioning from Mode B to Mode A to State 16, and then transition from State 16 in a subsequent Mode A transition.

[0180] To implement such a dual-mode waveform system, a means similar to dual waveform implementation option 3 may be considered. First, the controller must determine how to modify the next state of every pixel through a pixel-by-pixel examination of the pixel's initial and final image states, as well as its nearest neighbors. For pixels whose transition falls into waveform mode A, the new states of those pixels must be loaded into the image buffer, and the region updates for those pixels must then be commanded to use waveform mode A. One frame later, for pixels whose transition falls into waveform mode B, the new states of those pixels must be loaded into the image buffer, and the region updates for those pixels must then be commanded to use waveform mode B. In modern controller architectures, only controllers with a pixel-by-pixel pipeline architecture and / or no restrictions on rectangular region size will be able to perform the above procedure.

[0181] A third option is to use a controller architecture with separate final and initial image buffers (alternately loaded with successive images) with additional memory space for optional state information. These are fed into pipelined operators that can perform various operations on every pixel, taking into account the initial, final, and additional states of each pixel's nearest neighbors, and their effect on the pixel. The operators calculate a waveform table index for each pixel, store this in a separate memory location, and optionally modify the saved state information for the pixel. Alternatively, a memory format can be used whereby all of the memory buffers are combined into a single large word for each pixel. This reduces the number of reads from different memory locations for every pixel. In addition, a 32-bit word is proposed with a frame count timestamp field, which allows for optional entry into the waveform lookup table for any pixel (per-pixel pipelining). Finally, a pipelined structure for the operators is proposed, where three image rows are loaded into fast-access registers, allowing for efficient shifting of data into the operator structure.

[0182] The frame count timestamp and mode fields can be used to create unique specifiers in a mode lookup table to provide the illusion of a per-pixel pipeline. These two fields allow each pixel to be assigned one of 15 waveform modes (allowing one mode state to have no effect on the selected pixel) and one of 8,196 frames (currently well exceeding the number of frames required for display updates). The price for this added flexibility achieved by expanding the waveform index from 16 bits to 32 bits, as in prior art controller designs, is the speed at which the display can be scanned. In a 32-bit system, twice as many bits must be read from memory for each pixel, and the controller has limited memory bandwidth (the rate at which data can be read from memory). This limits the rate at which the panel can be scanned, since the entire waveform table index (currently consisting of a 32-bit word for each pixel) must be read for every single scan frame.

[0183] The operator may be a general purpose arithmetic logic unit (ALU) capable of performing simple operations on the pixel under examination and its nearest neighbors, such as: Bitwise logical operations (AND, NOT, OR, XOR) Integer arithmetic (addition, subtraction, optionally, multiplication, and division) Bit shift operations

[0184] The nearest neighboring pixels are identified in the dashed box that surrounds the pixel under examination. The instructions for the ALU can be hard-coded or stored in system non-volatile memory and loaded into the ALU instruction cache at startup. This architecture will allow for great flexibility in designing new waveforms and algorithms for image processing.

[0185] The image preprocessing required by the various methods of the present invention will now be considered. For dual-mode waveforms or waveforms using balanced pulse pairs, it may be necessary to map an n-bit image to n+1-bit states. Several approaches to this operation may be used: (a) Alpha blending may enable double transitions based on a transition map / mask. If a 1-bit pixel per alpha mask is maintained to identify regions associated with transition mode A and transition mode B, this map may be blended with an n-bit subsequent image to generate an n+1-bit transition mapped image, which may then use an n+1-bit waveform. A suitable algorithm is as follows: DP=∝IP+(1-∝)M {(If M=0, DP=0.5IP (specifies a 1-bit shift to the right for the IP data), If M=1, DP=IP (specifies no data shift) Where DP = Display Pixels IP = Image Pixels M = pixel mask (either 1 or 0) ∝=0.5 For the 5-bit example with 4-bit gray level image pixels discussed above, this algorithm would place pixels that fall within the transition mode A region (specified by 0 in the pixel mask) in the 16-31 range, and pixels that fall within the transition mode B region in the 0-15 range. (b) Simple raster operations may prove easier to implement: simply ORing the mask bits into the most significant bits of the image data will achieve the same result. (c) Additionally, adding 16 to the image pixels associated with one of the transition regions according to the transition map / mask would also solve the problem.

[0186] For waveforms using balanced pulse pairs, the above steps may be necessary, but not sufficient. If the dual-mode waveform has a fixed mask, BPP requires some complex calculations to generate the unique masks required for the proper transitions. This calculation step may eliminate the need for a separate mask step; the image analysis and display pixel calculations can incorporate the mask step.

[0187] The SEEPDS method, discussed in Part E above, involves additional complexity in the controller architecture, namely, the creation of "artificial" edges, i.e., edges that do not appear in the initial or final images, but are required to define intermediate images that occur during the transition, such as those shown in Figure 12B. Whereas prior art controller architectures only allow region updates to be performed within a single persistent rectangular boundary, the SEEPDS method (and potentially other drive methods) requires a controller architecture that allows multiple discontinuous regions of arbitrary shape and size to be updated in parallel, as illustrated in Figure 13.

[0188] A memory and controller architecture that meets this requirement maintains (region) bits in the image buffer memory to designate any pixel for inclusion within the region. The region bits are used as "gatekeepers" for update buffer modification and lookup table number assignment. The region bits may in fact comprise multiple bits that may be used to point to separate, concurrently updatable, arbitrarily shaped regions that may be assigned different waveform modes, thus allowing any region to be selected without creating a new waveform mode.

[0189] It will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative sense, and not in a restrictive sense.

Claims

1. 1. A method of driving an electro-optic display having a plurality of display pixels, the method comprising: determining a level of stress for a display pixel of the electro-optic display, said determining being based on at least one previous update to the optical state of the display pixel; receiving a request to update the optical state of the display pixel; (i) when a drive waveform from a first update scheme was used for the immediately preceding update of the display pixel and the level of the stress amount is not greater than the level of a first stress threshold; or (ii) when a drive waveform from a second update scheme was used for the immediately preceding update of the display pixel and the level of the stress amount is less than a second stress threshold level; applying drive waveforms from the first update scheme to the display pixels; (i) when a drive waveform from the first update scheme was used for the previous update of the display pixel and the level of the stress amount is greater than the first stress threshold level; or (ii) when a drive waveform from the second update scheme was used for the immediately preceding update of the display pixel and the level of the stress amount is not greater than the second stress threshold level; applying drive waveforms from the second update scheme to the display pixels; A method comprising:

2. The method of claim 1 , wherein the request to update the optical states of the display pixels of the electro-optic display is triggered by a user interaction with the electro-optic display.

3. The method of claim 2 , wherein the user interaction comprises swiping a screen surface of the electro-optic display.

4. The method of claim 2 , wherein the user interaction includes presenting an animation on the electro-optic display.

5. The method of claim 1 , wherein the first update scheme comprises a drive waveform that is DC unbalanced.

6. The method of claim 1 , wherein the second update scheme comprises a drive waveform that is DC balanced.

7. The method of claim 1 , wherein the second update scheme comprises a drive waveform that is longer in duration than a drive waveform of the first update scheme.

8. 8. The method of claim 7, wherein the drive waveforms of the second update scheme are approximately 350 ms to 500 ms longer in duration than the drive waveforms of the first update scheme.

9. 8. The method of claim 7, wherein the drive waveforms of the second update scheme are approximately 30% to 55% longer in duration than the drive waveforms of the first update scheme.

10. The method of claim 7 , wherein the drive waveforms of the second update scheme are approximately 50% to 70% longer in duration than the drive waveforms of the first update scheme.

11. 10. The method of claim 1, wherein the level of the stress amount is a numerical amount that comprises an approximation of the actual amount of residual voltage accumulated on the display pixel.

12. 2. The method of claim 1, wherein the level of the stress amount is a scalar amount having an index that indicates a growth or decay in the amount of remnant voltage accumulated on the display pixel.

13. Determining the level of the amount of stress for the display pixel is performed using the following formula: [Equation 3] where x(n) indicates the level of stress amount for update n, UT(n) indicates the duration in milliseconds of the drive waveform used for the previous update of the display pixel, DT(n) indicates the number of milliseconds of a post-drive discharge routine applied after the previous update of the display pixel, TAU represents a time constant for decay of the stress amount level x(n), x(n-1) is a stress amount level calculated based on the previous update of the display pixel, and B has a value that varies depending on the drive mode used for update n.

14. 14. The method of claim 13, wherein B is set to a non-zero positive value after applying a drive waveform from the first update scheme to the display pixel.

15. The method of claim 13 , wherein the value of B is set to zero after applying a drive waveform from the second update scheme to the display pixel.

16. The method of claim 1 , further comprising: interrupting a post-drive discharge routine after applying a drive waveform from the first update scheme.

17. The method of claim 1 , further comprising: implementing substantially no dwell time after applying a drive waveform from the first update scheme.

18. The method of claim 1 , further comprising: interrupting a post-drive discharge routine after applying a drive waveform from the second update scheme.

19. The method of claim 1 , further comprising: implementing substantially no dwell time after applying a drive waveform from the second update scheme.

20. 10. The method of claim 1, wherein the first stress threshold level indicates a limit on an acceptable level of stress for the electro-optic display.

21. An electro-optic display configured to carry out a method according to any one of claims 1-20.

Images