Printed image for security features and method of designing and producing the same, security feature, security document, non-transitory computer readable medium

Abstract

A method for designing a first layer of a printed image in a security feature is provided. The security feature includes an array of optical elements overlaying the printed image, the method comprising: receiving an original image, the original image comprising rows of pixels extending in an x-direction and columns of pixels extending in a y-direction; selecting a first portion of the original image; generating a first block by combining pixels of the first portion with pixels of the first portion mirrored in both the x-direction and the y-direction; and assigning the first block a position within the first layer of the printed image, the position corresponding to a position of the first portion within the original image. A method of producing a printed image, a printed image, a security feature, a security document, and a non-transitory computer readable medium are also provided.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for designing a printed image of a security feature. A portion of the original image used to produce the printed image can be mirrored in multiple directions, providing a more unique design. BACKGROUND

[0002] Many documents contain security features that help to identify a counterfeit or forged document. Many such documents contain security features that utilize micro-optics, and as such, such features are often difficult to replicate with the precision required to produce a convincing counterfeit.

[0003] The micro-optics used in these documents often include an array of optical elements overlaid on a printed image composed of pixels. Viewing the printed image through the array of optical elements distorts the printed image and can result in unique effects, particularly when the security feature is tilted to change the angle between the viewer's eye and the plane of the security feature.

[0004] The more unique the effects produced by a security feature, the more certain one can be that the security feature (and its associated document) is authentic. Furthermore, the effects produced by a security feature are the product of a complex interaction between the printed image and the array of optical elements. The arrangement of features in the printed image can produce many effects, including magnification, implied depth, and animation. It can be difficult to "reverse engineer" the interaction between the printed image and the array of optical elements that produces such effects, and thus it can be difficult to replicate these effects. However, over time and with access to the security feature, counterfeiting is possible.

[0005] The more effects of a printed image, the more complex the combination of these effects. For example, a combination of animation and magnification can be more complex visually and mathematically than either animation or magnification alone. In turn, it can be more difficult for a counterfeiter to identify a certain effect or combination of effects and derive the arrangement of the printed image that produces these effects. The greater the degrees of freedom that exist in the design of the printed image, the greater the range of possible security features. The greater the range of possible security features, the more difficult it is to determine the underlying printed image, which is beneficial in deterring and combating counterfeiting.

[0006] Accordingly, there is a need for improvements in printed image and security feature design to provide unique effects and underlying image / optical element interactions that are difficult to determine. SUMMARY

[0007] The present invention is defined by the independent claims appended hereto. Embodiments of the present invention are defined by the dependent claims.

[0008] In a first aspect, there is provided a method for designing a first layer of a printed image in a security feature, the security feature comprising an array of optical elements overlaying the printed image, the method comprising: receiving an original image, the original image comprising rows of pixels extending in an x-direction and columns of pixels extending in a y-direction; selecting a first portion of the original image; generating a first block by combining the pixels of the first portion with the pixels of the first portion mirrored in both the x-direction and the y-direction; and assigning a position to the first block within the first layer of the printed image, the position corresponding to the position of the first portion within the original image.

[0009] In this way, the method provides a printed image with a unique arrangement that can produce a unique effect when incorporated into a security feature. In particular, mirroring portions of the original image in both the x-direction and the y-direction can "correct" a phenomenon known as "frame skipping". Frame skipping will be discussed in more detail later. Briefly, frame skipping occurs when a security feature is tilted by an observer beyond a threshold angle, for example 30° from the normal. If this threshold angle is exceeded, there is a mismatch between the printed image and the array of micro-optical elements overlaying it. This mismatch can be so severe that each lens focuses on a portion of the printed image that is not directly beneath the relevant lens, but rather on adjacent portions of the printed image respectively. This results in the appearance of specific visual features in the security image.

[0010] By correcting for "frame skipping", a smoother animation can be achieved when a user rotates the security feature. This not only has the benefit of being unique, and therefore easy to quickly identify as a genuine security feature, but also provides another link in the chain between the printed image and the appearance of the security feature; a counterfeiter would have to break this chain in order to counterfeit the security feature.

[0011] Generating the first block of the printed image can comprise: i) mirroring the pixels of the first portion in the x-direction about the right edge of the first portion; ii) mirroring the pixels of the result of step i) in the y-direction about the lower edge of the result of step i); or i) mirroring the pixels of the first portion in the y-direction about the lower edge of the first portion; ii) mirroring the pixels of the result of step i) in the x-direction about the right edge of the result of step i). It will be appreciated that the mirroring can be equivalently performed about the left edge and the upper edge respectively, rather than the right edge and the lower edge.

[0012] The method can further comprise setting the dimensions of the first block such that the dimensions of the first block relative to the printed image are equal to the dimensions of the first portion relative to the original image.

[0013] This allows certain patterns in the original image to be reproduced, albeit with added features, but appearing at the same scale in the printed image. Maintaining the scale of recognizable objects in the original image helps viewers determine the content of the printed image, making it easier to identify the genuine security feature.

[0014] The first block may be sized to be covered by exactly one optical element in the array of optical elements.

[0015] The array of optical elements may include rows of optical elements extending in the x-direction and columns of optical elements extending in the y-direction, and further include: rotating the first layer of the printed image relative to the array of optical elements by a rotation angle so that the individual rows and columns of blocks in the first layer and the rows and columns of optical elements are offset by the rotation angle.

[0016] In this way, the method provides a more unique character to the printed image. By tilting the printed image relative to the array of optical elements, a looping effect is created for the security feature. Identifiable objects or patterns in the printed image are no longer aligned vertically or horizontally with the array of optical elements. Therefore, when the security feature is tilted, the viewer sees horizontally and vertically moving elements of the printed image, which also loop. This effect is demonstrated in the accompanying drawings and described below.

[0017] As with "frame skipping" correction, this additional effect helps to enhance the uniqueness of the security feature, which is beneficial in preventing fraud and identifying genuine security features.

[0018] The rotation angle may be between 0.1° and 5°, for example between 0.1° and 2°, between 0.1° and 1°, or preferably between 0.1° and 0.6°.These angles have been found to be particularly effective for generating a unique printed image and thus a unique security feature.

[0019] In particular, these ranges are most effective for creating vibration effects in security features that are visually similar to the movement of underwater objects. This underwater effect is particularly recognizable to many people and provides a new class of unique features to complement features such as magnification and animation.

[0020] The method may further include: selecting another portion of the original image; and, for each additional portion: generating blocks and assigning the blocks to positions within the printed image corresponding to respective positions of the portion in the original image using the same generating and assigning steps as applied to the first portion and first blocks. Each block may correspond to one optical element in the array.

[0021] In this way, the entire original image can be transferred to the printed image, thereby preserving the content of the original image, such as any objects or patterns.

[0022] The method may further include receiving another original image, designing a first layer of a printed image using the original image, and designing a second layer of the printed image from the other original image, wherein the rotation angle applied to the first layer of the printed image is different from the rotation angle applied to the second layer of the printed image. The method may further include combining the rotated first and second layers to form a printed image.

[0023] By varying the vibration levels in different areas of the printed image, more unique characteristics can be applied to the security feature. For example, background objects in a printed image can be designed to vibrate more than foreground objects, thereby suggesting depth in the image. This is exemplary only, and applying any different rotations to the layers of the printed image will produce a more unique security feature, helping to identify genuine features and deter counterfeiters. In fact, a rotation angle of 0° can be applied to one layer and a non-zero rotation angle to another layer, thereby enhancing the vibration effect of objects in the rotated layer.

[0024] As used herein, compositing layers to form a printed image encompasses any physical or computational process by which the contents of the layers are superimposed on one another and / or combined into a single printed image. For example, a physical process for compositing two layers is simply printing one layer on top of the other. Depending on the opacity of the ink used and other printing parameters selected by the designer, part or all of one layer may dominate, or the color values ​​of the layers may be combined. For example, computational compositing processes can be performed using image processing software so that a single printing process can be used to print the composite image. Any process that combines visual elements from more than one layer to produce a printed image is considered compositing those layers.

[0025] The original image may be an interlaced image. The interlaced image may be generated by interlacing an input image, wherein interlacing the input image comprises: generating a plurality of frames of a multi-frame image, each frame comprising the input image at a different position within the frame; defining an arrangement of the plurality of frames, the arrangement comprising a grid; and interlacing the frames with each other according to positions of the frames in the grid.

[0026] The first portion may be selected to contain only a portion of each interlaced frame.

[0027] The original image may be a multi-frame image including a plurality of frames, and the first portion of the original image may include one frame of the multi-frame image.

[0028] Each further portion can comprise a different frame of the multi-frame image, and the method can further comprise interleaving the generated plurality of tiles according to the assigned positions of the tiles to form the first layer. Generating the multi-frame image as the original image can comprise generating a plurality of frames, each frame comprising the input image at different positions within the frame, and defining an arrangement of the plurality of frames, the arrangement comprising a grid.

[0029] In the inventive method using interleaving, three key processes are actually performed: frame generation; interleaving of portions of frames with each other; and jump-frame correction by mirroring. Frame generation occurs before interleaving (as interleaving is based on the existence of multiple frames in the multi-frame image). While jump-frame correction by mirroring occurs after frame generation, it can be performed before or after interleaving. The content of the images of the frames of the multi-frame image (input images) and the selection of the first portion and the further portion to generate the images of the first tile and the further tile (original images) will differ depending on when the correction by mirroring is performed. In embodiments where the correction by mirroring is performed after interleaving is complete, the input images can be images that have not been processed in any way, while the original images are interleaved versions of the input images. In embodiments where the correction by mirroring is performed during interleaving, the input images are the same (images that have not been processed in any way), the original images are the multi-frame image generated from the input images, and the first portion is a frame of the multi-frame image (i.e. the first portion can consist of one frame), such that the first tile is a mirrored frame (mirrored in both the x-direction and the y-direction). The further tile is a further mirrored frame, and then interleaving is performed on the tiles that are mirrored frames.

[0030] It will be appreciated that defining an arrangement of a plurality of frames (the arrangement being a grid) can require either the grid to be created as an actual entity stored in memory, or it can require only a data flag to be assigned to each frame and its content so that the interleaving algorithm knows the position of each frame relative to the interleaving step. In other words, the arranged frames can exist as an arranged grid, e.g. a person observing the arrangement would recognise it as such, or the frames can have associated metadata to allow the interleaving algorithm to derive the position of the frame within a nominal grid in order to perform the interleaving step.

[0031] The first portion and / or the further portion can be selected to be square.

[0032] In a second aspect, there is provided a method of producing a printed image for a security feature, the method comprising printing a printed image designed according to the first aspect. Printing in this context comprises producing a physical representation of the printed image, the data of which can be stored on a computing device.

[0033] In a third aspect, there is provided a printed image for a security feature, the security feature comprising an array of optical elements overlaying the printed image, the printed image comprising: a first layer comprising a first block comprising pixels of a first portion of an original image mirrored in both the x-direction and the y-direction. The physically printed images designed according to the method of the first aspect are advantageous in that they allow for more unique security features than security features based on known printed images, for the reasons as described in relation to the first aspect.

[0034] The first layer can further comprise: one or more further blocks, each further block comprising pixels of a respective further portion of the original image mirrored in both the x-direction and the y-direction.

[0035] The printed image can further comprise: a second layer comprising a second block comprising pixels of a first portion of a second original image mirrored in both the x-direction and the y-direction.

[0036] The first layer and / or the second layer can be rotated by a rotation angle with respect to the x-direction and the y-direction, optionally wherein the rotation angle is between 0.1° and 5°, for example between 0.1° and 2°, between 0.1° and 1°, or preferably between 0.1° and 0.6°.

[0037] In a fourth aspect, there is provided a security feature comprising: a printed image as described in the third aspect; and an array of the same optical elements overlaying the printed image.

[0038] In a fifth aspect, there is provided a security document comprising a security feature as described in the fourth aspect. A security document in the present context can be any document for which authenticity marking can be useful or necessary for the document to serve its purpose.

[0039] The security document can be a banknote, a passport, a driver’s license, an identity card, etc.

[0040] In a sixth aspect, there is provided a non-transitory computer readable medium storing computer readable instructions that, when executed, cause a machine comprising a processor to perform the method of any of the first aspect. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 shows a plan view of a security feature according to the present application;

[0042] Figure 2 shows a cross-sectional view of a security feature according to the present application;

[0043] Figure 3 shows a plurality of images used in an interleaving method according to the present application;

[0044] Figure 4 showing the interleaving method according to the present application;

[0045] Figure 5 showing the sampling method according to the present application;

[0046] Figure 6 showing a graph of the number of frames versus the number of pixels to show the frame skipping problem in the uncorrected security feature;

[0047] Figure 7 showing a security feature with an uncorrected printed image at three simulated optical viewing angles;

[0048] Figure 8 showing a security feature with a corrected printed image at three simulated optical viewing angles according to the present application;

[0049] Figure 9 showing the method of frame skipping correction by mirroring according to the present application;

[0050] Figure 10 showing a graph of the number of frames versus the number of pixels to show the frame skipping effect is eliminated in the corrected security feature;

[0051] Figure 11A showing a security feature with an uncorrected and rotated printed image at three simulated optical viewing angles, showing only horizontal tilt;

[0052] Figure 11B showing a security feature with a corrected and rotated printed image at three simulated optical viewing angles according to the present application, showing only horizontal tilt;

[0053] Figure 12 showing a portion of a security feature with an array of optical elements covering a rotated printed image according to the present application;

[0054] Figure 13 showing a security feature with a corrected and rotated printed image at four simulated optical viewing angles according to the present application, showing horizontal and vertical tilt;

[0055] Figure 14A showing a security feature with a corrected and rotated printed image at four simulated optical viewing angles according to the present application, showing horizontal and vertical tilt;

[0056] Figure 14B showing a security feature at six alternative simulated optical viewing angles according to the present application; Figure 14A

[0057] Figures 15A to 15D showing the method of generating a first layer from an original image according to the present application; and ​

[0058] Figures 16A to 16C A method of generating a first layer from an original image according to the present application is shown. DETAILED DESCRIPTION

[0059] Certain example embodiments will now be described in order for a full and enabling disclosure of the principles of the structures, functions, manufacture, and use of the devices, systems, and methods being disclosed herein to be realized. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the devices, systems, and methods specifically described herein and illustrated in the drawings are non-limiting example embodiments and that the scope of the present application is defined solely by the claims. The features illustrated or described in connection with one example embodiment can be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present application.

[0060] Security feature

[0061] Figure 1 An example security feature 100 is shown that includes an array 110 of optical elements 114 having a width 115. Each array includes a plurality of optical elements 114 arranged in parallel rows 111 and columns 112. In some preferred embodiments, the optical elements can be lenses, and the array of optical elements can be an array of lenses. In another preferred embodiment, the lenses can be circular lenses. In another preferred embodiment, the lenses can be square lenses. In other embodiments, the lenses can be other shapes of tessellation, such as hexagonal lenses. In another embodiment, the lenses can be planar lenses. In some embodiments, the planar lenses can include Fresnel lenses, holographic lenses, or diffractive lenses. The use of multiple shapes of lenses is contemplated, and the above embodiments should not be construed as limiting.

[0062] Figure 2 An example security feature 200 is shown that includes an array 110 of optical elements 114 that covers a printed image 210, where the printed image includes a two-dimensional matrix of rows and columns of pixels 211. The printed image can include a series of frames, where the frames can be different frames of an animation or different perspectives of an image, and where the frames can be interleaved in both dimensions of the two-dimensional matrix. This interleaving means that a user will see pixels from different frames 212, 213, 214, 215 depending on the angle at which they view the security feature 200. In this way, as the user tilts the security feature (or otherwise changes their position relative to the security feature), they will see different frames, either giving the impression of an animated image if the frames are frames of an animation, or a false three-dimensional effect if the different frames are different perspectives of an image.

[0063] In general, a security feature can be manufactured by printing pixels onto a substrate to form a printed image, and then covering the substrate with an array of optical elements. In some embodiments, the security feature can include printing the printed image on a first side of a polymeric film, and applying the array of optical elements to the other side of the polymeric film. In some embodiments, the array of optical elements can be applied as a sheet or cast directly on top of the printed image. In some embodiments, the security feature can include an array of optical elements focused onto an internal surface of the security feature.

[0064] In principle, the ideal approach would be to produce optical elements and pixels that are exactly the same size as the design, and the width of the optical elements should be an integer multiple of the pixel width, so that an integer number of pixels fits exactly underneath each optical element. For example, a typical optical element might have a nominal design size of 70 microns, and a pixel might have a nominal design size of 2.5 microns, which would result in 28 rows (or columns) of pixels underneath each optical element. This would result in the observation experience described above, where the user would see different frames depending on the angle at which they observe the security feature 200, and they would see the same pixels in only one frame of the overall image.

[0065] The content of the printed image underneath the array of optical elements determines the appearance of the security feature when viewed through the optical elements. The integral image is an image created through the process of integral imaging, allowing the security feature to exhibit certain unique effects.

[0066] Generating an integral image

[0067] Integral imaging is a process that can suggest three-dimensionality from a two-dimensional object. Macro integral imaging uses photographic techniques with multiple lenses, and then arranges all the photographed images through a single array to produce a three-dimensional effect. Photographic interlacing requires a lot of labor, and the effects it can produce are limited. While computational integral imaging can be used to recreate a simplified version of a three-dimensional object, it also offers a great deal of design freedom, whereby the image can be further simplified. Printed or displayed objects are perceived as three-dimensional in two ways: first, in cases where they simultaneously present slightly different views to each eye; and second, in cases where the display medium appears to move in the opposite direction to the display medium as it moves relative to the observer.

[0068] The process of generating a three-dimensional view can be simplified as follows: if a two-dimensional object moves faster than its background or in the opposite direction, they will appear to be on a plane above or below the plane of the print. These objects will also satisfy the binocular three-dimensional requirement, as they will appear slightly different in each eye, but consistent with their motion. In this way, complex three-dimensional images composed of objects on different planes can be constructed.

[0069] Animating a flat object to create the effect of depth doesn't require multiple 3D views; it can be done with just a single image. There are two ways to convert a single image into this type of animation:

[0070] (a) Create multiple image frames in which each object is slightly moved relative to its previous frame, and then interleave them together. For example, a 3D animation using a 70-micron lens and a 10-micron frame size would require 49 such frames, with the object in each frame moving slightly.

[0071] (b) The image is "observed" through a series of "lenses," replicating the integral imaging process. In other words, portions of the image are sampled and cropped, and placed side by side on a new image. Sampling is performed by iterating the x / y origin of the sample window by an amount smaller than the sample size.

[0072] Two integral imaging processes are described in detail here: interleaving and sampling.

[0073] staggered

[0074] The interlacing process begins by generating multiple frames. The frames are selected to produce a specific effect in the security feature, as shown in the following example. Figure 2 This first stage is to create some frames; Figure 3 This process is illustrated in Figure 1. For ease of illustration, the frames are shaded, and only nine frames are shown for simplicity, but this is exemplary only; any number of frames can be used and their shading maintained uniformly. The center frame contains arrows, and the frames surrounding the center contain arrows that move to positions consistent with where the object is expected to move on the tilted micro-optics—these are animation frames, similar to those in a movie, but here they are associated with different viewing directions arranged in a grid.

[0075] Once the frames are generated, each frame is split into its component parts and then reassembled into the larger output image. Figure 4 This is shown in Figure 4 In the image, you can see that the shaded and numbered frames retain their relative positions to each other in the finished design, but are broken apart pixel by pixel and blended together to form a larger image (again, this is just a tiny 3×3 pixel image represented; the real image can be any size and could be much larger).

[0076] Each repeating image cycle can be designed to fit a single lens in the output design so that when a user views the optics from an angle, the same frame will be magnified by all lenses (e.g., looking from a normal direction, the observer will see all pixels of the image frame labeled "5").

[0077] A further step of pitch correction can be applied to correct for the fact that the lens and print pitches are unlikely to match, so the displayed frame can move as the observer moves over the image. However, this step is not essential and can in fact be deliberately omitted, as the mismatch between the array of lenses and the print increases the sense of depth.

[0078] In formal terms, this relationship can be written as:

[0079]

[0080] where:

[0081] Pij is a pixel on one of the image frames;

[0082] x and y are the column and row positions of the frame;

[0083] i and j are the column and row positions of the pixel;

[0084] P is the pixel at the calculated position on the new image; and

[0085] w is the width of each frame and h is the height of each frame (all frames are typically the same size).

[0086] The output image size is equal to the total width and height of all the individual frames.

[0087] Sampling

[0088] The sampling technique can also be used to produce the integral image. The advantage of the sampling technique is that it is faster and uses fewer system resources; in this case, rather than generating multiple frames by moving the object in the image, a single image is used and sampled at positions incremented at intervals smaller than the frame size. Figure 5 This example is shown. The first column of pictures to the right of the arrow is generated by cropping the frame shown on the leftmost image. It can be seen that the frame is large (100x100 pixels in this case) and moves down at half its size (i.e. each frame is offset by 50 pixels), so in this case it covers the height of the image in three slices. The same process occurs on the x-axis, where five slices are needed to cover the width of the image. For ease of illustration, the final image is shown with space between each part (although in practice there is no space between adjacent parts). As with interleaving, each part is equal to the size of a lens on the array of lenses. The number of slices and the number of pixels in the image are simplified here; there can be any number of slices, typically thousands.

[0089] The design created with this method is intended to be performed in the same way as the design created by interleaving a series of frames. The sampling method does not create any extra unwanted images and can be optimized to copy only specific colors (e.g. exclude white which is not needed to copy), thus using sampling is faster than using interleaving.

[0090] In form, this relationship is:

[0091]

[0092] Where:

[0093] P is a pixel on the new image; for a real design, the size of the output controls the design process - as the design determines the size needed;

[0094] P° is a pixel on the old image;

[0095] C w and C h are the slice width and height, which are equal to the diameter of the lenses of a normal two-dimensional lens system, or one of them is equal to the height of the output image of the array of lenses;

[0096] I x and I y are the increments between the slice parts in the x and y axes. Usually, this is expressed as the original magnification or scaling to the output size;

[0097] i and j are the row and column positions on the slice, with values 0 to C w and 0 to C h , respectively;

[0098] m is the lens column in the array of lenses, with values 0 to m max = w / F, where w is the width of P and F is the pitch of the lenses in the array; and

[0099] n is the lens row in the array of lenses, with values 0 to n max = h / F, where h is the height of P and F is the pitch of the lenses in the array.

[0100] The input image can need to be resized to fit the specific increment size. The size is: width (= m max I x +F) x height (= n max I y +F).

[0101] As can be seen from equation (2), this method is actually another interleaving method, but using a single image and interleaving with itself.

[0102] The sampling can introduce errors between the lens and the printed image, i.e. the degree of mismatch between the starting point of a pixel in the printed image and the starting point of a lens in the array of lenses is increasing; this can happen in both the x and y directions. If the error exceeds the width of one pixel, the sampling algorithm can skip a column or a row of the original image. In practice, while this has little effect on the final feature, it does not need to be addressed at all, but can be corrected by a simple method of copying the next row or column of pixels back.

[0103] When the following conditions are met, a correction point can be found:

[0104]

[0105] Where:

[0106] m is the lens number;

[0107] F is the actual size of the lens in microns; and

[0108] (int) is a computational term that essentially rounds the term that follows it down. In other words, when the error is lower than the error of the previous lens; the error is the difference between the original value and the rounded down value.

[0109] Finally, the direction of movement can be controlled by reversing the slices before they are placed; the reversal can be single or double axis, similar to the negative / positive magnification parameters in a Mool lens. The effect produced is a reversal of the motion relative to the single or double axis tilt - a rotational motion can be reversed from clockwise to counter-clockwise.

[0110] Correcting an integral image

[0111] Another problem is encountered with the computed integral image created in any way (e.g. by sampling and interleaving as described above): if the optics are tilted too much, the animation jumps back from the last frame to the first one. Figure 6 It is shown how the relationship between the frames and the pixels works in one dimension - of course, this relationship actually happens in both the x and y directions.

[0112] The jump frame phenomenon occurs when the security feature is tilted to an angle such that the viewer can see the information printed under the adjacent lens. The angle of tilt at which the jump frame or frame jump occurs is determined by the lens design; for micro-optics, this angle is typically + / - 30°, but can vary. Once the limit angle is exceeded, the viewer no longer sees the information printed directly under each lens, but each lens is now focused on the information printed under the adjacent lens.

[0113] Figure 7 and Figure 8Frame skipping is shown, using a safety feature depicting a clownfish. Figure 7 Simulates a printed image that has not been corrected to eliminate frame skipping. Figure 8 A corrected printed image is simulated.

[0114] Wherever references to simulated security features are made herein and where they are shown in the figures, they are represented by animated static images. The animation is a computational simulation of the changing appearance of the physical security feature as the "viewing angle" changes. In all cases described herein, the simulated observer is viewing the page perpendicularly, so the change in viewing angle is achieved by tilting the simulated security feature. However, it will be appreciated that the effect is the same for a stationary security feature and for an observer moving in an arc around the security feature.

[0115] Both figures show the animated progression of the security feature as the observer tilts it only in the horizontal plane from the normal (i.e., 0° for the observer), with the left side of the security feature rotating into the page and the right side rotating out of the page. There are slight differences between the leftmost and center static images of the uncorrected and corrected security features, which are caused by the correction. However, in the last frame, at the maximum rotation angle for the observer, the uncorrected security feature ( Figure 7 ) will jump in the opposite direction to the natural direction expected by the observer. Figure 7 and Figure 8 In the leftmost and center still images, the clownfish appears to span the frame as expected if the security feature were three-dimensional (i.e., if the clownfish were actually below the page and being viewed through the window). The clownfish appears to slide toward the right side of the still image. While this occurs to a limited extent, Figure 8 It has decreased somewhat, but is still noticeable.

[0116] However, in Figure 7 In the rightmost still, the clownfish "jumps" back toward the left side of the still, breaking the illusion of depth. This effect occurs because the lenses are now focusing on the rightmost edge of the frame adjacent to the frame they were designed to focus on.

[0117] Corrected safety features ( Figure 8 ) will not experience this frame skipping phenomenon, but will continue to transition smoothly. Figure 8 In the rightmost frame (at maximum simulated tilt), the clownfish is located on the far right. Although the lens of the corrected safety feature is still focused on the adjacent frame, Figure 7 The same security feature is used as in the case of the optical effect, but the printed content of the frame has been configured so that this optical effect does not produce frame skipping. The design method for achieving this will now be described.

[0118] The significance of jumping from the last frame to the first frame depends on the design. However, rather than rely on the design to correct this issue, a general condition can be created that can control it.

[0119] Rather than using the full 14 frames to make the animation (whose progression is shown in Figure 6 ), the number of frames can be halved and then mirrored. Figure 9 A schematic of a single slice, mirroring, and reassembly is shown. The size of the slice is half of the uncorrected slice, and it is mirrored in both the horizontal and vertical axes before reassembly.

[0120] This method can make a simpler animation with one quarter the number of frames of the original animation. The frame progression is shown in Figure 10 ; the effect is to halve the number of frames (in each dimension) and create a reverse loop in which the animation will return to the original point. As before, this can happen in two dimensions.

[0121] Equation (2) becomes a series of four equations, shown on the next page, where:

[0122] P and P° are the pixels on the new and original images, respectively;

[0123] F is the lens diameter;

[0124] m is the lens column index, which takes on values from 0 to m max = w / F, where w is the width of the output image;

[0125] n is the lens row index, which takes on values from 0 to n max = h / F, where h is the height of the output image;

[0126] i and j are the indices of the slice; and

[0127] I x and I y are the increments between slice positions.

[0128] The input image can need to be resized to: m max I x + (F / 2), n max I y + (F / 2).

[0129]

[0130] As before, the slices can be inverted to change the movement of the features, in a manner similar to the positive / negative magnification of moire. For the corrected integral image, this can be accomplished by reflecting the slices in one or both axes, or by simply assembling the slices in a different order; both methods will produce the same output design.

[0131] Applying a rotation

[0132] By interleaving or sampling, integral imaging changes the order of the frames in the animation from linear to circular; in two-dimensional animation, this happens in both axes. Rotating the design a little bit creates a double relationship between the frames and the positions, which creates a unique effect for the circular device.

[0133] The design rotation is done by a standard image rotation matrix transformation. The transformation matrix is shown in equation (8):

[0134]

[0135] This is a 2x2 matrix, where the coefficients are:

[0136]

[0137] Normally, the inverse matrix is used to perform the image transformation; the reason for this is that the transformed image is the destination image, and if some coordinates do not have a pixel, they will contain a tear when the image is transformed. The solution is to start from the completed image and find the pixels on the original image that, after transformation, match the pixels expected on the destination image.

[0138] The inverse of a 2x2 matrix can be calculated using the following equation:

[0139]

[0140] When applied to the x, y coordinates of the image:

[0141]

[0142] Solving the x' and y' matrices gives:

[0143]

[0144] Since we want to be able to control the center point of the transformation and the destination, we need to add adjustment parameters x s , y s (source for x and y) and x d , y d (destination for x and y).

[0145]

[0146] Referring back to equations (8) and (9):

[0147] a = cos 0, b = -sin 0, c = sin 0, and d = cos 0

[0148] Another advantage of using an inverse transform is that the standard form of the transform function is usually a forward transform, so there is no need to further invert the transform coefficients to add a new type of transform. Furthermore, the rotational transform is changed from anti-clockwise to clockwise.

[0149] If a small angle rotation (e.g. 0.2°) is implemented between the printed image and the array of lenses, the designed x and y axes will no longer match the x and y axes of the lenses. The designed x axis will interact at a small angle with the y axis of the lenses - this adds a second animation cycle to the animation. In a physical security feature, the rotation can affect the printed image, the array of lenses, or both. In Figure 11A and Figure 11B the effects of uncorrected and corrected security features can be seen. The static images shown in these figures are static images of simulated animations made in the same way as described in Figure 7 and Figure 8 .

[0150] The uncorrected grid ( Figure 11A ) runs along the diagonal direction and resets itself once the jump from the last frame to the first frame occurs. A vertical "screen wipe" effect occurs because the next set of frames are focused gradually, rather than simultaneously. The corrected version ( Figure 11B ) cycles back and forth in a triangular cycle that is flat enough to appear serpentine to the observer. Figure 11A and Figure 11B show a pattern with horizontal and vertical lines; here the printed image under the optical elements is a simple grid. It can be seen that tilting the optical elements along one axis causes the lines of only one axis to move. In other words, when the security feature is tilted in the horizontal plane (i.e. the horizontal centre of the top and bottom edges of the security feature remains the same distance from the observer), the grid lines closer to the horizontal do not move. Tilting the security feature vertically causes the horizontal grid lines to appear to vibrate, while the vertical grid lines remain stationary (although no longer straight).

[0151] The reason for this second axis relationship can be seen in Figure 12 ; the horizontal line passes through the schematic and is covered by the lenses (in this case, the image is rotated 4° relative to the lens array, the effect is exaggerated for the effect shown). As the line moves from left to right of the image, it descends relative to the line of the lenses. If the optics designed in this way are tilted vertically, the vertical position of the lens focal point will move up and down; at the same time, the point at which the printed line intersects this will move left and right (the opposite of what can be seen in Figure 4 ).

[0152] In practice, an observer will tilt the security feature in a rolling fashion along multiple axes. Partly this is because it is a common method of viewing such features, and also because it is unlikely that an observer could manually apply a motion precise enough to tilt the feature in only one axis.

[0153] Figure 13 and Figure 14A The effect of applying a horizontal tilt to a security feature with printed images that have been corrected for judder and also have a rotation applied so that the printed images and the array of optical elements are offset by a rotation angle is shown. Of course, a rotation could also be applied to the optical array to achieve the same effect; the important thing is the rotational offset between the printed images and the overlapping array of optical elements.

[0154] Although Figure 13 The same grid pattern as depicted in Figure 11A and Figure 11B is shown, but now both the horizontal and vertical lines are wiggling in a sinuous fashion.

[0155] Figure 14A The clownfish from Figure 7 and Figure 8 is shown, but now a rotation has been applied to the printed images. The clownfish can now be seen to have a sinuous motion, similar to how it would appear to be viewed underwater where the observer’s perspective would be distorted by the moving water / air boundary. In Figure 13 and Figure 14A four still images of an animated simulation of the security feature are shown. In both cases, the top left corner still shows the top left corner of the feature “off-page”, the top right corner still shows the top right corner of the feature “off-page”, and so on.

[0156] Figure 14AIt is also highlighted how different layers of printed images can be superimposed or composited to produce more unique security features, including more complex motion. A group of smaller clownfish (1402) in the image background has been produced as a second layer of the printed image. The individual clownfish and the group of clownfish were originally started as separate original images, then interleaved or sampled, corrected, then rotated. Thus, the rotation was applied independently to each image, and in this case a different rotation angle was applied. As a result, each image appears to have a different degree of wobble or "undulation"; the group of fish was applied a larger rotation angle, and thus appears to wobble more. This can be appropriate in this case, for example, because the group of fish is in the background, so their greater wobble provides an enhanced perception of depth to the image. However, in general, the ability to produce security features in which organisms appear to wobble when tilted can provide more unique security features that are more difficult to counterfeit by reverse-engineering the printed image. The difficulty of counterfeiting is exacerbated by the more complex security features in which multiple layers with different amplitudes of wobble are composited to form the printed image.

[0157] Figure 14B The same simulated security feature as shown in Figure 14A is shown. However, in Figure 14B the simulated perspective change between each static image has been minimised (i.e. each static image represents a smaller amount of tilt from the position of its neighbouring static image). Figure 14B The progression of the corrected and rotated security feature is shown as the security feature is tilted. The meandering effect produced by the appropriate processing of the printed image according to the method of the present invention is visible in the gradual changes in shape and size of the features of the clownfish and group of clownfish in each static image. For example, the dorsal fin (top) of the clownfish appears to be compressed and tilted to the right, and the position of each fish in the group of clownfish relative to each other is different. As described in detail herein, a counterfeiter can only access the animated security feature, and must "reverse-engineer" the underlying printed image to counterfeit the security feature. Complex animation effects (e.g. meandering, flowing water, wobble) can produce security features that are more difficult to reverse-engineer. Furthermore, the unique nature of the effect is easily recognisable and distinguishable from other animation effects. This means that the speed and difficulty of verifying a genuine security feature can be increased.

[0158] Exemplary method

[0159] Figure 15A The original image 1502 is shown. A first portion 1504, here the top left corner, is selected, shown as a dashed line in the figure. Figure 15B The mirror image of the first portion 1504 in the x-direction and the y-direction is shown. The x-direction and the y-direction can be defined as the intended horizontal and vertical axes of the original image.

[0160] The combination of the content of the first portion 1504 and the mirrored portion forms a first block 1506. The first block 1506 is shown with a box around the content of the first portion 1504 and the mirrored portion. It will be appreciated that this is to show the construction of the first block 1506 and that the box lines can not be reproduced in the first block 1506 during operation of the method.

[0161] The first block 1506 can be sized and positioned in a first layer 1508 of the printed image, the position corresponding to the position of the first portion within the original image, as shown in Figure 15C According to the same method, one or more further blocks can be generated from one or more portions of the original image. Thus, the method represents a form of sampling. The portions of the original image can overlap each other, as shown in Figure 5

[0162] A rotation angle can then be applied to the first layer 1508, as shown in Figure 15D In Figure 15D , the first layer 1508 comprises only the content of the first block 1506, so this is all the content that has been rotated. The rotated first layer 1508 is contained within a printed image 1510. An array of optical elements arranged to overlay the printed image 1510 will then be offset from the printed image by the rotation angle. In other words, the array of optical elements will be offset from the x and y directions of the printed image, where the x and y directions of the printed image are the directions before the first layer 1508 was rotated, i.e. the directions in which the first portion 1504 was mirrored.

[0163] In an alternative embodiment, using the jump-frame correction method according to the present application, the original image is a multi-frame image comprising a plurality of frames. To illustrate this embodiment, Figure 3 and Figure 4 are used as Figures 16A to 16C .

[0164] Figure 16A A multi-frame image 1602 is shown, here generated from an arrow positioned at the centre of the image. This is the central frame 1604, and additional frames are generated to surround the central frame - these surrounding frames contain the arrow moved to a position consistent with the position of the object expected to move on the tilted micro-optics - these are animation frames, and are similar to animation frames in a movie, but here the frames of the animation are associated with different viewing directions arranged in a grid, as referenced in Figure 3 ​As described. For ease of explanation, the frames are shaded, and for simplicity, only nine frames are shown, but this is merely exemplary, and any number of frames can be used and their shading can be kept uniform. In this embodiment, the first portion 1606 is selected to consist of one frame of the multi-frame image 1602. In this example, the upper left frame is selected as the first portion, but it will be understood that any frame can be selected.

[0165] Now it will be applied to Figure 15A The same process as for the first portion 1504 is applied to the first portion 1606, as shown in FIG. Figure 16B . The first portion 1606 is mirrored in both the x-direction and the y-direction to form a first block 1608. As with the frame skipping method described herein, the mirroring in the x-direction and the y-direction is performed in one of two orders: either first in the x-direction or first in the y-direction to form the first block 1608. The mirroring process can then be performed on additional portions of the original (multi-frame) image 1602, each additional portion also consisting of a frame.

[0166] Figure 16C Showing multiple blocks generated according to the above process, such as Figure 16B Each block is now represented by a grid of numbers to show the interleaving process that follows. In reality, the content of each block is determined by the frame from which it originated and the Figure 16B , is determined by a mirroring process. For example, first block 1608 is represented by a grid of multiple "1's." The blocks are arranged in the same manner as the multiple frames of multi-frame image 1602. As shown, the blocks are interleaved to form first layer 1610. The interleaving used in this example selects a portion of each block and arranges the selected portion into the same arrangement defined by the block. For example, the upper left portion of each block (a single "1" starting from the upper left corner of the upper left block, a single "2" starting from the upper left corner of the upper middle block, and so on) is arranged according to the block arrangement, forming a 3×3 portion in the upper left corner of printed image 1610, which contains each digit from "1" to "9" arranged from left to right and from top to bottom, just like the multiple blocks. This process is repeated until first layer 1610 contains the contents of multiple blocks.

[0167] It will be appreciated that the interleaving forming the first layer 1610 can be performed in any order, i.e., the first layer 1610 can fill portions of the plurality of blocks in any order. As described with respect to other embodiments of the present invention, the first layer 1610 can be included in a printed image that overlaps an array of optical elements to create a security feature. Furthermore, the first layer 1610 can be rotated by an angle to create an offset between the printed image and the overlapping optical elements.

Claims

1. A method for designing a printed image in a security feature, the security feature comprising an array of optical elements overlying the printed image, the method comprising: receiving an original image, the original image comprising rows of pixels extending in an x-direction and columns of pixels extending in a y-direction; selecting a first portion of the original image; generating a first block by combining pixels of the first portion with pixels of the first portion mirrored in both the x-direction and the y-direction; as well as The first block is assigned a position within a first layer of the printed image that corresponds to the position of the first portion within the original image.

2. The method of claim 1 , wherein generating the first block of the printed image comprises: i) mirroring the pixels of the first portion in the x-direction about the right edge of the first portion; ii) mirroring the pixels of the result of step i in the y direction about the bottom edge of the result of step i; or i) mirroring the pixels of the first portion in the y direction with respect to the lower edge of the first portion; ii) Mirror the pixels of the result of step i in the x direction about the right edge of the result of step i.

3. The method according to claim 1 or claim 2, further comprising: The size of the first block is set so that the size of the first block relative to the printed image is equal to the size of the first portion relative to the original image.

4. The method of claim 3, wherein the first block is sized to be covered by exactly one optical element in the array of optical elements.

5. The method of claim 4 , wherein the array of optical elements comprises rows of optical elements extending in the x-direction and columns of optical elements extending in the y-direction, the method further comprising: The first layer of the printed image is rotated relative to the array of optical elements by a rotation angle such that the individual rows and columns of blocks in the first layer and the rows and columns of optical elements are offset by the rotation angle. The method according to claim 5 , wherein the rotation angle is between 0.1° and 5°.

7. The method according to claim 1 or claim 2, further comprising: selecting another portion of the original image; and, for each additional portion: Blocks are generated and assigned positions within the printed image corresponding to the respective positions of the further portion in the original image using the same generation and assignment steps as applied to the first portion and the first block. The method of claim 7 , wherein each block corresponds to an optical element in the array.

9. The method according to claim 5 or claim 6, further comprising: receiving another original image; as well as The step of designing a first layer of the printed image using the step of designing a second layer of the printed image from the other original image, wherein the rotation angle applied to the first layer of the printed image is different from the rotation angle applied to the second layer of the printed image.

10. The method according to claim 9, further comprising: The rotated first and second layers are composited to form the printed image.

11. The method according to claim 1 or claim 2, wherein the original image is an interlaced image.

12. The method of claim 11 , wherein the interlaced image is generated by interlacing the input image, and wherein interlacing the input image comprises: generating a plurality of frames of a multi-frame image, each frame including the input image at a different position within the frame; defining an arrangement of the plurality of frames, the arrangement comprising a grid; and The frames are interleaved with each other according to their positions in the grid.

13. The method of claim 12, wherein the first portion is selected to include only a portion of each interlaced frame.

14. The method according to claim 1 or claim 2, wherein the original image is a multi-frame image including a plurality of frames, and wherein the first part of the original image includes one frame of the multi-frame image.

15. The method of claim 7 , wherein the original image is a multi-frame image comprising a plurality of frames, and wherein the first portion of the original image comprises one frame of the multi-frame image, and wherein each additional portion comprises a different frame of the multi-frame image, the method further comprising: Interleave the generated multiple blocks.

16. The method of claim 7, wherein the first portion and / or the further portion is selected to be square.

17. The method according to claim 6, wherein The rotation angle is between 0.1° and 2°.

18. The method according to claim 6, wherein The rotation angle is between 0.1° and 1°.

19. The method according to claim 6, wherein The rotation angle is between 0.1° and 0.6°.

20. A method of producing a printed image for a security feature, the method comprising: Printing of a printed image designed according to any one of claims 1 to 19.

21. A printed image for a security feature, the security feature comprising an array of optical elements overlying the printed image, the printed image comprising: A first layer includes a first block including pixels of a first portion of an original image mirrored in both an x-direction and a y-direction.

22. The printed image of claim 21, wherein the first layer further comprises: One or more further blocks, each further block comprising pixels of a respective further portion of the original image mirrored in both the x-direction and the y-direction.

23. A printed image according to claim 21 or claim 22, further comprising: A second layer includes a second block including pixels of a first portion of the second original image mirrored in both the x-direction and the y-direction.

24. The printed image according to claim 23, wherein the first layer and / or the second layer is rotated relative to the x-direction and the y-direction by a rotation angle, wherein the rotation angle is between 0.1° and 5°.

25. The printed image of claim 24, wherein the rotation angle is between 0.1° and 2°.

26. The printed image of claim 24, wherein the rotation angle is between 0.1° and 1°.

27. The printed image of claim 24, wherein the rotation angle is between 0.1° and 0.6°.

28. A security feature comprising: A printed image as claimed in any one of claims 21 to 27; as well as An array of identical optical elements covering the printed image.

29. A security document comprising a security feature as claimed in claim 28.

30. The security document of claim 29, wherein the security document is one of a banknote, a passport, a driver's license and an identity card.

31. A non-transitory computer-readable medium storing computer-readable instructions which, when executed, cause a machine comprising a processor to perform the method of any one of claims 1 to 19.

Images