WO2011042395A1

WO2011042395A1 - System and method for post-processing an anti-aliased object

Info

Publication number: WO2011042395A1
Application number: PCT/EP2010/064759
Authority: WO
Inventors: Johannes Petrus Van De Kamer
Original assignee: Irex Technologies B.V.; Irx Innovations B.V.
Priority date: 2009-10-05
Filing date: 2010-10-04
Publication date: 2011-04-14

Abstract

The invention relates to a system (10) for post-processing an anti-aliased object (20a). The invention further relates to a display device (100), a method of post-processing and a computer program product. The system is configured for post-processing an anti-aliased object before the anti-aliased object is being rendered on a pixelized screen (30). The object (20) has an object color (g0) and is configured for being rendered in a background having a background color (g15). The anti-aliased object comprises intermediate pixels (24) constituted by the anti-aliasing of the object in which the intermediate pixels comprise an intermediate color (g1, g2..., g14) being intermediate between the object color and the background color. The system is configured for receiving the anti-aliased object and is configured for adapting the intermediate color of a preselected range (R1, R2, R3) of intermediate pixels for enhancing a contrast of the anti-aliased object.

Description

SYSTEM AND METHOD FOR POST-PROCESSING AN ANTI-ALIASED OBJECT

Field of the invention

The invention relates to a system for post-processing an anti-aliased object.

The invention further relates to a display device, a method of postprocessing the anti-aliased object and.

Background of the invention

Pixelated display devices are well known and used for displaying still and moving images. They are inter alia used as monitors for displaying information from computers, games, mobile terminals and other man-machine interfaces. The pixelated display devices are also used as monitors for television applications and advertising bill-boards. For rendering an image on the pixelated display device, an image-rendering engine is used which determines which pixels of the pixelated display device has to emit which color such that the plurality of pixels of the pixelated display devices represent the image to be shown. Such image-rendering engine receives all kinds of different objects having, for example, different object-formats and converts these objects into a single image to be displayed on the pixelated display device.

In rendering the image on the pixelated display device, aliasing has been an issue. Aliasing occurs when a high-resolution image has to be rendered on a pixelated display device having lower resolution. The result is that edges of objects in images, when the edge is drawn at an angle, are displayed with a stair-step effect rather than a smooth line. The known technique of anti-aliasing is used to minimize this stair-step effect. With anti-aliasing, signal components having a higher frequency than the frequency which may be rendered on the pixelated display device are removed such that image may be properly displayed on the screen. In a practical implementation of such anti-aliasing technique, a sharp edge between an object and its surroundings may be softened by introducing a plurality of intermediate pixels having a color somewhere between the color of the object and the color of the background. For example, when having to render a black thick line at a certain angle, on a white background the aliasing effect results in a stair-step line defined by the intersection of the black thick line with individual pixels in the pixelated display device. When anti-aliasing such line, the sharp contrast between the black thick line and the white background is reduced by introducing intermediate pixels having a grey-value somewhere in between the black of the black thick line and the white of the background to blur the edge of the black thick line. Due to this blurring of the edge of the black thick line, no stair-step effect is visible which makes it more pleasant to people to view the object.

When a rendering engine has to render an object meant for rendering at a first size onto a smaller sized display device, the resulting contrast of the object is too poor.

Object and summary of the invention

It is an object of the invention to provide a system for improving the contrast in an anti-aliased object.

A first aspect of the invention relates to system according to claim 1 . A second aspect of the invention relates to a display device according to claim 10. A third aspect of the invention relates to a method according to claim 1 1. A fourth aspect of the invention relates to a computer program produce according to claim 15.

According to the first aspect of the invention, the object is achieved by a system for post-processing an anti-aliased object before the anti-aliased object is being rendered on a pixelized screen. The object has an object color and is configured for being rendered in a background having a background color. The anti- aliased object comprises intermediate pixels constituted by the anti-aliasing of the object. The intermediate pixels comprise an intermediate color being intermediate between the object color and the background color. The system is configured for receiving the anti-aliased object and is configured for adapting the intermediate color of a preselected range of intermediate pixels for enhancing a contrast of the anti- aliased object.

Color in the current interpretation represents a point in an RGB color model and represents, for example, a combination of the hue, saturation and lightness, further also referred to as an HSL representation of color. Alternatively, color represents, for example, hue, saturation and value, further also referred to as HSV of the pixel. Each of these representations of the RGB color model may be used to fully represent a perceived color of a pixel. In a monochrome representation of pixels, the color of the pixel represents different saturation and lightness of the pixels all having the same hue. Consequently, for example, having a black object in a white background or having a white object in a black background, the color of the object and the color of the background both have the same hue and different saturation and/or lightness. In such a representation, the intermediate color represents an intermediate saturation value and/or an intermediate lightness value of the intermediate pixels - or, said differently, the intermediate color represents different grey-values in between the object color and the background color.

Consequently, in a black-and-white pixelized screen, anti-aliasing an object results in intermediate pixels having different levels of grey while the object is constituted of black lines on a white background, or is constituted of white lines on a black or dark- grey background.

An effect of the measures according to the invention is that the number of intermediate levels due to the post-processing by the system is reduced which reduces the width of the blurred line generated by the anti-aliasing process. Due to this reduced width of the blurred line, the contrast is improved. When, for example, the object is text to be rendered on a pixelized display device, the text may comprise stair-step artifacts. Due to the anti-aliasing of the text, the edges of the lines constituting the text are blurred such that the stair-step artifacts are no longer visible. Such anti-aliased text is perceived by a human viewing the text on the pixelated display device as being a high-contrast object. When, however, the text was initially intended to be displayed on, for example, an A4 document size while the pixelated display device has a smaller size, the anti-aliased object has to be rendered on a smaller sized display device than originally anticipated. In such configuration, the dimensions of the text may be further decreased and may merge with the intermediate pixels generated by the anti-aliasing. As a result, the text is not perceived as black text on a white background, but is perceived as a grey text on a white background - and consequently the contrast is reduced due to the displaying of the object on a smaller size than originally intended. The system for postprocessing according to the current invention selects a preselected range of intermediate pixels and adapts the intermediate color of the intermediate pixels within the preselected range to enhance the contrast of the anti-aliased object. The preselected range may, for example, comprise intermediate pixels having an intermediate color which lies within a specific range of intermediate colors. In such an embodiment of the system, pixels having an intermediate color near the object color may be altered such that the intermediate color changed to be equal to the object color or to be closer to the object color. Returning to the previous example in which the object is text, the blurred edges due to the anti-aliasing process are reduced and the dimensions of the text are increased. Furthermore, the postprocessing reduces the merging of the object with the intermediate pixels which causes the text to be perceived grey, which also enhances the contrast.

Alternatively, the preselected range may, for example, comprise pixels which are geometrically positioned within a specific distance from the original object. By adapting the intermediate color of the intermediate pixels located in the vicinity of the original object and by shifting the intermediate color of these intermediate pixels in the preselected range to be closer to the object color or even equal to the object color, again the width of the blurring edge is reduced which improves the contrast of the anti-aliased object. Furthermore, when the object is text of which the text-color after rendering has merged with the intermediate pixels, the shifting of the intermediate color reduces this merging of the text-color and improves the contrast on the pixelated display device.

Especially when the object is a vector based object the system for post-processing the anti-aliased object may be used to beneficially enhance the contrast. Vector based objects are ideal for varying the actual dimensions of the object without losing detail information. For example, the font-size of text being constituted by vector based objects may be varied at will, without losing details in the vector based objects. However, when trying to render such text having a relatively small font-size, anti-aliasing is required to ensure that the rendered image on the pixelated display device does not comprise the stair-step artifacts. However, when text of font-size 12 is to be displayed on a mobile device such as a mobile phone, a laptop, a notebook or an e-reader, the display typically is much smaller. To be able to display a certain amount of text on this smaller display, the text which is originally intended to be displayed in font-size 12 is reduced in size. The anti-aliasing applied to the text was also intended for font-size 12 and by reducing the dimensions of the text such that a certain amount of the text is visible on the screen of the portable device the contrast of the anti-aliased text is reduced significantly, resulting in text appearing grey. Applying the post-processing to the anti-aliased vector based object via the system according to the invention ensures that the rendering of the anti- aliased vector based object still comprises sufficient contrast and ensures that the text is still perceived as black. In an embodiment of the system, the preselected range of

intermediate pixels comprises intermediate pixels comprising an intermediate color within a predefined range of intermediate colors. In such an embodiment

intermediate pixels having an intermediate color which is, for example, close to the object color may, for example, be adapted such that the intermediate color for these pixels corresponds to the object color. Consequently, the dimension of the object color within the post-processed object compared to the anti-aliased object is increased, increasing the contrast. Even in embodiments in which the anti-aliasing has caused the object color to merge with the intermediate colors such that substantially no object color is present anymore in the object - giving the object a grey appearance - the adapting of the pixels having the intermediate color near the object color into pixels having the object color causes to increase the object color within the post-processed object compared to the anti-aliased object which increases the contrast. Because the anti-aliasing effect creates a blurring edge, this selection of the predefined range may be used to reduce the dimensions of the blurring edge which enhances the contrast of the anti-aliased object. Consequently, when imaging an anti-aliased object on a screen which is too small, the post-processing ensures that the contrast visible on this too small screen is still good.

Additionally and/or alternatively, intermediate pixels having an intermediate color which is close to the background color may, for example, be adapted such that the intermediate color for these pixels corresponds to the background color. In such an embodiment, the width of the blurring edge may again be reduced which improves the contrast in of the object in the background.

In an embodiment of the system, the preselected range of intermediate pixels comprises intermediate pixels comprising a predefined geometrical position relative to the anti-aliased object. This predefined geometrical position may, for example, be a position at the edge of the blurring line. Again, the intermediate color of such intermediate pixels may be adapted to enhance the contrast as previously indicated. Changing the intermediate color of the intermediate pixels near the object towards the object color, the amount of object color in the post- processed object is increased compared to the amount of object color in the anti- aliased object which increases the contrast between the post-processed object and its background. Additionally and/or alternatively changing the intermediate color of the intermediate pixels near the background towards the background color, the width of the blurring edge of the post-processed object is reduced compared anti-aliased object which also increases the contrast between the post-processed object and its background.

In an embodiment of the system, the system is configured for postprocessing a text-portion of the anti-aliased object only. Typically, the text-portion of an object comprises vector based objects of which a specific level of contrast is preferred for the readability by humans. Because the system according to the current embodiment is configured for only post-processing the text-portion of the anti-aliased object, the quality and the readability of the text is ensured via the system for postprocessing, not depending on the size of the display device on which the text is displayed. Images which may be part of the object are not altered. The system according to the current embodiment has the further advantage that the processing time for performing the post-processing is reduced because not the full object has to be post-process but only the text-portion.

In an embodiment of the system, the preselected range of intermediate pixels comprising a first preselected range of intermediate pixels having an intermediate color being closer to the object color than to the background color, wherein the adapting of the intermediate color comprises shifting the intermediate color of intermediate pixels of the first preselected range toward the object color for enhancing the object color and improving the contrast. As indicated before, the shifting of the intermediate color of the intermediate pixels more towards the object color causes to increase the object color within the post-processed object compared to the anti-aliased object which increases the contrast. Furthermore, because the anti-aliasing effect creates a blurring edge, this selection of the predefined range may be used to reduce the dimensions of the blurring edge which enhances the contrast of the anti-aliased object. Consequently, when imaging an anti-aliased object on a screen which is too small, the post-processing ensures that the contrast visible on this too small screen is still good.

In an embodiment of the system, the preselected range of intermediate pixels comprising a second preselected range of intermediate pixels having an intermediate color being closer to the background color than to the object color, wherein the adapting of the intermediate color comprises shifting the intermediate color of intermediate pixels of the second preselected range toward the background color for improving the contrast. As indicated before, changing the intermediate color of the intermediate pixels near the background towards the background color, the width of the blurring edge of the anti-aliased object is reduced which also increases the contrast between the post-processed object and its background compared to the anti-aliased object and its background.

In an embodiment of the system, the preselected range of intermediate pixels comprising a third preselected range of intermediate pixels having an intermediate color between the first preselected range of intermediate pixels and the background color, wherein the adapting of the intermediate color comprises shifting the intermediate color of intermediate pixels of the third

preselected range toward the object color, the shifting of the first preselected range of intermediate pixels being larger than the shifting of the third preselected range of intermediate pixels. In the current embodiment the shifting of the intermediate color of the first preselected range is larger than the shifting of the intermediate color of the third preselected range. For example, the intermediate pixels of the first preselected range may all be changed to the object color for enhancing the object color in the anti-aliased object, while the intermediate pixels of the third preselected range may be changed such that each intermediate color is shifted toward the object color but not changed into the object color. As such, both the amount of object color in the post-processed object compared to the anti-aliased object is increased and the width of the blurring edge caused by the anti-aliasing is reduced which both lead to improvement in contrast of the object. Typically, the intermediate pixels comprise different levels of grey, for example, 16 levels of grey between the black text and the white background. Grey-level "0" represents, for example, black, while grey-level "15" represents white. The first preselected range of intermediate pixels may, for example, comprise pixels having grey-levels which range from "1 " to "3", while the third preselected range of intermediate pixels may, for example, comprise pixels having grey-levels which range from "4" to "7". The post-processing according to the invention may require the intermediate pixels of the first preselected range to be shifted in color toward the object color, for example, by 2 grey-levels, while the intermediate pixels of the third preselected range are shifted in color toward the object color, for example, by 1 grey-level. As such, the intermediate pixel which has after anti-aliasing a grey-level "1 " will be converted due to the post-processing into the rendered pixel having grey-level "0" (being the object color - there is no other darker color than black, so no further reduction of the intermediate color is possible), the intermediate pixel which has after anti-aliasing a grey-level "2" will be converted due to the post-processing into the rendered pixel having grey-level "0", and the intermediate pixel which has after anti-aliasing a grey-level "3" will be converted due to the post-processing into the rendered pixel having grey-level "1 ". Furthermore, the intermediate pixels which have after anti-aliasing a grey-level "4", "5" or "6" will be converted due to the post-processing into the rendered pixel having grey-levels "3", "4" or "5", respectively.

In an embodiment of the system, the adapting of the intermediate color of the intermediate pixels comprises adapting the intermediate color according to a color-transformation map defining for every intermediate color an adapted intermediate color. In the previous examples, the adapting of the intermediate color is done via step-wise altering of the intermediate color. However, often in color conversion, so called color-transformation maps are used. Known color- transformation maps are, for example, used for gamma-correction in which deviations of the display device are corrected such that the correct color is displayed. These color-transformation maps may be defined by a graphical representation of the color-transformation, for example, using a functional description between the intermediate color and the adapted intermediate color. For example, an S-shaped curve may be used to define how the intermediate color should be adapted to obtain the adapted color. Such an S-shaped curve may, for example, shift the intermediate color of intermediate pixels having the intermediate color near the object color more towards the object color, shift the intermediate color of intermediate pixels having the intermediate color near the background color more towards the background color, and maintain the intermediate color of the remainder of the pixels substantially unaltered. The use of such a graphical representation of the color-transformation map has the advantage that it may be altered using only a few parameters for altering the shape of the curve defining the color-transformation. Consequently, the manual adaptation of the post-processing may be done via only a few parameters. Alternatively, the color-transformation maps may be defined by a look-up-table providing the adapted intermediate color for every intermediate color of the anti- aliased object.

In an embodiment of the system, the system is configured for receiving a trigger for initiating the post-processing of the anti-aliased object, the trigger comprising a zooming-trigger for reducing a rendering size of the anti-aliased object. The zooming-trigger may be generated by a user wishing, for example, to have a complete overview of the document at hand. In such a situation, the contrast of the text should preferably be relatively high, while the contrast in images in the document may not need to be altered. Consequently, by post-processing the anti- aliased object, the object may appear at high contrast to a viewer, even while zoomed to generate an overview of the object.

In an embodiment of the system, the system is configured for receiving a trigger for initiating the post-processing of the anti-aliased object, the trigger comprising a manual-trigger for enhancing the contrast in the rendered anti- aliased object. The manual trigger may, for example, be a result of the image preferences as provided by the user to the display device.

In an embodiment of the system, the system is configured for receiving a trigger for initiating the post-processing of the anti-aliased object, the trigger comprising a rescale-trigger for rendering the anti-aliased object on a rendering-surface-dimension of the pixelized screen being smaller than an original- surface-dimension of the anti-aliased object. This trigger may be generated by the display device in that the display device receives an object which is intended to be displayed at a certain dimension but which, for example, does not match the screen- size of the display device. This mismatch may be used as a trigger for initiating the post-processing such that the anti-aliased object, when viewed on the smaller screen-size of the display device, still has sufficient contrast. Such a situation may occur, for example, when using a 5 inch e-reader which displays a document in pdf format which was intended for an A4-document size. The e-reader, being the display device, will notice the mismatch between the pdf document size and the e-reader display size and will start post-processing the anti-aliased pdf document to ensure that the pdf document displayed on the e-reader has sufficient contrast.

In an embodiment of the system, the object color is black, the background color is white or the other way around and the intermediate color comprises a plurality of grey-values.

In the previous part of the document, mainly black objects on a white background are used to explain the principle of the post-processing. It will be directly apparent to the person skilled in the art that the similar examples as shown above may use different colored objects in differently colored background achieving improved contrast without departing from the scope of the invention. For example, blue objects in a white background will cause different shades of blue in the intermediate pixels due to the anti-aliasing. Alternatively, using white objects or yellow objects in a blue background will generate different shade of blue or differently colored intermediate pixels. Still, the post-processing as explained above will lead to improved contrast when rendering an object intended for a relatively large display on a relatively small display.

According to the second aspect of the invention, the object is achieved with a display device comprising a pixelized screen for rendering an object and comprising the system according to the invention.

According to the third aspect of the invention, the object is achieved with a method of post-processing an anti-aliased object before the anti-aliased object is being rendered on a pixelized screen. The method according to the invention comprises the steps of:

- receiving the anti-aliased object,

- selecting a preselected range of intermediate pixels,

- adapting the intermediate color of intermediate pixels of the preselected range for enhancing a contrast of the anti-aliased object, and

- providing the post-processed object to be rendered on the pixelized

screen.

This method may, for example, be performed in a microprocessor which is available in the display device and which determines the rendering of the object on the screen of the display device. Alternatively, the method may be performed in a

microprocessor which is part of the operating system of a computer, for example, a microprocessor which is used to generate the output signal to the display device for creating the object on the display.

In an embodiment of the method, the step of selecting the preselected range comprises selecting a first preselected range of intermediate pixels having an intermediate color being closer to the object color than to the background color, and wherein the step of adapting the intermediate color comprises shifting the

intermediate color of intermediate pixels of the first preselected range toward the object color for enhancing the object color and improving the contrast. As indicated before, by shifting the intermediate color of the first preselected range of

intermediate pixels toward the object color, an increase of the object color in the post-processed object compared with the anti-aliased object occurs which increases the contrast. Furthermore, the dimensions of the blurring edge are reduced which enhances the contrast of the anti-aliased object. In an embodiment of the method, the step of selecting the preselected range comprises selecting a second preselected range of intermediate pixels having an intermediate color being closer to the background color than to the object color, and wherein the step of adapting the intermediate color comprises shifting the intermediate color of intermediate pixels of the second preselected range toward the background color for improving the contrast. By changing the intermediate color of the intermediate pixels near the background towards the background color, the width of the blurring edge of the post-processed object compared to the anti-aliased object is reduced which also increases the contrast between the anti-aliased object and its background.

In an embodiment of the method, the step of adapting the intermediate color comprises adapting the intermediate color according to a color- transformation map defining for every intermediate color an adapted intermediate color.

According to the fourth aspect of the invention, the object is achieved with a computer program product comprising instructions for causing a processor system to perform the method according to the invention.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Brief description of the drawings

In the drawings:

Fig. 1 is a schematic representation of the display device comprising the system according to the invention,

Figs. 2A and 2B are graphs comprising color-transformation maps indicating the shifting of the intermediate color of the intermediate pixels in the preselected ranges, and

Fig. 3 is a flow-diagram showing steps to be performed for performing the method of post-processing according to the invention.

The figures are purely diagrammatic and not drawn to scale.

Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the figures are denoted by the same reference numerals as much as possible. Detailed description

Fig. 1 is a schematic representation of the display device 100 comprising the system 10 according to the invention. The system 10 is configured for post-processing an anti-aliased object 20a before the anti-aliased object 20a is being rendered on a pixelized screen 30.

Rendering the image on the pixelated screen 30 of a display device 100, aliasing has been used to reduce for example, the stair-step effect as shown in the detail indicated with reference number 22 in Fig. 1. Aliasing occurs when a high- resolution image has to be rendered on a pixelated screen 30 having lower resolution. The result is that part of the object 20 is displayed with a stair-step effect rather than a smooth line. Often, display devices 100 comprise an anti-aliasing system 40 for converting the object 20 into an anti-aliased object 20a. In such anti- aliased object 20a, for example, sharp edges between the object 20 and its background may be softened by introducing a plurality of intermediate pixels 24 having an intermediate color g1 , g2,..., g14 (see Figs. 2A and 2B) somewhere between the object color gO of the object 20 and the background color g15 of the background.

A first part of Fig. 1 shows the rendering of a black thick line 23 at a certain angle, on a white background the aliasing effect results in a stair-step line as shown in the detail 22. The stair-step line is defined by the intersection of the black thick line with individual pixels in the pixelated screen 30. When anti-aliasing such line 23 to produce the anti-aliased object 20a via an anti-aliasing system 40, the sharp contrast between the black thick line 23 and the white background is reduced by introducing intermediate pixels 24 having a grey-value g1 , g2, g14 somewhere in between the black gO of the black thick line 23 and the white g15 of the

background to blur the edge of the black thick line 23 - as shown in the detail 22a. Due to this blurring of the edge of the black thick line 23, no stair-step effect is visible on the pixelized screen 30 when rendering the anti-aliased object 20a which makes it more pleasant to people to view.

The display device 100 according to the invention comprises the system 10 according to the invention. The system 10 is configured for receiving the anti-aliased object 20a and is configured for adapting the intermediate color g1 , g2, enhancing a contrast of the anti-aliased object 20a. Due to the post-processing by the system 10 the number of intermediate levels is reduced which reduces the width of the blurred line (see detail 22a in Fig. 1 ) generated by the anti-aliasing process. Due to this reduced width of the blurred line, the contrast is improved.

Especially when the object 20 is a vector based object 20 such as characters in text, the system 10 for post-processing the anti-aliased object 20a may be used to beneficially enhance the contrast. Vector based objects 20 are ideal for varying the actual dimensions of the object 20 without losing detail information. For example, the font-size of text 20 being constituted by vector based objects may be varied at will, without losing details in the vector based objects 20. However, when trying to render such text having a relatively small font-size, anti-aliasing is required to ensure that the rendered image on the pixelated screen 30 does not comprise the stair-step artifacts. However, when text of font-size 12 is to be displayed on, for example, a mobile device such as a mobile phone, a laptop, a notebook or an e- reader, the pixelated screen 30 typically is much smaller. To still be able to display a certain amount of text 20 on this smaller pixelated screen 30, the text 20 which is originally intended to be displayed in font-size 12 is reduced in size. The anti-aliasing applied to the text 20 was also intended for font-size 12 and by reducing the dimensions of the text 20 such that a certain amount of the text 20 is visible on the pixelated screen 30 of the portable device, the contrast of the anti-aliased text 20a is reduced significantly, resulting in the anti-aliased text 20a rendered on the pixelated screen 30 to appear grey. By applying the post-processing to the anti-aliased vector based object 20a via the system 10 according to the invention the contrast in the anti-aliased text 20a is improved which ensures that the rendered text still is perceived as black, even when displayed on a pixelated screen 30 which is smaller than the screen on which the anti-aliased object 20a was intended to be displayed.

The preselected range R1 , R2, R3 (see Fig. 2A) of intermediate pixels 24 may, for example, comprise intermediate pixels comprising an intermediate color g1 , g2, g14 within a predefined range R1 , R2, R3 of intermediate colors g1 , g2, g14. The preselected range R1 , R2, R3 of intermediate pixels 24 may, for example, comprise intermediate pixels comprising a predefined geometrical position relative to the anti-aliased object 20a. This predefined geometrical position may, for example, be a position at the edge of the blurring line or, for example, in the center of the anti-aliased object 20a or anti-aliased line 20a. Again, the intermediate color g1 , g2, g14 of such intermediate pixels 24 may be adapted to enhance the contrast. Changing, for example, the intermediate color g1 , g2, g14 of the intermediate pixels 24 near the object 20 towards the object color gO, the amount of object color gO in the post-processed object 20p is increased compared to the amount of object color gO in the anti-aliased object 20a which increases the contrast between the post-processed object 20p and its background. Additionally and/or alternatively changing the intermediate color g1 , g2, g14 of the intermediate pixels 24 near the background towards the background color g15, the width of the blurring edge of the post-processed object 20p is reduced compared anti-aliased object 20a which also increases the contrast between the post-processed object 20p and its background.

The preselected range R1 , R2, R3 may also comprise a first preselected range R1 (see Fig. 2A) of intermediate pixels 24 having a first range of intermediate colors g1 , g2, g3 being closer to the object color gO than to the background color g15. The adapting of the intermediate color g1 , g2, g3 of the first range of intermediate colors g1 , g2, g3 may, for example, comprise shifting the intermediate color g1 , g2, g3 of intermediate pixels 24 of the first preselected range R1 toward the object color gO for enhancing the object color gO and improving the contrast. The preselected range R1 , R2, R3 may also comprise a second

preselected range R2 (see Fig. 2A) of intermediate pixels 24 having an intermediate color g12, g13, g14 being closer to the background color g15 than to the object color gO. The adapting of the intermediate color g12, g13, g14 comprises shifting the intermediate color g12, g13, g14 of the intermediate pixels 24 of the second preselected range R2 toward the background color g15 for improving the contrast. the preselected range R1 , R2, R3 may also comprise a third preselected range R3 (see Fig. 2A) of intermediate pixels 24 having an intermediate color g4, g5, g6, g7 between the first preselected range R1 of intermediate pixels 24 and the background color g15. The adapting of the intermediate color g4, g5, g6, g7 of intermediate pixels 24 of the third preselected range R3 may be different compared to the adapting of the intermediate color g1 , g2, g3 of intermediate pixels 24 of the first preselected range R1. Of course, also further preselected ranges (not shown) may be present within which the intermediate color gl , g2, g14 is adapted to enhance the contrast of the anti-aliased object 20a to generate the post-processed object 20p.

Alternatively, the intermediate color may be adapted according to a color-transformation map 50, 52 (see Figs. 2A and 2B). This color-transformation map 50, 52 may be a step-wise color-transformation map 50 comprising different preselected ranges R1 , R2, R3 within which the adapting of the intermediate color g1 , g2, g14 is defined for the specific preselected range R1 ; R2; R3 (as is shown in Fig. 2A) or may be a continuous color-transformation map 52 in which a continuous curve 52 defines for every intermediate color g1 , g2, g14 an adapted intermediate color gal , ga2, ga14. The color-transformation may also be defined by a look-up-table (not shown) providing the adapted intermediate color gal , ga2, ga14 for every intermediate color g1 , g2, g14 of the anti-aliased object 20a.

The system 10 according to the invention may be configured for receiving a trigger 12 for initiating the post-processing of the anti-aliased object 20a. The trigger 12 may, for example, comprise a zooming-trigger 12 for reducing a rendering size of the anti-aliased object 20a. The zooming-trigger 12 may be generated by a user wishing, for example, to have a complete overview of the document at hand. Alternatively, the trigger 12 may, for example, comprise a manual-trigger 12 for enhancing the contrast in the rendered anti-aliased object 20a. The trigger may also comprise, for example, a rescale-trigger 12 for rendering the anti-aliased object 20a on a rendering-surface-dimension of the pixelized screen 30 being smaller than an original-surface-dimension of the anti-aliased object 20a. This trigger 12 may be generated by the display device 100 in that the display device 100 receives an object 20 which is intended to be displayed at a certain dimension but which, for example, does not match the screen-size of the pixelized screen 30 display device 100. The mismatch may be used as the trigger 12 for initiating the post-processing by the system 10 such that the anti-aliased object 20a, when viewed on the smaller screen-size of the display device 100, still has sufficient contrast. Such a situation may occur, for example, when using a 5 inch e-reader 100 as display device 100 which displays a document 20 in pdf format which was intended for an A4-document size. The e-reader 100 will notice the mismatch between the pdf document size 20 and the e-reader display size 30 and will start post-processing the anti-aliased pdf document 20a via the system 10 to ensure that the post-processed pdf document 20p has sufficient contrast when rendered on the e-reader 100. The mismatch may be quantified by having a predetermined reduction of pixels between the intended size of the object and the reduced size of the object on the pixelized screen 30. For example, when less than 80% of the pixels are still available to render the anti-aliased object 20a on the screen 30 compared to the intended number of pixels of the anti-aliased object 20a, the trigger 12 is generated for activating the post-processing of the system 10 to generate the post-processed object 20p having enhanced contrast.

Figs. 2A and 2B are graphs comprising color-transformation maps 50, 52 indicating the shifting of the intermediate color g1 , g2, g14 of the intermediate pixels 24 in the preselected ranges. On the horizontal axis of the graph, the color range available on the display device 100 are indicated in which each color is coded via a grey-value, ranging from black gO via grey g1 , g2, g14 to white g15. Of course, instead of black, grey and white also other colors may be used. On the vertical axis of the graph, the adapted color range gaO, ga15 is indicated. In the current example, also the adapted color range gaO, g15 ranges from black gaO via grey gal , ga2,..., ga14 to white ga15. Of course, also other colors may be used, even colors different from the intermediate color range gO, g15.

Fig. 2A shows a color-transformation map 50 in which the adapting of the intermediate color gO, g15 is done in predefined steps related to specific preselected ranges R1 , R2, R3. In Fig. 2A, a first preselected range R1 of intermediate pixels 24 comprises intermediate pixels 24 having an intermediate color g1 , g2, g3 being close to the object color gO. The step-size in the current

embodiment in the shifting of the intermediate color g1 , g2, g3 of the first preselected range R1 is chosen to result in two grey-values toward the object color gO. A second preselected range R2 of intermediate pixels 24 comprises intermediate pixels 24 having an intermediate color g12, g13, g14 being close to the background color g15. The step-size in the current embodiment in the shifting of the intermediate color g12, g13, g14 of the second preselected range R2 is chosen to result in one grey-value toward the background color g15. A third preselected range R3 of intermediate pixels 24 comprises intermediate pixels 24 having an intermediate color g4, g5, g6, g7 being in between the first preselected range R1 and the second preselected range R2. The step-size in the current embodiment in the shifting of the intermediate color g4, g5, g6, g7 of the third preselected range R2 is chosen to result in one grey-value toward the object color gO. As a result, both the amount of object color gO in the post-processed object 20p compared to the anti-aliased object 20a is increased and the width of the blurring edge caused by the anti-aliasing is reduced which both lead to improvement in contrast of the object 20.

When the object is black text 20 on a white background, the intermediate pixels 24 comprise different levels of grey, for example, 16 levels between the black text 20 and the white background. Grey-level "0" represents, for example, black gO, while grey-level "15" represents white g15. The first preselected range R1 of intermediate pixels 24 may, for example, comprise pixels having grey- levels which range from "1 " to "3" (indicated in Fig. 2A with g1 , g2, g3), while the third preselected range R3 of intermediate pixels 24 may, for example, comprise pixels having grey-levels which range from "4" to "7" (indicated in Fig. 2A with g4, g5, g6, g7). The post-processing according to the invention may require the intermediate pixels 24 of the first preselected range R1 to be shifted in color toward the object color gO by two grey-levels, while the intermediate pixels 24 of the third preselected range R3 are shifted in color toward the object color gO by one grey-level. As such, the intermediate pixel 24 which has after anti-aliasing a grey-level "1 " g1 will be converted due to the post-processing into the rendered pixel having grey-level "0" gaO (being the object color gaO - there is no other darker color than black gaO, so no further reduction of the intermediate color is possible), the intermediate pixel 24 which has after anti-aliasing a grey-level "2" g2 will be converted due to the postprocessing into the rendered pixel having grey-level "0" gaO, and the intermediate pixel 24 which has after anti-aliasing a grey-level "3" g3 will be converted due to the post-processing into the rendered pixel having grey-level "1 " gal . Furthermore, the intermediate pixels which have after anti-aliasing a grey-level "4", "5" or "6" g4, g5, g6 will be converted due to the post-processing into the rendered pixel having grey- levels "3", "4" or "5" ga3, ga4, ga5, respectively. The adapting of the color is illustrated for the intermediate color g5 being converted into the adapted

intermediate color ga4 via the color-transformation map 50 via the dash-dotted arrow in Fig. 2A.

Fig. 2B shows a color-transformation map 52 in which the adapting of the intermediate color gO, g15 is done via a smooth curve defining the required color transformation from the intermediate colors g1 , g2, g14 of the anti-aliased object 20a into the adapted intermediate colors gaO, ga15 of the post-processed object 20p. Using the continuous color-transformation map 52, the preselected range corresponds to the whole range of colors gO, ... g15. Because typically only an integer range of well defined adapted intermediate colors gaO, ga15 are available, the adapted intermediate color gaO, gal 5 resulting from the

transformation using the color-transformation map 52 as shown in Fig. 2B is found by rounding the adapted intermediate color gO, g15 to the closest integer adapted intermediate color gO, g15. Fig. 2B shows an S-shaped curved which may be used to define how the intermediate color g1 , g14 is to be adapted to obtain the adapted intermediate color gaO, ga15. Such an S-shaped curve may, for example, shift the intermediate color g1 , g7 of intermediate pixels 24 having the intermediate color gl , g7 near the object color gO more towards the object color gO, shift the intermediate color g9, g14 of intermediate pixels 24 having the intermediate color g9, g14 near the background color g15 more towards the background color gl 5, and maintain the intermediate color g8 of the remainder of the intermediate pixels 24 substantially unaltered. The adapting of the color is illustrated for the intermediate color g5 being converted into the adapted intermediate color ga2 via the color-transformation map 52 via the dash-dotted arrow in Fig. 2B.

These color-transformation maps 50, 52 may be defined by a graphical representation of the color-transformation as shown in Figs. 2A and 2B, for example, using a functional description between the intermediate color gl , g2, g14 and the adapted intermediate color gal , ga2, ga14. Alternatively, the color- transformation may also be defined by a look-up-table (not shown) providing the adapted intermediate color gal , ga2, ga14 for every intermediate color g1 , g2, g14 of the anti-aliased object 20a.

Fig. 3 is a flow-diagram showing steps to be performed for performing the method of post-processing according to the invention. The method of postprocessing starts with a step "start method of post-processing" 200. The starting of the post-processing may be initiated via a trigger 220 which may be a manual trigger 222 or an automated trigger 224. The manual trigger 222 may result from a request of a user to improve the contrast in the rendered image. The automated trigger 224 may result from a rendering of the anti-aliased object 20a on a pixelized screen 30 which is smaller than a surface for which the anti-aliased object 20a was intended. Such automated trigger 224 may result from using fewer pixels to render the anti- aliased object 20a, for example, when using less than 80% of the pixels which were initially intended for the anti-aliased object 20a, the post-processing may be initiated to ensure sufficient contrast. The method continues by receiving the anti-aliased object 20a in the step of "receiving anti-aliased object" 210. Subsequently the range of intermediate pixels 24 which have to be adapted by the post-processing are selected in the step "selecting preselected range" 230. In a specific example, only the text-portion of the anti-aliased object 20a has to be post-processed and thus an optional step of "select text-portion of anti-aliased object" 240 may be present to reduce the overall post-processing time by only applying the post-processing to the part of the anti-aliased object 20a which contains the text. Next the color

transformation is performed in the step "adapt intermediate color according to generate post-processed object" 250. Input for the generation of the post-processed object 20p may be a transformation map 252 and/or a look-up table 254.

Subsequently the post-processed object 20p is rendered on the pixelized screen 30 in the step "render post-processed object on the pixelized screen" 260. Next, the method checks whether there may be additional anti-aliased objects 20a to be post- processed in the step "Another object to be rendered?" 270. If yes "Y", the method returns to step "receiving anti-aliased object" 210 after which the post-processing of the additional anti-aliased object 20a continues. If no "N" the method stops.

It should be noted that items which have the same reference numbers in different Figures, have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1 . A system (10) for post-processing an anti-aliased object (20a) before the anti- aliased object (20a) is being rendered on a pixelized screen (30), the object (20) having an object color (gO) and being configured for being rendered in a background having a background color (g15), the anti-aliased object (20a) comprising intermediate pixels (24) constituted by the anti-aliasing of the object (20), the intermediate pixels (24) comprising an intermediate color (g1 , g2..., g14) being intermediate between the object color (gO) and the background color (g15), the system (10) being configured for receiving the anti-aliased object (20a) and being configured for adapting the intermediate color (g1 , g2..., g14) of a preselected range (R1 , R2, R3) of intermediate pixels (24) for enhancing a contrast of the anti-aliased object (20a). 2. The system (10) as claimed in claim 1 , wherein the preselected range (R1 ,

R2, R3) of intermediate pixels (24) comprises intermediate pixels (24) comprising an intermediate color (g1 , g2..., g14) within a predefined range of intermediate colors (g1 , g2..., g14), and/or comprises intermediate pixels (24) comprising a predefined geometrical position relative to the anti-aliased object (20a).

3. The system (10) as claimed in claim 1 or 2, wherein the system (10) is

configured for post-processing a text-portion of the anti-aliased object only. 4. The system (10) as claimed in claim 1 , 2 or 3, the preselected range (R1 , R2,

R3) of intermediate pixels (24) comprising a first preselected range (R1 ) of intermediate pixels (24) having an intermediate color (g1 , g2..., g14) being closer to the object color (gO) than to the background color (g15), wherein the adapting of the intermediate color (g1 , g2..., g14) comprises shifting the intermediate color (g1 , g2..., g14) of intermediate pixels (24) of the first preselected range (R1 ) toward the object color (gO) for enhancing the object color (gO) and improving the contrast.

5. The system (10) as claimed in claim 1 , 2, 3 or 4, the preselected range (R1 , R2, R3) of intermediate pixels (24) comprising a second preselected range (R2) of intermediate pixels (24) having an intermediate color (g1 , g2..., g14) being closer to the background color (g15) than to the object color (gO), wherein the adapting of the intermediate color (g1 , g2..., g14) comprises shifting the intermediate color (g1 , g2..., g14) of intermediate pixels (24) of the second preselected range (R2) toward the background color (g15) for improving the contrast.

The system (10) as claimed in claim 4, the preselected range (R1 , R2, R3) of intermediate pixels (24) comprising a third preselected range (R3) of intermediate pixels (24) having an intermediate color (g1 , g2..., g14) between the first preselected range (R1 ) of intermediate pixels (24) and the background color (g15), wherein the adapting of the intermediate color (g1 , g2..., g14) comprises shifting the intermediate color (g1 , g2..., g14) of intermediate pixels (24) of the third preselected range (R3) toward the object color (gO), the shifting of the first preselected range (R1 ) of intermediate pixels (24) being larger than the shifting of the third preselected range (R3) of intermediate pixels (24).

The system (10) as claimed in claim 1 , 2 or 3, wherein the adapting of the intermediate color (g1 , g2..., g14) of the intermediate pixels comprises adapting the intermediate color (g1 , g2..., g14) according to a color- transformation map (50, 52) defining for every intermediate color (g1 , g2..., g14) an adapted intermediate color (gal , ga2..., ga14).

The system (10) as claimed in any of the previous claims, wherein the system (10) is configured for receiving a trigger (12) for initiating the post-processing of the anti-aliased object (20a), the trigger (12) comprising:

- a zooming-trigger (12) for reducing a rendering size of the anti-aliased object (20a), and/or

- a manual-trigger (12) for enhancing the contrast in the rendered anti- aliased object (20a), and/or

- a rescale-trigger (12) for rendering the anti-aliased object (20a) on a

rendering-surface-dimension of the pixelized screen (30) being smaller than an original-surface-dimension of the anti-aliased object (20a).

9. The system (10) as claimed in any of the previous claims, wherein the object color (gO) is black, the background color (g15) is white or the other way around, and the intermediate color (g1 , g2..., g14) comprises a plurality of grey-values.

10. A display device (100) comprising a pixelized screen (30) for rendering an object (20) and comprising the system (10) according to any of the claims 1 to 9.

1 1 . A method of post-processing an anti-aliased object (20a) before the anti- aliased object (20a) is being rendered on a pixelized screen (30), the object (20) having an object color (gO) and being configured for being rendered in a background having a background color (g15), the anti-aliased object (20a) comprising intermediate pixels (24) constituted by the anti-aliasing of the object (20), the intermediate pixels (24) comprising an intermediate color (g1 , g2..., g14) being intermediate between the object color (gO) and the background color (g15), the method comprising the steps of:

- receiving (210) the anti-aliased object (20a),

- selecting (230) a preselected range (R1 , R2, R3) of intermediate pixels (24),

- adapting the intermediate color (g1 , g2..., g14) of intermediate pixels (24) of the preselected range (R1 , R2, R3) for enhancing a contrast of the anti- aliased object (20a), and

- providing the post-processed object (20p) to be rendered on the pixelized screen (30).

12. The method according to claim 1 1 , wherein the step of selecting the

preselected range (R1 , R2, R3) comprises selecting a first preselected range (R1 ) of intermediate pixels (24) having an intermediate color (g1 , g2..., g14) being closer to the object color (gO) than to the background color (g15), and wherein the step of adapting the intermediate color (g1 , g2..., g14) comprises shifting the intermediate color (g1 , g2..., g14) of intermediate pixels (24) of the first preselected range (R1 ) toward the object color (gO) for enhancing the object color (gO) and improving the contrast.

13. The method according to claim 1 1 or 12, wherein the step of selecting the preselected range (R1 , R2, R3) comprises selecting a second preselected range (R2) of intermediate pixels (24) having an intermediate color (g1 , g2..., g14) being closer to the background color (g15) than to the object color (gO), and wherein the step of adapting the intermediate color (g1 , g2..., g14) comprises shifting the intermediate color (g1 , g2..., g14) of intermediate pixels (24) of the second preselected range (R2) toward the background color

(G15) for improving the contrast.

14. The method according to claim 1 1 , wherein the step of adapting the

intermediate color (g1 , g2..., g14) comprises adapting the intermediate color (g1 , g2..., g14) according to a color-transformation map (50, 52) defining for every intermediate color (g1 , g2..., g14) an adapted intermediate color (gal , ga2..., ga14).

15. A computer program product comprising instructions for causing a processor system to perform the method according to any of the claims 1 1 to 14.