Color sets for low resolution dithering in reflective color displays

Abstract

A method of rendering an image on a reflective display is provided in which each pixel is capable of rendering a limited number of colors, each color being rendered by a predetermined set of waveforms stored in a waveform lookup table. In addition, the present invention provides a method of rendering an image using a color selected for optimal color rendering. The invention also provides for presenting a color image formed by a plurality of pixels on a reflective display, where each pixel has a color selected from the group consisting of red, green, blue, cyan, magenta, yellow, black, and white.

Description

[0001] This application is a divisional application of an application with a filing date of September 25, 2015, application number 201580063225.4, and invention name “Low-resolution dithered color sets for reflective color displays” (filing date of May 8, 2019, application number 201910380947.2) and a divisional application (filing date of November 11, 2021, application number 202111330798.2).

[0002] Related Applications

[0003] This application claims the benefit of co-pending U.S. Application No. 62 / 056,302, filed September 26, 2014.

[0004] This application is related to co-pending U.S. Application No. 14 / 849,658, filed on September 10, 2015. Technical Field

[0005] The present invention provides a method for presenting a color image on a reflective display, wherein at each pixel location one of a predetermined palette of colors can be presented. Background Art

[0006] The term color as used herein includes black and white. White particles are generally of the light scattering type.

[0007] The term gray state is used herein in its conventional sense in the field of imaging technology, referring to a state between two extreme optical states of a pixel, but does not necessarily mean a black-white transition between these two extreme states. For example, several patents and published applications of Iink Corporation referenced below describe electrophoretic displays in which the extreme states are white and dark blue, such that the intermediate gray state is actually light blue. In fact, as already mentioned, the change in optical state may not be a color change at all. The terms black and white used below refer to the two extreme optical states of the display, and should be understood to generally include, but not be limited to, the extreme optical states of black and white, such as the white and dark blue states mentioned above.

[0008] Several types of electro-optical displays are known. One type of electro-optical display is a rotating two-color element type, such as described in U.S. Patent Nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467 and 6,147,791 (although this type of display is generally referred to as a "rotating two-color ball" display, the term "rotating two-color element" is more accurate because in some of the above-mentioned patents, the rotating element is not spherical). This display uses a large number of small bodies (usually spherical or cylindrical), and the small body has two or more parts and internal dipoles with different optical properties. These bodies are suspended in a liquid-filled vacuole in a matrix, and the vacuole is filled with liquid so that the body can rotate freely. By applying an electric field to the display, the body is rotated to different positions and the part of the body seen by the viewing surface is changed, and the appearance of the display changes. Electro-optical media of this type are usually bistable.

[0009] Another type of electro-optical display uses an electrochromic medium, such as an electrochromic medium in the form of a nanochromic film, which includes an electrode formed at least in part by a semiconductor metal oxide and a plurality of dye particles attached to the electrode that can reversibly change color; 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. This type of nanochromic film is 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 generally bistable.

[0010] Yet another type of electro-optical display is the electrowetting display developed by Philips and described in "Video-Speed ​​Electronic Paper Based on Electrowetting" by Hayes, RA et al., Nature, 425, 383-385 (2003). U.S. Patent No. 7,420,549 shows that such an electrowetting display can be bistable.

[0011] Particle-based electrophoretic displays have been the subject of intensive research and development for many years. In such displays, multiple charged particles (sometimes also called pigment particles) move through a fluid under the influence of an electric field. Electrophoretic displays have the following properties compared to liquid crystal displays: good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, 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, making the service life of these displays insufficient.

[0012] As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but electrophoretic media can be made with gaseous fluids; see, for example, 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. Patents Nos. 7,321,459 and 7,236,291. When such gas-based electrophoretic media are used in a direction that allows deposition (e.g., in a signboard where the media is arranged in a vertical plane), the media appears to be susceptible to the same type of problems as liquid-based electrophoretic media due to particle deposition. In fact, particle deposition appears 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 liquid suspending fluid allows for more rapid deposition of electrophoretic particles.

[0013] 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-optical media. Such encapsulated media include a plurality of small capsules, each of which itself includes 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 in a polymeric binder to form a coherent layer located between two electrodes. These techniques are described in patents and applications including the following:

[0014] (a) electrophoretic particles, fluids, and fluid additives, see, for example, U.S. Pat. Nos. 7,002,728 and 7,679,814;

[0015] (b) capsules, adhesives and encapsulation processes, see, e.g., U.S. Pat. Nos. 6,922,276 and 7,411,719;

[0016] (c) films and subassemblies containing electro-optical materials, see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564;

[0017] (d) backplanes, adhesive layers and other auxiliary layers and methods for displays, see, for example, U.S. Patent Nos. 7,116,318 and 7,535,624;

[0018] (e) Color formation and color adjustment, see, e.g., U.S. Patent Nos. 6,017,584, 6,664,944, 6,864,875, 7,075,502, 7,167,155, 7,667,684, 7,791,789, 7,956,841, 8,040,594, 8,054,526, 8,098,418, 8,213,076, and 8,363,299; and U.S. Patent Application Publication Nos. 2004 / 0263947、2007 / 0109219、2007 / 0223079、2008 / 0023332、2008 / 0043318、2008 / 0048970、2009 / 0004442、2009 / 0225398、2010 / 0103502、2010 / 0156780、2011 / 0164307、 2011 / 0195629, 2011 / 0310461, 2012 / 0008188, 2012 / 0019898, 2012 / 0075687, 2012 / 0081779, 2012 / 0134009, 2012 / 0182597, 2012 / 0212462, 2012 / 0157269 and 2012 / 0326957;

[0019] (f) Methods for driving displays, see, e.g., 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, 7,312,794, 7,327,511, 7,453,445, 7,492,339, 7, 528,822, 7,545,358, 7,583,251, 7,602,374, 7,612,760, 7,679,599, 7,688,297, 7,729,039, 7,733,311, 7,733,335, 7,787,169, 7,952,557, 7,956,841, 7,999,787, 8,077,141, 8,125,501, 8,139,050, 8,174,490, 8,289,250, 8,300,006, and 8,314,784; and U.S. Patent Application Publication Nos. 2003 / 0102858, 2005 / 0122284, 2005 / 0179642, 2005 / 0253777, 2007 / 0091418, 2007 / 0103427, 2008 / 0024429, 2008 / 0024482, 2008 / 0136774, 2008 / 0150888, 2008 / 0291129, 2009 / 0174651 , 2009 / 0179923, 2009 / 0195568, 2009 / 0322721, 2010 / 0045592, 2010 / 0220121, 2010 / 0220122, 2010 / 0265561, 2011 / 0187684, 2011 / 0193840, 2011 / 0193841, 2011 / 0199671 and 2011 / 0285754 (these patents and applications may be referred to hereinafter as MEDEOD (Method for Driving Electro-Optical Display) applications);

[0020] (g) Display applications, see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348; and

[0021] (h) Non-electrophoretic displays, such as those described in U.S. Patent Nos. 6,241,921, 6,950,220, 7,420,549, and 8,319,759 and U.S. Patent Application Publication No. 2012 / 0293858.

[0022] Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium includes a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymer material, and recognize that the discrete droplets of electrophoretic fluid in such a polymer-dispersed electrophoretic display can be considered to be capsules or microcapsules, even though there is no discrete capsule membrane associated with each individual droplet. See, e.g., U.S. Patent No. 6,866,760. Therefore, for purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.

[0023] 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 are held in a plurality of cavities formed in a carrier medium (usually a polymer film); see, for example, U.S. Patent Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.

[0024] Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block visible light from being transmitted through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called shutter mode, in which one display state is substantially opaque and one display state is light-transmissive. See, for example, U.S. Patents 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 are similar to electrophoretic displays, but rely on changes in electric field strength, and can operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optical displays can also operate in shutter modes. Electro-optical media operating in a shutter mode are useful for multilayer structures for full color displays; in such structures, at least one layer near the viewing surface of the display is operated in a shutter mode to expose or hide a second layer further away from the viewing surface.

[0025] Packaged electrophoretic displays are generally not subject to the aggregation and deposition failure modes of conventional electrophoretic devices and offer additional benefits, such as the ability to print or coat the display on a variety of flexible and rigid substrates. (The use of the word "printing" is intended to include all forms of printing and coating, including, but not limited to: pre-metered coating such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coating such as roller blade coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing process, electrostatic printing process, thermal printing process, inkjet printing process, electrophoretic deposition (see U.S. Patent No. 7,339,715), and other similar technologies). Thus, the resulting display can be flexible. In addition, because the display medium can be printed (using a variety of methods), the display itself can be inexpensively made.

[0026] The aforementioned U.S. Patent No. 6,982,178 describes a method of assembling solid electro-optical displays (including encapsulated electrophoretic displays) that is well suited for mass production. In essence, the patent describes a so-called "front plane laminate" ("FPL") that includes, in sequence, a light-transmitting conductive layer, a solid electro-optic medium layer in electrical contact with the conductive layer, a bonding layer, and a release sheet. Typically, the light-transmitting conductive layer will be carried on a light-transmitting substrate, which is preferably flexible in the sense that the substrate can be manually wound around a cylinder of, say, 10 inches (254 mm) in diameter without permanent deformation. The term "light-transmitting" is used in this patent and herein to mean that the layer so designated transmits sufficient light to enable an observer to observe through the layer a change in the display state of the electro-optic medium, which is typically observed through the conductive layer and the adjacent substrate (if present); in the case where the electro-optic medium displays a change in reflectivity of non-visible wavelengths, the term "light-transmitting" should of course be interpreted as referring to the transmission of the relevant non-visible wavelengths. The substrate is typically a polymer film and typically has a thickness in the range of about 1 to about 25 mils (25 to 634 μm), preferably about 2 to about 10 mils (51 to 254 μm). The conductive layer is conveniently a thin metal or metal oxide layer such as aluminum or ITO, or may be a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are commercially available, such as "aluminized Mylar" ("Mylar" is a registered trademark) from EI du Pont de Nemours & Company, Wilmington DE, and this commercial material can produce good results in front plane lamination.

[0027] Assembly of an electro-optical display using such a front plane laminate may be accomplished by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions that effectively bond the adhesive layer to the backplane, thereby securing the adhesive layer, electro-optical dielectric layer, and conductive layer to the backplane. This method is well suited for mass production because the front plane laminate may be mass produced, typically using roll-to-roll coating techniques, and then cut into sheets of any size required for a particular backplane.

[0028] U.S. Patent No. 7,561,324 describes a so-called double release sheet, which is essentially a simplified version of the front plane lamination of the aforementioned U.S. Patent No. 6,982,178. One form of the double release sheet includes a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of the double release sheet includes a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of double release films are intended to be used in a process substantially similar to the process of assembling an electro-optic display from a front plane lamination already described, but involve two separate laminations; typically, in the first lamination, the double release sheet is laminated to the front electrode to form the front subassembly, and then in the second lamination, the front subassembly is laminated to the backplane to form the final display, however, if desired, the order of the two laminations can be reversed.

[0029] U.S. Patent No. 7,839,564 describes a so-called reverse front plane lamination, which is a variation of the front plane lamination described in the aforementioned U.S. Patent No. 6,982,178. The reverse front plane lamination comprises, in order: at least one of a light-transmitting protective layer and a light-transmitting conductive layer, a layer of adhesive, a layer of a solid electro-optic medium, and a release sheet. The reverse front plane lamination is used to form an electro-optic display having a layer of laminating adhesive between the electro-optic layer and the front electrode or front substrate; a second layer of adhesive, which is usually thinner, may or may not be present between the electro-optic layer and the backplane. Such an electro-optic display can have both good resolution and good low temperature performance.

[0030] As mentioned above, most simple prior art electrophoretic media display only two colors in essence. Such electrophoretic media either use a single type of electrophoretic particles having a first color in a colored fluid having a different second color (in which case the first color is displayed when the particles are close to the viewing surface of the display, and the second color is displayed when the particles are far from the viewing surface); or use first and second types of electrophoretic particles having different first and second colors in a colorless fluid (in which case the first color is displayed when the first type of particles are close to the viewing surface of the display, and the second color is displayed when the second type of particles are close to the viewing surface). The two colors are usually black and white. If a full color display is required, a color filter array can be deposited on the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to produce color stimuli. The available display area is shared between three or four primary colors, such as red / green / blue (RGB) or red / green / blue / white (RGBW), and the filters can be arranged in a one-dimensional (strip) or two-dimensional (2×2) repeating pattern. Other choices of primary colors or beyond the three primary colors are also known in the art. The three (in the case of an RGB display) or four (in the case of an RGBW display) sub-pixels are chosen to be small enough so that, at the expected viewing distance, they visually blend together into a single pixel with a uniform color stimulus ('color mixing'). The inherent drawback of area sharing is that the colorant is always present, and the color can only be adjusted by converting the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue and white primary colors occupies a quarter of the display area (one of the four sub-pixels), where the white sub-pixel is as bright as the potential monochrome display white, and each colored sub-pixel is no brighter than one-third of the monochrome display white. The brightness of the white shown on the display is generally no greater than half the brightness of the white sub-pixel (the white area on the display is produced by displaying one white sub-pixel out of every four sub-pixels, plus each colored sub-pixel in the form of its color equal to one-third of the white sub-pixel, so the three colored sub-pixels together contribute no more than one white sub-pixel). By sharing the area with the color pixels converted to black, the brightness and saturation of the color are reduced. Area sharing is particularly problematic when mixing yellow, because it is brighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Converting blue pixels (a quarter of the display area) to black makes yellow too dark.

[0031] Multilayer stacked electrophoretic displays are known in the art; see J. Heikenfeld, P. Drzaic, J-SYeo and T. Koch, Journal of the SID, Vol. 19, No. 2, pp. 129-156, 2011. In such displays, ambient light passes through the image in each of the three primary colors in a subtractive manner, exactly like conventional color printing. U.S. Patent No. 6,727,873 describes a stacked electrophoretic display in which three layers of switchable cells are placed on a reflective background. Similar displays are known in which the colored particles move laterally (see International Application No. WO 2008 / 065605) or the colored particles are isolated in micropits using a combination of longitudinal and lateral movement. In both cases, each layer has an electrode for gathering or dispersing the colored particles on a pixel-by-pixel basis, so each of the three layers requires a thin film transistor (TFT) layer (two of the three TFT layers must be substantially transparent) and a light-transmitting counter electrode. Such complex configurations of electrodes are expensive to produce, and in the current state of the art it is difficult to provide a sufficiently transparent plane for the pixel electrodes, particularly since the white state of the display must be viewed through multiple electrode layers. Multilayer displays also suffer from parallax problems as the thickness of the display stack approaches or exceeds the pixel size.

[0032] U.S. Patent Publication Nos. 2012 / 0008188 and 2012 / 0134009 describe a multicolor electrophoretic display with a single backplane, which includes independently addressable pixel electrodes and a common light-transmitting front electrode. Multiple electrophoretic layers are arranged between the backplane and the front electrode. The displays described in these applications are capable of displaying any primary color (red, green, blue, cyan, magenta, yellow, white and black) at any pixel position. However, the use of multiple electrophoretic layers located between a single set of addressing electrodes has disadvantages. The electric field experienced by particles in a particular layer is lower than that of a single electrophoretic layer addressed with the same voltage. In addition, light losses in the electrophoretic layer closest to the viewing surface (e.g., caused by light scattering or undesired absorption) may affect the appearance of the image formed in the electrophoretic layer below.

[0033] Attempts have been made to provide a full-color electrophoretic display using a single electrophoretic layer. For example, U.S. Patent Application Publication No. 2013 / 0208338 describes a color display including an electrophoretic fluid containing one or two types of pigment particles dispersed in a transparent colorless or colored solvent, the electrophoretic fluid being sandwiched between a common electrode and a plurality of drive electrodes. The drive electrodes are held at a distance to expose a background layer. U.S. Patent Application Publication No. 2014 / 0177031 describes a method for driving a display unit filled with an electrophoretic fluid containing two types of charged particles carrying opposite charge polarities and having two contrasting colors. The two types of pigment particles are dispersed in a colored solvent or in a solvent in which uncharged or slightly charged colored particles are dispersed. The method includes driving the display unit to display the color of the solvent or the color of the uncharged or slightly charged colored particles by applying a drive voltage (about 1 to about 20% of the full drive voltage). U.S. Patent Application Publication Nos. 2014 / 0092465 and 2014 / 0092466 describe an electrophoretic fluid and a method for driving an electrophoretic display. The fluid includes first, second, and third types of pigment particles, all of which are dispersed in a solvent or solvent mixture. The first and second types of pigment particles carry opposite charge polarities, and the charge level of the third type of pigment particles is less than about 50% of the charge level of the first type or the second type. The three types of pigment particles have different threshold voltage levels or different mobility levels, or both. None of these patent applications discloses a full-color display in the sense of the term used hereinafter.

[0034] U.S. Patent Application Publication No. 2007 / 0031031 describes a kind of image processing equipment, and it is used for processing image data so that each pixel can display white, black and another color on the display medium display image. U.S. Patent Application Publication Nos. 2008 / 0151355, 2010 / 0188732 and 2011 / 0279885 describe the color display that wherein mobile particle moves through porous structure. U.S. Patent Application Publication Nos. 2008 / 0303779 and 2010 / 0020384 describe the display medium that comprises the first, second and third particles of different colors. The first and second particles can form aggregates, and the smaller third particle can move through the hole that stays between the first and second particles of aggregation. U.S. Patent Application Publication No. 2011 / 0134506 describes a kind of display equipment, and it comprises electrophoresis display element and voltage control part. The electrophoretic display element includes a plurality of types of particles enclosed between a pair of substrates, at least one substrate is translucent, and each of the corresponding plurality of types of particles carries a charge of the same polarity, but has different optical properties, and has different migration speeds and / or electric field thresholds for movement; a translucent display side electrode provided on the substrate side where the translucent substrate is arranged; a first backside electrode provided on the side of the other substrate opposite to the display side electrode; and a second backside electrode provided on the side of the other substrate opposite to the display side electrode. The voltage control part controls the voltage applied to the display side electrode, the first backside electrode, and the second backside electrode, so that the particle type with the fastest migration rate among the plurality of types of particles, or the particle type with the lowest threshold among the plurality of types of particles moves to the first backside electrode or the second backside electrode in sequence via each of the different types of particles, and then the particles moved to the first backside electrode move to the display side electrode. U.S. Patent Application Publication Nos. 2011 / 0175939, 2011 / 0298835, 2012 / 0327504 and 2012 / 0139966 describe color displays that rely on the aggregation of multiple particles and threshold voltage. U.S. Patent Application Publication No. 2013 / 0222884 describes an electrophoretic particle that includes: a colored particle that includes a polymer containing a charged group and a colorant; and a branched silicone-based polymer that is attached to the colored particle and contains a reactive monomer as a copolymer component and at least one monomer selected from a specific monomer group. U.S. Patent Application Publication No. 2013 / 0222885 describes a dispersion for an electrophoretic display, comprising a dispersion medium, a group of colored electrophoretic particles dispersed in the dispersion medium and migrating in an electric field, a group of non-electrophoretic particles that do not migrate and have a color different from that of the electrophoretic particle group, and a compound having a neutral polar group and a hydrophobic group, the compound being contained in the dispersion medium in a mass ratio of about 0.01% to about 1% of the entire dispersion.U.S. Patent Application Publication No. 2013 / 0222886 describes a dispersion for a display, comprising: floating particles including core particles including a colorant and a hydrophilic resin; and a shell covering the surface of each core particle and comprising a solubility parameter difference of 7.95 (J / cm. 3 ) 1 / 2 or more of a hydrophobic resin. U.S. Patent Application Publication Nos. 2013 / 0222887 and 2013 / 0222888 describe electrophoretic particles having a specific chemical composition. Finally, U.S. Patent Application Publication No. 2014 / 0104675 describes a particle dispersion including first and second colored particles that move in response to an electric field and a dispersion medium, the second colored particles having a larger diameter than the first colored particles and a charging characteristic that is the same as the charging characteristic of the first colored particles, and wherein the ratio of the charge amount Cs of the first colored particles to the charge amount Cl of the second colored particles per unit area of ​​the display (Cs / Cl) is less than or equal to 5. Some of the aforementioned displays do provide full color, but at the expense of requiring lengthy and cumbersome addressing methods.

[0035] U.S. Patent Application Publication Nos. describes an electrophoretic device comprising a plurality of first and second electrophoretic particles contained in an insulating liquid, the first and second particles having different charging characteristics from each other; the device also includes a porous layer contained in the insulating liquid and formed of a fiber structure. These patent applications are not full-color displays in the sense of the term used below.

[0036] See also US Patent Application Publication No. 2011 / 0134506 and aforementioned Application No. 14 / 277,107; the latter describes a full color display using three different types of particles in a colored fluid, but the presence of the colored fluid limits the quality of the white state achievable by the display.

[0037] In summary, in the prior art, there are several embodiments of color reflective displays with different mechanisms for producing color. Although such displays are generally capable of presenting multiple colors at each pixel location (e.g., black, white, three subtractive primary colors (cyan, magenta, and yellow), and three additive primary colors (red, green, and blue)), in the prior art, they are unable to present colors corresponding to 256 RGB levels at each pixel location. This is in contrast to typical emissive displays (e.g., liquid crystal displays or displays using light emitting diodes), which are capable of providing at least 256 different intensity levels in the red, green, and blue channels at each pixel location, for a total of 2 24 Different colors.

[0038] The present invention is directed to a method for presenting a full color image on a reflective display that is capable of providing a color palette at each pixel location that includes fewer colors than are required to present the image on the entire display (i.e., the colors available at the pixel level are quantized at a coarser level than are required to present an image of sufficient quality for a particular application). The method of the present invention involves dithering between the colors available in the color palette at each pixel so that the spatial resolution of the image presented on the display is reduced, but the color depth is increased. Such dithering is well known in the printing art. When the dithered image is viewed at a sufficient distance, the individual color pixels are merged by the human visual system into a perceived uniform color. Due to the tradeoff between color depth and spatial resolution, a closely observed dithered image has a typical grainy state compared to an image in which the color palette available at each pixel location has the same depth as is required to present the image on the entire display.

[0039] Algorithms have been developed for assigning specific colors to specific pixels to avoid annoying patterns and textures that can appear in images due to dithering. Such algorithms may involve error diffusion, a technique in which the error caused by the difference between the desired color at a specific pixel and the closest color in a per-pixel palette (i.e., quantization residuals) is assigned to neighboring pixels that have not yet been processed. European Patent No. 0677950 describes this technique in detail, while U.S. Patent No. 5,880,857 describes a metric for comparing dithering techniques.

[0040] In the present invention, a method is proposed to select which colors should be optimally selected for the (reduced) color palette available at each pixel position in a reflective display. Summary of the invention

[0041] The present invention provides a method for presenting an image on a reflective display, wherein three different color sets are involved. The first color set contains a specific number of colors, each color is presented by one of a predetermined set of waveforms stored in a waveform lookup table. The second color set is defined by the image to be presented on the display (usually including 2 24 The invention provides a method for presenting an image by assigning from the third color set (those colors to form the first color set) those colors selected for optimal color presentation.

[0042] Furthermore, the present invention provides an image formed by a plurality of pixels on a reflective display, wherein each pixel has a color selected from the group consisting of: red, green, blue, cyan, magenta, yellow, black and white.

[0043] In addition, the present invention provides an image formed by multiple pixels on a reflective display, wherein each pixel has a color selected from the group consisting of: red, green, blue, cyan, magenta, yellow, black, white, light red, light green, light blue, dark cyan, dark magenta, dark yellow, light gray and dark gray.

[0044] Still further, the present invention provides an image formed on a reflective display, wherein each pixel has a color selected from the group consisting of red, green, blue, cyan, magenta, yellow, black, white, orange, pink, green, purple, cyan grey, and pure blue.

[0045] In another aspect, the present invention provides an image formed on a reflective display wherein each pixel has a color selected from the group consisting of light, intermediate, and dark versions of the following colors: red, green, blue, cyan, magenta, yellow, black, and white. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 Schematic cross-sectional view showing the positions of various particles in the electrophoretic medium of the present invention when black, white, three subtractive primary colors, and three additive primary colors are displayed.

[0047] Figure 2A and 2B The following table shows the Figure 1 Waveforms showing an electrophoretic medium driven to its black and white states.

[0048] Figure 3A and 3B Shown for Figure 1 Waveforms showing the electrophoretic medium driven to its magenta and blue states.

[0049] Figure 3C and 3D Shown for Figure 1 Waveforms showing the electrophoretic medium driven to its yellow and green states.

[0050] Figure 4A and 4B The following table shows the Figure 1 Waveforms showing the electrophoretic medium driven to its red and cyan states.

[0051] Figure 5-6 Show that it can be replaced Figure 2A-2B , 3A-3D and 4A-4B are shown to Figure 1 Waveforms showing an electrophoretic medium driven to all of its color states.

[0052] Figure Fig. 7A , 7B 7A and 7C show three general methods of rendering color on reflective media.

[0053] Figure 8-14 RGB values ​​are shown for various choices of per-pixel palettes according to the present invention.

[0054] Fig.15A , 15B 15C are schematic diagrams showing the colors of a preferred per-pixel palette as defined in the CIELab color space, where the various colors are shown projected onto the a* / b* plane. DETAILED DESCRIPTION

[0055] One type of reflective display is an electrophoretic display comprising an electrophoretic medium. The electrophoretic medium comprises a fluid, first light scattering particles (typically white) and second, third and fourth particles having three subtractive primary colors (typically magenta, cyan and yellow); at least two of these colored particles are non-light scattering. The first and second particles carry a polymer coating so that the electric field required to separate aggregates formed by the third and fourth particles is greater than the electric field required to separate aggregates formed by any other two types of particles. Methods for driving the medium to produce white ("W"), black ("K"), magenta ("M"), cyan ("C"), yellow ("Y"), red ("R"), green ("G") and blue ("B") are also described.

[0056] In the attached figure Figure 1 Schematic cross-sectional view showing the positions of various particles in the electrophoretic medium of the present invention when displaying black, white, three subtractive primary colors, and three additive primary colors. Figure 1 In the above, it is assumed that the viewing surface of the display is at the top (as shown in the figure), i.e., the user views the display from this direction and the light is incident from this direction. As already noted, in the preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, while Figure 1 In the example, the particle is assumed to be a white pigment. Basically, the light scattering white particle forms a white reflector, and any particle above the white particle (such as Figure 1The second, third and fourth particles are all observed at the viewing surface of the display. Light entering the viewing surface of the display passes through these particles, reflects from the white particles, passes back through these particles and emerges from the display. Therefore, the particles above the white particles can absorb various colors, and the color seen by the user is produced by the combination of the particles above the white particles. Any particles distributed below the white particles (behind the user's viewing point) are masked by the white particles and do not affect the displayed color. Since the second, third and fourth particles are essentially non-light scattering, their order or arrangement relative to each other is unimportant, but their order or arrangement relative to the white (light scattering) particles is critical for the reasons already explained.

[0057] More specifically, when the cyan, magenta, and yellow particles are below the white particles ( Figure 1 In case [A]), there are no particles above the white particles, and the pixel simply displays white. When a single particle is above the white particle, the color of that single particle is displayed. Figure 1 In cases [B], [D], and [F], the colors are yellow, magenta, and cyan, respectively. When two particles are located above a white particle, the color displayed is a combination of the two particles; Figure 1 In case [C], the magenta and yellow particles display red color, in case [E], the cyan and magenta particles display blue color, and in case [G], the yellow and cyan particles display green color. Finally, when all three colored particles are located above the white particle ( Figure 1 In the case [H] in Figure 1, all incident light is absorbed by the three subtractive primary color particles and the pixel displays black.

[0058] It is possible that one of the subtractive primary colors can be rendered by particles that scatter light, so that the display contains two types of light scattering particles, one of which is white and the other colored. However, in this case, the position of the light scattering colored particles relative to the other colored particles covering the white particles will be important. For example, when displaying black (when all three colored particles are located above the white particles), the scattering colored particles cannot be located above the non-scattering colored particles (otherwise the non-scattering colored particles will be partially or completely hidden behind the scattering particles and the rendered color will be the color of the scattering colored particles, rather than black).

[0059] It's not easy to render black if there's more than one type of colored particles scattering light.

[0060] Figure 1The ideal case where the color is not contaminated (i.e., the light scattering white particles completely mask any particles located behind the white particles) is shown. In practice, the masking of the white particles may be imperfect, so there may be some small light absorption of the particles that are ideally completely masked. This contamination usually reduces the brightness and chromaticity of the color presented. In the electrophoretic medium of the present invention, this color contamination should be minimized so that the formed color is comparable to the industry standard for color presentation. The standard that is particularly preferred is SNAP (the standard for newspaper advertising production), which specifies the L*, a* and b* values ​​of each of the above eight primary colors. The electrophoretic fluid of the present invention can be used to construct a display device in several ways known in the prior art. The electrophoretic fluid can be encapsulated in a microcapsule or contained in a microcell structure that is subsequently sealed with a polymer layer. The microcapsule or microcell layer can be coated or embossed on a plastic substrate or plastic film with a transparent coating of a conductive material. The component can be laminated to a backplane carrying pixel electrodes using a conductive adhesive.

[0061] Now, the method for implementing the Figure 1 A first embodiment of a waveform for each of the particle arrangements shown. Hereinafter. This drive method will be referred to as the "first drive scheme" of the present invention. In this discussion, it is assumed that the first group of particles are white and negatively charged, the second group of particles are cyan and positively charged, the third group of particles are yellow and negatively charged, and the fourth group of particles are magenta and positively charged. Those skilled in the art will understand how the color transitions will change if these assignments of particle color are changed. Similarly, the polarity of the charge on all particles can be similarly reversed, and the electrophoretic medium will still function in the same manner, as long as the polarity of the waveform used to drive the medium (see next paragraph) is reversed.

[0062] In the following discussion, the waveforms (voltage-time curves) applied to the pixel electrodes of the display backplane of the present invention are described and plotted assuming that the front electrode is grounded (i.e., at zero potential). The electric field experienced by the electrophoretic medium is of course determined by the potential difference between the backplane and the front electrode and the distance separating them. The display is usually viewed through its front electrode so that the particles adjacent to the front electrode control the color displayed by the pixel, and it is sometimes easier to understand the optical transitions involved if the potential of the front electrode relative to the backplane is considered; this can be done simply by reversing the waveforms discussed below.

[0063] These waveforms require that each pixel of the display can be driven at five different addressing voltages, which are nominally +V 高 , + V 低 , 0, -V 低 and -V 高, shown in 2-4 as 30V, 15V, 0, -15V and -30V. In practice, it is preferred to use a larger number of addressing voltages. If only three voltages are available (i.e. + V 高 , 0, and -V 高 ), then by using a voltage of V 高 However, pulse addressing with a duty cycle of 1 / n can be achieved with lower voltages (e.g., V 高 / n, where n is a positive integer > 1).

[0064] The waveform used in the present invention may include three phases: a DC balance phase, in which a DC imbalance resulting from a previous waveform applied to the pixel is corrected, or a DC imbalance to be generated in a subsequent color rendering transition is corrected (as is known in the art); a "reset" phase, in which the pixel is returned to a starting configuration, which is at least approximately the same regardless of the pixel's previous optical state; and a "color rendering" phase, as described below. The DC balance and reset phases are optional and may be omitted, depending on the requirements of the specific application. If applied, the "reset" phase may be the same as the magenta rendering waveform described below, or may include continuous driving of the maximum possible positive and negative voltages, or may be some other pulse pattern, as long as it returns the display to a state where subsequent colors can be reproducibly obtained.

[0065] Figure 2A and 2B Typical color rendering stages of the waveforms used to produce black and white states in the display of the present invention are shown in idealized form. Figure 2A and 2B The graph in shows the voltage applied to the backplane (pixel) electrodes of the display, while the transparent common electrode on the top plate is grounded. The x-axis represents time measured in arbitrary units, while the y-axis is the applied voltage in volts. Driving the display to black ( Figure 2A ) or white ( Figure 2B ) state is achieved by a sequence of positive or negative pulses, preferably at a voltage V 低 , because as mentioned above, in the case corresponding to V 低In the field (or current) of , magenta and yellow pigments gather together. Therefore, white and cyan pigments move, while magenta and yellow pigments remain stationary (or move at a much lower speed), and the display switches between a white state and a state corresponding to absorption by cyan, magenta and yellow pigments (commonly referred to as "composite black" in the art). The length of the pulses driven to black and white can vary between about 10-1000 milliseconds, and the pulses can be separated by rests (zero applied voltage) of length in the range of 10-1000 milliseconds. Although FIG. 2 shows pulses of positive and negative voltages that produce black and white, respectively, these pulses are separated by "rests" that provide zero voltage, it is sometimes preferred that these "rest" periods include pulses of opposite polarity to the drive pulses, but with lower impulse (i.e., with a shorter duration or lower applied voltage than the main drive pulse, or both).

[0066] Figures 3A-3D The color scheme for producing magenta and blue ( Figure 3A and 3B ) and yellow and green ( Figure 3C and 3D ) is a typical color rendering stage of the waveform. Figure 3A In the waveform, the waveform oscillates between positive and negative impulses, but the length of the positive impulse (t p ) is shorter than the length of the negative impulse (t n ), while the voltage applied in the positive pulse (V p ) is greater than the voltage applied during the negative impulse (V n ). When V p t p = V n t n When the waveform is overall "DC balanced", the period of one cycle of positive and negative impulses can be in the range of about 30-1000 milliseconds.

[0067] When the positive impulse ends, the display is in the blue state, and at the end of the negative impulse, the display is in the magenta state. This is consistent with the change in optical density corresponding to the motion of the cyan pigment being greater than the change corresponding to the motion of the magenta or yellow pigments (relative to the white pigment). Based on the above assumptions, this is expected if the interaction between the magenta pigment and the white pigment is stronger than the interaction between the cyan pigment and the white pigment. The relative mobility of the yellow and white pigments (both negatively charged) is much lower than the relative mobility of the cyan and white pigments (oppositely charged). Therefore, in the preferred waveforms for producing magenta or blue, it is preferred to include at least one V n t n Follow V p t p The impulse sequence of the cycle, where Vp > V n and t p <t n When blue is required, the sequence starts with V p When magenta is required, the sequence ends with V n Finish.

[0068] Figure 3B An alternative waveform is shown that uses only three voltage levels to produce the magenta and blue states. In this alternative waveform, it is preferred that at least one V n t n Follow V p t p cycle, where V p = V n = V 高 And t n <t p The sequence cannot be DC balanced. When blue is required, the sequence starts with V p When magenta is required, the sequence ends with V n Finish.

[0069] Figure 3C and 3D The waveforms shown are Figure 3A and 3B The inverse of the waveform shown, and produces the corresponding complementary yellow and green. In a preferred waveform that produces yellow or green, as Figure 3C As shown, use a n t n Follow V p t p The impulse sequence of the cycle, where V p <V n And t p > t n When green is required, the sequence starts with V p When yellow is needed, the sequence ends with V n Finish.

[0070] Figure 3D Another preferred waveform is shown using only three voltage levels to produce yellow or green. In this case, at least one V n t n Follow V p t p cycle, where V p =V n =V 高 And t n > t p The sequence cannot be DC balanced. When green is required, the sequence starts with Vp When yellow is needed, the sequence ends with V n Finish.

[0071] Figure 4A and 4B The color rendering phase of the waveforms used to render red and cyan on the display of the present invention is shown. These waveforms also oscillate between positive and negative impulses, but they are different from Figures 3A-3D The waveforms differ in that the period of one cycle of the positive and negative impulses is usually longer, and the addressing voltage used may (but is not necessarily) be lower. Figure 4A The red waveform includes the pulses that produce the black (+V 低 ) (similar to Figure 2A ), followed by a shorter pulse of opposite polarity (-V 低 ), which removes the cyan particles and turns black into red, which is the complementary color of cyan. The cyan waveform is the inverse of the red waveform, with a portion that produces white (-V 低 ), followed by a short pulse (V 低 ), which causes the cyan particles to move near the observation surface. Figures 3A-3D As in the waveform shown in FIG, the cyan pigment moves faster relative to the white pigment than the magenta or yellow pigment relative to the white pigment. However, in contrast to the waveform of FIG3, the yellow pigment in the waveform of FIG4 remains on the same side of the white particle as the magenta particle.

[0072] The waveforms described above with reference to Figures 2-4 use a five-level drive scheme, i.e., a drive scheme in which, at any given time, the pixel electrode can be at any one of two different positive voltages, two different negative voltages, or zero volts relative to the common front electrode. In the specific waveforms shown in Figures 2-4, the five levels are 0, ±15V, and ±30V. However, it has been found that it is advantageous to use a seven-level drive scheme, which uses seven different voltages: three positive, three negative, and zero, at least in some cases. The seven-level drive scheme may be referred to as the "second drive scheme" of the present invention hereinafter. The selection of the number of voltages for addressing the display should take into account the limitations of the electronic components used to drive the display. Generally, a larger number of drive voltages will provide greater flexibility in addressing different colors, but complicates the arrangement necessary to provide a larger number of drive voltages for conventional device display drivers. The inventors have found that the use of seven different voltages provides a good compromise between the complexity of the display structure and the color gamut.

[0073] Now refer to Figure 5 To describe the use of a display (e.g. Figure 1The second drive scheme (shown in Figure 2-4) produces the general principle of eight primary colors (white, black, cyan, magenta, yellow, red, green and blue). As shown in Figures 2-4, assume that the first pigment is white, the second is cyan, the third is yellow and the fourth is magenta. It will be clear to those skilled in the art that if the distribution of pigment colors is changed, the colors displayed by the display will change.

[0074] The maximum positive and negative voltages applied to the pixel electrode (in Figure 5 Indicated as ±V 最大 ) produce the colors formed by the mixture of the second and fourth particles (cyan and magenta, producing the blue color, see Figure 1 [E]), or the color formed by a third particle (yellow, see Figure 1 In the case [B], the white pigment scatters light and is located between the colored pigments). These blue and yellow colors are not necessarily the best blue and yellow colors that can be achieved by the display. The intermediate levels of positive and negative voltages applied to the pixel electrodes ( Figure 5 Indicated as ±V 中间 ) produce black and white colors respectively.

[0075] The other four primary colors can be obtained from these blue, yellow, black, or white optical states by simply moving the second particles (in this case, the cyan particles) relative to the first particles (in this case, the white particles), which uses the lowest applied voltage ( Figure 5 ±V 最小 ). Therefore, cyan is separated from blue (by applying -V to the pixel electrode 最小 ) produces a magenta color (see ) for blue and magenta, respectively Figure 1 cases [E] and [D]); shifting cyan into yellow (by applying +V 最小 ) provides green (see for yellow and green respectively Figure 1 cases [B] and [G]); shifting cyan out of black (by applying -V to the pixel electrode 最小 ) available in red (see separate for black and red Figure 1 cases [H] and [C]), and shifting cyan into white (by applying +V 最小 ) provides cyan (see the Figure 1 cases [A] and [F]).

[0076] While these general principles are useful in the construction of waveforms that produce specific colors in displays of the present invention, in practice the ideal behavior described above may not be observed and it may be desirable to employ modifications to the basic scheme.

[0077] exist Figure 5 A modified general waveform embodying the above basic principle is shown in , where the abscissa represents time (in arbitrary units) and the ordinate represents the voltage difference between the pixel electrode and the common front electrode. Figure 5 The three positive voltages used in the driving scheme shown may have magnitudes between approximately +3V and +30V, and the three negative voltages may have magnitudes between approximately -3V and -30V. In an empirically preferred embodiment, the highest positive voltage +V 最大 +24V, the middle positive voltage is +V 中间 is 12V, and the minimum positive voltage +V 最小 is 5V. In a similar manner, the negative voltage -V 最大 , -V 中间 and -V 最小 In the preferred embodiment, this is -24 V, -12 V and -9 V. For any of the three voltage levels, it is not necessary that the magnitude of the voltage |+V|= |-V|, but may be preferred in some cases.

[0078] exist Figure 5 There are four distinct phases in the generic waveform shown. In the first phase ( Figure 5 "A"), providing the +V 最大 and -V 最大 The pulses (where "pulse" means a unipolar square wave, i.e., a constant voltage applied for a predetermined period of time) used to erase the previous image presented on the display (i.e., "reset" the display). The lengths of these pulses (t1 and t3) and the rest (i.e., the zero voltage period between the pulses (t2 and t4)) can be selected so that the entire waveform (i.e., as Figure 5 The integral of the voltage over time over the entire waveform shown) is DC balanced (i.e., the integral is approximately zero). DC balance can be achieved by adjusting the lengths of the pulses and rests in phase A so that the net impulse provided in that phase is equal in magnitude and opposite in sign to the net impulse provided in the combination of phases B and C, during which the display will be switched to a particular desired color, as described below.

[0079] Figure 5 The waveforms shown are purely for the purpose of illustrating the structure of a general waveform and are not intended to limit the scope of the present invention in any way. Figure 5 In the embodiment, in phase A, the negative pulse is shown as being before the positive pulse, but this is not a requirement of the present invention. It is not required that there is only a single negative pulse and a single positive pulse in phase A either.

[0080] As mentioned above, the general waveform is inherently DC balanced, and this may be preferred in certain embodiments of the invention. Alternatively, the pulses in phase A may provide DC balance for a series of color transitions rather than a single transition, in a manner similar to that provided in certain black and white displays of the prior art, see, for example, U.S. Patent No. 7,453,445.

[0081] In the second phase of the waveform ( Figure 5 In phase B) of the embodiment, pulses using maximum and intermediate voltage amplitudes are provided. In this phase, white, black, magenta, red and yellow are preferably presented in the manner described above with reference to Figures 2-4.

[0082] As mentioned above (see Figure 2B and related descriptions), which can be obtained by using -V 中间 In some cases, however, the white color produced in this way may be contaminated by yellow pigments and appear yellowish. In order to correct this color contamination, it may be necessary to introduce some pulses of positive polarity. Thus, for example, white may be obtained by a single instance or a repetition of a pulse sequence comprising a pulse of length T1 and an amplitude of + V 最大 or +V 中间 A pulse of length T2 and amplitude -V 中间 pulses, where T2 > T1. The last pulse should be a negative pulse. Figure 5 In the figure, +V 最大 Duration t5 and subsequent -V 中间 Four repetitions of the sequence of duration t6. During this pulse sequence, the appearance of the display oscillates between magenta (but usually not ideal magenta) and white (i.e., it will go through states with lower L* and higher a* than the final white state before reaching white). This is consistent with Figure 3A The pulse train shown is similar, where oscillations between magenta and blue are observed. The difference here is that the net impulse ratio of the pulse train Figure 3A The pulse train shown is more negative, so the oscillation is biased towards the negatively charged white pigment.

[0083] As mentioned above (see Figure 3A and related descriptions), can be achieved by 中间 One or more pulses at 100 Hz separated by periods of zero voltage are used to obtain black.

[0084] As mentioned above (see Figure 3A and 3B and related descriptions), magenta can be obtained by a single instance or repetition of a pulse sequence, the pulse sequence comprising a pulse of length T3 and an amplitude of +V 最大 or +V 中间A pulse of length T4 and amplitude -V 中间 The display will then oscillate between states that are substantially blue and magenta. Prior to magenta, it will go through states that are more negative in a* and lower in L* than the final magenta state.

[0085] As mentioned above (see Figure 4A and related descriptions), the red color can be obtained by a single instance or repetition of a pulse sequence, which includes a pulse sequence of length T5 and an amplitude of +V 最大 or +V 中间 A pulse of length T6 and amplitude -V 最大 or -V 中间 To produce red, the net impulse should be more positive than for producing white or yellow. Preferably, to produce red, the positive and negative voltages used have substantially the same magnitude (both V 最大 Or both V 中间 ), the length of the positive pulse is longer than the length of the negative pulse, and the final pulse is a negative pulse. During the pulse sequence for producing red, the display will oscillate between substantially black and red states. Before red it will go through states with lower L*, lower a*, and lower b* than the final red state.

[0086] Yellow can be obtained by a single instance of the pulse sequence or by repetition of instances (see Figure 3C and 3D and related description), the pulse sequence includes a length of T7 and an amplitude of +V 最大 or +V 中间 A pulse of length T8 and amplitude -V 最大 The final pulse should be a negative pulse. Alternatively, as mentioned above, the yellow color can be obtained by 最大 It can be obtained by using a single pulse or multiple pulses.

[0087] In the third phase of the waveform ( Figure 5In stage C of the waveform, a pulse using an intermediate and minimum voltage amplitude is provided. At this stage of the waveform, blue and cyan are produced after driving to white in the second stage of the waveform, and green is produced after driving to yellow in the second stage of the waveform. Therefore, when the waveform transient phenomenon of the display of the present invention is observed, a color in which b* is more positive than the b* value of the final cyan or blue will be experienced before blue and cyan, and a more yellow color in which L* is higher than the L* of the final green and a* and b* are more positive than the a* and b* of the final green will be experienced before green. More generally, when the display of the present invention presents a color corresponding to a colored particle in the first and second particles, a substantially white state (i.e., C* is less than about 5) will be experienced before this state. When the display of the present invention presents a color corresponding to a combination of a colored particle in the first and second particles and a particle of the opposite polarity to the particle in the third and fourth particles, the display will first substantially present the color of the particle of the third and fourth particles with the opposite polarity to the colored particle in the first and second particles.

[0088] Typically, cyan and green will be passed through the +V 最小 This is because only at this minimum positive voltage can the cyan pigment move independently of the magenta and yellow pigments relative to the white pigment. This movement of the cyan pigment is necessary to produce cyan from white or green from yellow.

[0089] Finally, in the fourth phase of the waveform ( Figure 5 In phase D), zero voltage is provided.

[0090] Although the display of the present invention has been described as producing eight primary colors, in practice, it is preferred to produce as many colors as possible at the pixel level. A full-color grayscale image can then be presented by dithering between these colors using techniques well known to those skilled in the art of imaging technology and described in more detail below. For example, in addition to the eight primary colors produced as described above, the display can be configured to present another eight colors. In one embodiment, these additional colors are: light red, light green, light blue, dark cyan, dark magenta, dark yellow, and two levels of gray between black and white. The terms "light" and "dark" used in this article refer to colors that have substantially the same hue angle as a reference color in a color space such as CIE L* a*b*, but have a higher or lower L*, respectively.

[0091] Generally, light colors are obtained in the same manner as dark colors, but using waveforms with slightly different net impulses in phases B and C. Thus, for example, the light red, light green, and light blue waveforms have more negative net impulses in phases B and C than the corresponding red, green, and blue waveforms, while dark cyan, dark magenta, and dark yellow have more positive net impulses in phases B and C than the corresponding cyan, magenta, and yellow waveforms. The change in net impulse can be achieved by varying the pulse length, number of pulses, or pulse amplitude in phases B and C.

[0092] Gray colors are usually achieved by a sequence of pulses oscillating between low or medium voltages.

[0093] It will be clear to those skilled in the art that in the display of the present invention driven by a thin film transistor (TFT) array, Figure 5 The available time increments on the abscissa of are usually quantified by the frame rate of the display. Similarly, it will be apparent that the display is addressed by varying the potential of the pixel electrode relative to the front electrode, and this can be achieved by varying the potential of either the pixel electrode or the front electrode or both. In the prior art, there is usually a matrix of pixel electrodes on the backplane, with the front electrode being common to all pixels. Therefore, when the potential of the front electrode is varied, the addressing of all pixels is affected. Regardless of whether a varying voltage is applied to the front electrode, the above reference to Figure 5 The basic structure of the waveforms described is the same.

[0094] Figure 5 The general waveform shown requires the drive electronics to provide up to seven different voltages to the data lines during the update of a selected row of the display. Although multi-level source drivers are available that can deliver seven different voltages, many commercially available source drivers for electrophoretic displays only allow for the delivery of three different voltages (usually positive, zero, and negative) during a single frame. The term "frame" in this article refers to a single update of all rows in the display. Figure 8 The generic waveform can be adapted to a three-level source driver structure as long as the three voltages supplied to the panel (usually +V, 0, and -V) can change from one frame to the next (i.e., for example, in frame n, the voltage supplied (+V 最大 , 0, -V 最小 ) and in frame n+1 a voltage (+V 中间 , 0, -V 最大 )).

[0095] Since changes in the voltage supplied to the source driver will affect each pixel, the waveform needs to be modified accordingly so that the waveform used to produce each color must be aligned with the supplied voltage. Figure 6 Shows the Figure 5In Phase A, no changes are required because only three voltages (+V 最大 , 0, -V 最大 ). Phase B is replaced by sub-phases B1 and B2 of lengths defined as L1 and L2, respectively, using a specific set of three voltages in each sub-phase. Figure 6 In phase B1, the voltage (+V 最大 , 0, -V 最大 ) is available, while in phase B2, the voltage +V 中间 , 0, -V 中间 Available. Figure 6 As shown, in sub-phase B1, the waveform requires +V 最大 A pulse of duration t5. Sub-phase B1 is longer than time t5 (e.g., to accommodate a waveform of another color that may require a pulse longer than t5), so a zero voltage is provided at time L1-t5. The position of the pulse of length t5 and the one or more zero pulses of length L1-t5 within sub-phase B1 can be adjusted as desired (i.e., sub-phase B1 does not have to start with a pulse of length t5 as shown). By subdividing phases B and C into sub-phases, in which one of three positive voltages, one of three negative voltages, and zero can be selected, the same optical result as obtained using a multi-level source driver can be achieved, although at the expense of a longer waveform (to accommodate the necessary zero pulse).

[0096] When top panel switching is used in combination with a three-level source driver, the same Figure 6 The same general principle applies when the source driver cannot provide the preferred V 最大 At equally high voltages, top-plane switching may be preferred. Methods of driving electrophoretic displays using top-plane switching are well known in the art.

[0097] Typical waveforms for the second drive scheme according to the present invention are shown in Table 1 below, where the numbers in brackets correspond to the number of frames driven using the indicated backplate voltage (relative to the top plate which is assumed to be at zero potential).

[0098] Table 1

[0099]

[0100] During the reset phase, a pulse of maximum negative and positive voltage is provided to erase the previous state of the display. The number of frames at each voltage is offset to compensate for the amount of net impulse in the high / mid voltage and low / mid voltage phases where the color is presented (denoted as Δ for color x) x )). In order to achieve DC balance, Δ xis chosen to be half the net impulse. The reset phase need not be implemented exactly as shown in the table; for example, when top plate switching is used, a specific number of frames may need to be allocated to the negative and positive drives. In this case, it is preferred to provide the maximum number of high voltage pulses consistent with achieving DC balance (i.e., subtracting 2Δ from the negative or positive frames as appropriate). x ).

[0101] In the high / medium voltage phase, as described above, a sequence of N repetitions of a pulse sequence for each color is provided, where N can be 1-20. As shown, the sequence includes 14 frames, which are assigned amplitudes of V 最大 or V 中间 The pulse sequence shown is consistent with the above discussion. It can be seen that at this stage of the waveform, the pulse sequence to render white, blue, and cyan is identical (because blue and cyan start from the white state in this case, as described above). Likewise, at this stage, the pulse sequence to render yellow and green is identical (because green starts from the yellow state, as described above).

[0102] In the low / medium voltage stage, blue and cyan are derived from white, while green is derived from yellow.

[0103] Typical results for driving a display of the invention are shown in Table 2. The waveforms used were similar to those shown in Table 1 for N=18 and the display was addressed at 65 frames per second using the preferred voltages as described above.

[0104] Table 2

[0105]

[0106] As will be clear from Table 1, a typical waveform for rendering a particular color on a display of the present invention may use 100-1000 frames, with a choice of 3-7 different voltages that may be applied to a pixel during each frame. Thus, there is a large number of possible waveforms that may be used. However, in practice, only a relatively small number of waveforms are likely to be stored in a waveform lookup table. In the prior art, 16 (4-bit) or 32 (5-bit) different color states are typically supported (although, as will be clear to those skilled in the art, a greater number of waveforms are stored because for each color state there may be a different waveform required depending on, for example, the previous state of the display, the temperature, the nature of the update (global or local), etc. Thus, a smaller set of colors (a minimum of 8, but preferably 16 or 32) is required to be rendered at the pixel level from the countless possible colors that may be rendered by the plethora of possible waveforms. As is known in the art, a full color image is produced by dithering between these pixel colors. Rendering colors in this manner is different from rendering colors in a typical emissive display, or in conventional color printing.

[0107] Fig. 7A The conventional method of rendering a full-color image on a reflective medium (e.g. white paper) using a printer such as an inkjet printer or a CMYK halftoning process is shown. Transparent or semi-transparent inks are available in three subtractive primary colors and optionally black. The dots of ink are applied successively so that they can be overlapped. Thus, for example, red is obtained by overlapping magenta and yellow dots. A full-color image requires no more than dots of the three subtractive primary colors.

[0108] Figure 7B The case of using a reflective display (e.g., a reflective electrophoretic display) that renders black, white, and gray, covered with a color filter array, is shown. A full-color image is a mix of modulated red, green, blue, and optionally black and white pixels, the various pixels being separated from each other and arranged side by side. It is not possible to render a specific color over the entire area of ​​such a display. For example, to render red, the electrophoretic ink behind the blue and green filters must be switched to black, and the black and white pixels may be black, white, or gray (depending on the brightness of the red required).

[0109] Figure 7C The electrophoretic display of the present invention is shown, which can present at least red, green, blue, cyan, magenta, yellow, white and black at each pixel location. The precise configuration of the pigments required to achieve these colors is described above. Note that in this case, it is not sufficient to apply only subtractive primaries (as in printing) because the pixels are still arranged side by side and the subtractive primaries cannot be covered to form additive primaries. It is also not sufficient to apply only additive primaries because in this case the subtractive primaries cannot be presented with 100% area coverage.

[0110] Electrophoretic displays of the present invention are generally capable of presenting many different colors at each pixel location, but these colors will not be selectable by independent manipulation of three control variables (in contrast to displays having a color filter array, as described in reference to FIG. Figure 1 As described above, in the electrophoretic display of the present invention, colors are typically rendered by controlling the movement of pigment particles (which may be white, cyan, magenta, and yellow) within a single layer of electrophoretic material. The movement of these pigment particles is typically not independently controllable (i.e., an electrical impulse that causes movement of one pigment particle will also move other pigment particles). Therefore, in the display of the present invention, certain RGB combinations can be rendered at the pixel level, while others cannot.

[0111] In one embodiment of the present invention, a full color image is obtained by dithering a pixel-level color palette consisting of red, green, blue, cyan, magenta, yellow, white and black corresponding to RGB values ​​in the source space (i.e., in the color space of the image to be rendered) of [255,0,0], [0,255,0], [0,0,255], [0,255,255], [255,0,255], [255,255,0], [255,255,255] and [0,0,0]. These primary colors are Figure 8 , and are referred to hereinafter as per-pixel palette primaries, also referred to herein as “device primaries” or “first color set”.

[0112] For example, the allocation of specific colors at the display pixel level that can be used for the present invention as device primaries can be performed by sampling all colors available for the display at the pixel level and selecting the eight colors that are closest to the standard of the specific color space. For example, the colors presented by the display can be measured in the CIELab color space, and the colors with the closest vector distances to those colors can be selected as the eight device primaries.

[0113] In fact, it is impossible to Figure 8 The ideal colors shown must be matched exactly, and some method must be provided for determining which colors are closest to the ideal colors. One method is to address the display with a waveform whose waveform parameters (such as those shown in Table 1) vary and measure the colors presented. From this set of presented colors, those colors that are closest to the standard in a particular color space can be identified. This method is used to generate the colors shown in Table 2.

[0114] Table 3 shows, in units of optical density, some of the colors obtained using a display addressed as described above with respect to Tables 1 and 2.

[0115] Table 3

[0116]

[0117] For red, green and blue, the optical density measured at 650nm, 550nm and 450nm respectively (shown in bold in the table) should be at least 0.2OD lower than the average of the optical density measured at the other two of the three wavelengths, and at least 0.1OD lower than any one of the optical densities measured at the other two of the three wavelengths. It can be seen that the green color presented in this particular display is not within this basic preferred specification.

[0118] For cyan, magenta and yellow, the optical density measured at 650 nm, 550 nm and 450 nm, respectively (as underlined in the table) should be at least 0.2 OD higher than the average of the optical densities measured at the other two of the three wavelengths, and at least 0.1 OD higher than any one of the optical densities measured at the other two of the three wavelengths.

[0119] For black (K) and white, the difference in optical density measured at 650 nm, 550 nm, and 450 nm, respectively, should not exceed 0.2 OD.

[0120] When the device primaries differ significantly from the destination colors in the source space (such as those shown in Table 3), the following steps can be used to render the image on a display.

[0121] First, the L* a* b* (CIELAB 1978, D65 / 2) values ​​of each color are measured. These L* a* b* values ​​are converted to the sRGB (0-255) color space using a known transformation matrix. The result is a set of points representing the actual device primaries in sRGB space.

[0122] This set of points can be arbitrarily transformed in order to facilitate dithering for rendering a color image. For example, the measured sRGB values ​​of the primary colors can be moved to be closer to the target points in the source space. The target image in the source space can also be transformed, such as by linear scaling to correspond to the measured black and white state of the display (i.e., each point in the image can be normalized to the measured dynamic range of the display).

[0123] After such a transformation, image dithering may be performed using algorithms known in the art, such as Floyd-Steinberg dithering.

[0124] If the resolution of the display is high enough and the viewing distance is long enough, dithering between only eight device primaries is sufficient. However, in practice, it may be necessary to provide more per-pixel palette compositions than just these eight primaries.

[0125] For example, for an information sign designed to be viewed at a distance of 1 meter, individual pixels become unobtrusive to a typical observer when the pixels are square with a side length of 0.25 mm. However, rendering an image at this pixel resolution using only eight device primaries results in unacceptable image artifacts, regardless of which of the prior art dithering algorithms are employed. Typically, in the grayscale ramp (i.e., gradient of brightness) between light and dark colors, sharp transitions may result in a blotchy appearance of mid-tones. This problem can be addressed by using a higher pixel resolution (e.g., using square pixels with a side length of less than, for example, 0.125 mm). However, such a display would require more drive circuitry, which would increase its cost.

[0126] According to the present invention, it is preferred to maintain the resolution of the display and increase the number of colors available in the per-pixel palette. As described above, the per-pixel palette consists of 8 (i.e., 2 3 However, with a four-bit waveform, 16 per-pixel colors may be addressable, and with a five-bit waveform, 32 per-pixel colors may be addressable, so the per-pixel palette may be expanded.

[0127] Fig. 9 A solution or embodiment to the above-mentioned problem of spots in the grayscale ramp is shown. Using only eight device primaries, grayscale can only be obtained by dithering between black and white. Introducing a single intermediate gray point, such as Fig. 9 As shown, having RGB values ​​(or actually L* values ​​between black and white states) in the [127,127,127] source space alleviates this problem. Since there are actually eight (8) additional addressable states obtainable using a four-bit waveform according to an embodiment of the present invention, they are allocated between [0,0,0] and [255,255,255] (i.e., along Fig. 9 The black / white diagonal line separating the middle gray levels in the image.

[0128] The actual assignment of the intermediate colors in the per-pixel palette is performed using the same color space as that used to assign the device primaries. For example, using the CIELab color space, these colors are different levels of gray, so none of them should have C*>5 (where C* is the distance from the L* axis and is defined as: ).

[0129] Fig.10An alternative embodiment of intermediate colors in a per-pixel palette for use with a four-bit waveform according to the present invention is shown. In addition to the device primaries defined above, there are six additional intermediate colors that include the centers of the faces of the color cube. These six intermediate colors correspond to RGB values ​​in the source space of [127, 127, 0] (dark yellow), [127, 0, 127] (dark magenta), [0, 127, 127] (dark cyan), [127, 127, 255] (light red), [127, 255, 127] (light green), and [255, 127, 127] (light blue). The remaining two intermediate colors can be assigned along the white / black diagonal (i.e., using the CIELab color space, these colors are different levels of gray and therefore should not have C*>5).

[0130] The actual color of the intermediate colors of the per-pixel palette according to the present invention can be measured in CIELab (or other) color space with reference to any of the eight device primaries described above. For example, the intermediate color in the white / yellow / red / magenta face of the color cube should be approximately equidistant (i.e., differing by no more than 10 dE* units from the equidistant distance) from each pair of diagonally opposite corners of the primary (corner) points so that it is approximately located in the center of a face of the color cube.

[0131] Table 4 shows the optical density of intermediate colors obtained using the same display of the present invention as described above to obtain the colors shown in Table 3.

[0132] Table 4

[0133]

[0134] The same criteria as described above with reference to Table 3 apply to the lighter and darker colors designated "red", "green", "blue", "cyan", "magenta", and "yellow". The lighter versions of red, green, and blue should have an optical density measured at 650nm, 550nm, and 450nm, respectively, that is at least 0.1 OD lower than the optical density of "red", "green", and "blue".

[0135] The optical density of the dark versions of "Cyan", "Magenta", and "Yellow" should be at least 0.1 OD higher than the optical density of "Cyan", "Magenta", and "Yellow".

[0136] Yet another alternative for the intermediate colors of the per-pixel palette according to the present invention is Figure 1In this case, a second color cube with a side length one-third of the primary color cube is used. The coordinates of the second color cube correspond to the RGB values ​​in the source space of [170,85,85], [85,170,85], [85,85,170], [85,170,170], [170,85,170], [170,170,85], [170,170,170], and [85,85,85].

[0137] exist Fig.10 and Fig.11 The color palette of the present invention is shown with only 3 and 4 points on the grey axis respectively. If a five-bit addressing waveform is available, additional points along the grey axis can be added to Fig.10 and Fig.11 . was added to Fig.11 Such a single point in Fig.12 Shown in.

[0138] Another alternative for intermediate colors in accordance with the present invention for a per-pixel palette utilizing five-bit addressing (32 states) is in Fig.13 In this case, in addition to the per-pixel palette primary color composition defined above, there are 19 intermediate colors corresponding to the midpoints of each face and edge of the color cube. These coordinates correspond to [0,0,127], [0,127,0], [0,127,127], [0,127,255], [0,255,127], [127,0,0,], [127,0,127], [127,0,255], [127,127,0], [127,127,127], [127,127,255], [127,255,0], [127,255,127], [127,255,255], [255,0,127], [255,127,0], [255,127,127], [255,127,255], and [255,255,127] RGB values ​​in the source space. These corresponding colors are navy, office green, turquoise, pure blue, livid, maroon, purple, violet, olive, gray, light blue, green, light livid, light cyan, pink, orange, light red, light magenta, and light yellow. There are another 5 colors available which can be selected to be along the gray axis mentioned above, or can be assigned to other intermediate colors.

[0139] In practice, it may happen that a display is able to render colors corresponding to the center of an edge or face of a color cube that is actually outside the polyhedron in CIElab space, which includes the convex hull constructed from the eight per-pixel palette primary compositions. In this case, the hue angle and brightness of the intermediate colors should be maintained as consistent with the above, but chromaticity expansion should be allowed. For example, the hue angle and brightness of an intermediate color defined as the midpoint of an edge should be substantially equal to the hue angle and brightness of the color of a point equidistant from two per-pixel palette primary compositions (i.e., within 10 degrees and 10dE*, respectively).

[0140] If more than five bits of addressing are possible, additional intermediate colors can be added to the per-pixel palette. An example of an extended per-pixel palette requiring six bits of addressing is Fig.14 shown.

[0141] Fig.15A -C is a schematic diagram depicting the colors of a preferred per-pixel palette as defined in the CIELab color space, where the colors are shown projected onto the a* / b* plane (a* is the horizontal axis; b* is the vertical axis). The L* axis is perpendicular to the a* / b* plane. The hue angle (h°) is measured counterclockwise from the horizontal axis and is calculated by the following formula:

[0142]

[0143] exist Fig.15A , the per-pixel palette is defined by eight (8) colors, which correspond to the above reference Figure 8 A color palette is described in which the source space is sRGB. Of the 8 colors, six (6) colors have a C* value greater than 10 and two (2) colors have a C* value less than 5. The two colors with C* values ​​less than 5 are preferably white and black. Preferably, the six colors with C* values ​​greater than 10 are distributed approximately equally in the CIELab color space so that one (1) color is assigned to each of the six sectors of the hue angle. For example, one color has an h° between 1 and 60 degrees, one color between 60 and 120 degrees, one color has an h° between 120 and 180 degrees, one color has an h° between 180 and 240 degrees, one color has an h° between 240 and 310 degrees, and one color has an h° between 310 and 360 degrees.

[0144] exist Fig. 15B , the per-pixel palette is defined by sixteen (16) colors, as described above with reference to Fig.10The 16 colors are preferably arranged in a CIELab color space such that 2 colors are allocated in each of the six sectors of the hue angle. For example, two colors have an angle of h° between 1 degree and 60 degrees, two colors have an angle of h° between 60 degrees and 120 degrees, two colors have an angle of h° between 120 and 180 degrees, two colors have an angle of h° between 180 and 240 degrees, two colors have an angle of h° between 240 and 310 degrees, and two colors have an angle of h° between 310 and 360 degrees. The L* of the color pairs in each segment differ from one another (eg, providing lighter and darker reds, as described above).

[0145] exist Fig. 15C , the per-pixel palette is defined by thirty-two (32) colors, as described above with reference to Fig.13 The 32 colors are described, wherein the source space is sRGB. Of the 32 colors, twenty-four (24) colors have C* values ​​greater than 10, and eight (8) colors have C* values ​​less than 5. Preferably, two of the eight colors with C* values ​​less than 5 are white and black, and the other six are distributed between black and white, preferably distributed between black and white with approximately equal spacing of L*. The 24 colors with C* values ​​greater than 10 are preferably distributed in the CIELab color space so that all colors are assigned to each of the seven sectors of the hue angle. For example, three colors have h° between angles of 15 and 45 degrees, one color has h° between 45 and 75 degrees, three colors have h° between 75 and 110 degrees, five colors have h° between 110 and 180 degrees, three colors have h° between 180 and 250 degrees, eight colors have h° between 250 and 345 degrees, one color has h° between 345 and 15 degrees, and the sector in which it is located includes h° = 360°.

[0146] In the present invention, then, an image is presented on a reflective display by dithering between per-pixel palette colors. Colors in the presented image that correspond to per-pixel palette colors can be presented without dithering. Thus, when the display is instructed to display one of the per-pixel palette colors over an extended area of ​​much more than one pixel, that entire area will be the per-pixel palette color without any pixels of the other color. However, in complex images with highly diverse colors (i.e., the second color set), image pixels may not be assigned their per-pixel palette colors (i.e., the first color set) due to errors introduced by the dithering algorithm that requires rendering other colors in neighboring pixels.

Claims

1. An electrophoretic display comprising a plurality of pixels for producing a color image, each pixel being capable of presenting not less than 8 colors and not more than 16 colors, wherein: The 16 colors are: red, green, blue, cyan, magenta, yellow, white, black, dark yellow, dark magenta, dark cyan, light red, light green, light blue, light gray and dark gray.

2. The electrophoretic display according to claim 1, wherein: Red, green and blue have optical densities that can be measured at three wavelengths: 650nm, 550nm and 450nm, respectively, where: When measured at 650nm, the optical density of red is: At least 0.2 OD lower than the average of its OD when measured at 550 nm and 450 nm, and at least 0.1 OD lower than its OD when measured at either 550 nm and 450 nm; When measured at 550nm, the optical density of green is: At least 0.2 OD lower than the average of its OD when measured at 650nm and 450nm, and at least 0.1 OD lower than its OD when measured at either 650 nm and 450 nm; and When measured at 450nm, the optical density of blue is: At least 0.2 OD lower than the average of its OD when measured at 650nm and 550nm, and is at least 0.1 OD lower than its OD when measured at either 650 nm and 550 nm.

3. The electrophoretic display according to claim 2, wherein: Cyan, magenta and yellow have optical densities that can be measured at three wavelengths: 650nm, 550nm and 450nm, respectively, where: When measured at 650nm, the optical density of cyan is: At least 0.2 OD higher than the average of its OD when measured at 550 nm and 450 nm, and at least 0.1 OD higher than its OD when measured at either 550 nm and 450 nm; The optical density of magenta when measured at 550nm: At least 0.2 OD higher than the average of its OD when measured at 650nm and 450nm, and at least 0.1 OD higher than its OD when measured at either 650 nm and 450 nm; and When measured at 450nm, the optical density of yellow is: At least 0.2 OD higher than the average of its OD when measured at 650nm and 550nm, and at least 0.1 OD higher than its OD when measured at either 650 nm and 550 nm.

4. The electrophoretic display according to claim 3, wherein: The black and white colors have optical densities that can be measured at three wavelengths, 650 nm, 550 nm and 450 nm, respectively, wherein: The optical density of black measured at 650 nm, 550 nm and 450 nm does not differ by more than 0.2 OD; and The difference in the optical density of white measured at 650 nm, 550 nm and 450 nm does not exceed 0.2 OD.

5. The electrophoretic display according to claim 4, wherein: Light red, light green and light blue have optical densities that can be measured at three wavelengths: 650nm, 550nm and 450nm, respectively, where: The optical density of the light red measured at 650 nm is at least 0.1 OD lower than the optical density of the red measured at 650 nm; The optical density of the light green color measured at 550 nm is at least 0.1 OD lower than the optical density of the green color measured at 550 nm; and The optical density of the light blue measured at 450 nm is at least 0.1 OD lower than the optical densities of the blue measured at 650 nm, 550 nm and 450 nm, respectively.

6. The electrophoretic display according to claim 5, wherein: Dark cyan, dark magenta, and dark yellow have optical densities that can be measured at three wavelengths, 650nm, 550nm, and 450nm, respectively, where: The optical density of dark cyan measured at 650 nm is at least 0.1 OD higher than the optical density of cyan measured at 650 nm; The optical density of dark magenta measured at 550 nm is at least 0.1 OD higher than the optical density of magenta measured at 550 nm; and The optical density of dark yellow measured at 450 nm is at least 0.1 OD higher than the optical density of yellow measured at 450 nm.

Images