EP0635155B1

EP0635155B1 - Process for producing shaded color images on display screens

Info

Publication number: EP0635155B1
Application number: EP93912159A
Authority: EP
Inventors: Robin S. Han
Original assignee: Cirrus Logic Inc
Current assignee: Cirrus Logic Inc
Priority date: 1992-04-07
Filing date: 1993-04-07
Publication date: 1998-06-17
Anticipated expiration: 2013-04-07
Also published as: DE69319236T2; EP0635155A1; JPH07508839A; JP2727029B2; DE69319236D1; WO1993020549A1; EP0635155A4; ATE167586T1; TW225079B

Abstract

A process for producing a wide range of colors in images that are presented in successive frames on image fields (13) on color opto-electronic display means having separate illumination elements each of a different color at each pixel location. Each pixel location, for example, may have a red, a green and a blue illumination element. The display is divided into uniformly-sized display neighborhoods (17). The process includes, for a given pixel location (a-p) within any one of the uniformly-sized display neighborhoods, producing a given color at that pixel location by selecting a frame sequence for illuminating the illumination elements at the pixel location during presentation of an image wherein the number of times that any given illumination element at the pixel location is illuminated within a given frame sequence is controlled to create an appearance of color shading of that pixel location relative to other pixel locations and wherein adjacent pixel locations that have the same color within any one of the display neighborhoods have their illumination elements illuminated with different frame sequences to minimize display noise.

Description

BACKGROUND OF THE INVENTION Field of the Invention:

The present invention generally relates to processes for providing multi-color images on opto-electronic display devices; more particularly, the present invention relates to processes for producing multi-color shaded images in successive frames of video information on opto-electronic display devices such as flat-panel LCDs (liquid crystal diodes) and similar display devices.

State of the Art:

In recent years, the computer industry has given significant attention to laptop computer components and, more particularly, to providing laptop computer components with the same functionality as desktop computers. One particular challenge has been the opto-electronic displays, such as flat-panel LCDs (liquid crystal diodes) and similar display devices that are employed with laptop computers.

LCDs and other flat-panel display devices differ from CRT devices in two important aspects. First, in operation of a CRT device, an electron beam is driven to scan rapidly back and forth across a screen to sequentially energize selected picture-element or "pixel" locations along horizontal scanning lines; the net effect of a complete raster of scans is to reproduce snapshot-like "frames" that each contain video data as to the state of each pixel location on each scanning line. The horizontal scanning lines are organized by synchronizing signals, with each frame containing a fixed number of horizontal lines. The frames are reproduced at a standard rate; for example, the frame repetition rate might be sixty frames per second.

In operation of LCDs and similar flat-panel display devices, there is no back and forth scanning of an electron beam -- in fact, there is no electron beam. Instead, such display devices employ arrays of shift registers, with the result that locations anywhere on a screen can be illuminated simultaneously -- i.e., at exactly the same instant. Nevertheless, in flat-panel display devices as in CRT devices that are employed with microprocessor-based computers, video information is still presented in frames. Each frame normally comprises a field which is 640 pixel locations wide by 480 pixel locations high, and the typical frame repetition rate is sixty frames per second (i.e., 60 hertz).

Also, LCDs and similar flat-panel display screens differ from CRT devices in that the illumination intensity (i.e., brightness) at the pixel locations cannot be varied. Instead, the illumination intensity at pixel locations on a flat-panel display screen is either "on" or "off". (For present purposes, a pixel location will be considered "on" when the pixel location is illuminated, and, conversely, a pixel location will be considered "off" when it is not illuminated.) Thus, when a flat-panel display screen is fully illuminated -- that is, each pixel location is in its "on" state -- the screen will have uniform brightness. (In the following, the term "binary display device" refers to display devices whose picture elements have only two display states -- either an "on" or an "off" state.)

Because pixel locations on flat-panel display screens only have an "on" or "off" state, shading effects cannot be directly produced for images that appear on the screens. To overcome this problem, frame modulation techniques have been employed for simulating grey scale shading of images on binary display devices. Frame modulation techniques basically employ the principle that the frequency with which a pixel location is illuminated determines its perceived brightness and, therefore, its perceived shading. For example, to display a 25% black tone using simple frame modulation, a display element is made active (or inactive) in one-quarter of the frames; similarly, to display a tone of 75% black, a display element would be made active (or inactive) in three-quarters of the frames. Thus, frame modulation techniques are based upon the principle that, for a picture element having only an active state and an inactive state, when the picture element is made active (or inactive) in a certain fraction of successive frames occurring within a short period of time, the human eye will perceive the picture element as having a tone which is intermediate to tones that are presented when the display elements were constantly active (or constantly inactive). The intermediate tones are determined by the percentage of frames in which the display element is active (or inactive). Accordingly, when modulation is performed over a sixteen-frame period, then sixteen different tones are simulated.

In summary, it can be said that frame modulation techniques take advantage of persistence and averaging properties of human vision according to which a display element turned on and off at a sufficiently rapid rate is perceived as being continually on and as having a display intensity proportional to the on/off duty cycle of the display element. In conventional practice, frame modulation techniques for producing shading on binary display devices tend to create displays in which the human eye detects considerable turbulence or "display noise".

One such frame modulation technique is disclosed in WO-A-90/12388.

SUMMARY OF THE INVENTION

The present invention, generally speaking, relates to processes for producing color shading in multi-color images that are presented in successive frames of video information on flat-panel LCD (liquid crystal diode) displays and similar binary display devices. More particularly, the present invention, as it is defined in

independent Claims

1 and 10, provides a method for simulating various color shades in images on a display device that has an array of picture elements each having only two display states, an "on" state and an "off" state, and a corresponding display device.

In the preferred embodiment, the present invention provides a process for producing shading in multi-color images that are presented in successive frames of video information on flat-panel LCD (liquid crystal diode) displays and similar binary display devices while reducing display noise to a minimum. Each pixel location includes illumination elements each of a different color, for example, red, green and blue illumination elements. The method of the present invention is accomplished by modulating an on/off duty cycle of one or more illumination elements of each picture element of the array of picture elements during a multi-frame display sequence according to attribute information of respective picture element data to be displayed. The timing of on/off and off/on state transitions of the illumination elements is coordinated within predetermined neighborhoods throughout the array of picture elements such that the state transitions occur substantially uniformly in space and time within a display neighborhood during the multi-frame display sequence. Accordingly, the present invention takes further advantage of the visual averaging property by causing state transitions to occur substantially uniformly in space and time within each neighborhood throughout the array of picture elements during a multi-frame display sequence. In use of the present invention, no individual state transitions, which by themselves constitute only display noise, are perceived; instead, a coherent pattern of state transitions blending is seen that effectively simulates non-monochrome image displays.

The process of the present invention can be employed such that any pixel location can have any one of 4056 different color shades. This is accomplished even though each illumination element at each pixel location can have one of two states (i.e., either "on" or "off") at any given instant, and, therefore, each pixel location can have any one of eight colors (i.e., 2³ colors) at any given instant.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be further understood with reference to the following description in conjunction with the appended drawings. In the drawings:

Figure 1 is a pictorial representation of a display screen having an image field;

Figure 2A shows a display neighborhood of the image field of the display screen of Figure 1, with the display neighborhood being drawn to a highly enlarged scale for purpose of convenience in describing the process of the present invention;

Figure 2B shows in greater detail the display neighborhood of Figure 2A, in particular showing the different illumination elements included in each picture element;

Figure 3 shows an example of a look-up table for determining an entire frame modulation sequence for each of a number of display tones within a display neighborhood as in Figure 2; and

Figure 4 shows the display neighborhood of Figure 2 and a preferred pixel transition order within each neighborhood according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows an image field 13 that appears on the display screen of a flat-panel LCD or similar binary display device. These display devices, as mentioned above, are characterized by the fact that their pixel locations have only two display states -- that is, the pixel locations are either illuminated or are not illuminated. To produce shading in images that are presented in successive frames of video information on such display screens, the image field is subdivided into two-dimensional, uniformly-sized display neighborhoods, such as will be discussed below in conjunction with Figures 2-4.

For convenience of discussion, the display neighborhood 17 in Figure 2A is shown to be four pixels wide by four pixels high; in other words, display neighborhood 17 is a square that encompasses sixteen pixel locations. Also for convenience of discussion, the sixteen pixel locations in display neighborhood 17 are labelled as locations "a" through "p". In the case of a multi-color opto-electronic display device, there are three illumination elements -- namely a red, green and blue illumination element -- at each of the pixel locations, as seen in Figure 2B.

Figure 3 shows an example of a look-up table for determining a temporal pattern for illuminating pixel locations in the display neighborhoods to produce selected shades. In practice, the temporal pattern over which a given illumination element at a pixel location is illuminated is expressed in terms of a "frame sequence". Within a frame sequence, the number of times that a given illumination element at a pixel location is illuminated will determine its brightness and, therefore, will create an appearance of its shade relative to other pixel locations.

The look-up table in Figure 3 is used in conjunction with a frame modulation process whereby the frequency with which a pixel location is illuminated will determine its perceived brightness and, therefore, its shading. For example, if pixel location "a" in Figure 2A is illuminated only once over a sequence of sixteen frames, that pixel location will appear as a dark shade relative to other pixel locations that are illuminated more frequently over the same frame sequence. In a similar way, if pixel location "e" is illuminated three times over a sequence of sixteen frames, that pixel location will appear as a lighter shade (brighter) relative to pixel location "a". Likewise, if pixel location "b" is illuminated four times over a sequence of sixteen frames, that pixel location will appear as a still lighter shade relative to pixel locations "a" and "e". In practice, it is convenient to employ a frame sequence that comprises sixteen frames with the frame sequence being repeated between sixty and one hundred thirty times per second.

In the look-up table in Figure 3, the vertical axis indicates shading, from light to dark, over sixteen different shades. In particular, the upper rows of the look-up table show pixel illumination patterns that provide the appearance of darker shades. The pixel illumination patterns in the lower rows of the look-up table, conversely, provide the appearance of lighter shades. The darkest shade will be referred to as shade #1, the next darkest shade will be referred to as shade #2, and so forth.

The horizontal axis in the look-up table in Figure 3 indicates the frame number. Because a sixteen-frame sequence has been selected in this example, the first column in the table represents the first frame of the sixteen-frame sequence, the second column represents the second frame of the sixteen-frame sequence, and so forth.

Each square area in the look-up table in Figure 3 shows the state of the pixel locations in the display neighborhood for a selected shading at a given frame number. For example, the look-up table indicates that shade #1 is produced at pixel location "a" by illuminating that pixel location only during the eighth frame of a sixteen-frame sequence. Similarly, the look-up table indicates that shade #1 is produced at pixel location "f" by illuminating that pixel location only during the fifteenth frame of the sixteen-frame sequence. Or, shade #1 is produced at pixel location "d" by illuminating that pixel location only during the sixteenth frame.

As still another example, the look-up table in Figure 3 indicates that shade #3 is produced at pixel location "e" by illuminating that pixel location during the fourth, tenth, and fifteenth frames of the sixteen-frame sequence. The look-up table similarly indicates that shade #4 is produced at pixel location "b" by illuminating that pixel location during the first, fifth, ninth and thirteenth frames of the sixteen-frame sequence. Thus, for this example, pixel location "e" will appear lighter than pixel location "a", and pixel location "b" will appear as still lighter -- and this is a result of the fact that pixel location "a" is illuminated once in the sixteen-frame sequence, while pixel location "e" is illuminated three times in the sixteen-frame sequence, and pixel location "b" is illuminated four times in the sixteen-frame sequence. The limit, obviously, is to illuminate a pixel location "b" sixteen times in the sixteen-frame sequence.

Upon examination of the look-up table in Figure 3, it will be seen that, as a general rule, adjacent pixel locations that have the same shade within any one of the display neighborhoods are illuminated with different temporal patterns over a frame sequence. Thus, continuing with the example above for producing shade #1, the look-up table indicates that pixel location "a" is illuminated only during the eighth frame of the sixteen-frame sequence and that pixel location "b" is illuminated only during the first frame of the sequence. Similarly, for producing shade #3, the look-up table indicates that pixel location "e" is illuminated during the fourth, tenth, and fifteenth frames of the sixteen-frame sequence, while pixel location "f" is illuminated during the fifth, eleventh, and sixteenth frames to produce the same shade.

The conditions under which a given display neighborhood is to be uniformly shaded can now be readily understood. For instance, if an entire display neighborhood is to have shade #3, the look-up table in Figure 3 indicates that the three pixel locations "b", "h" and "o" are to be illuminated during the first frame of the sixteen-frame sequence; that the three pixel locations "g", "i" and "p" are to be illuminated during the second frame; that pixel locations "a", "c" and "j" are to be illuminated during the third frame; and so forth. This example can be extended so that a display neighborhood can have any one of sixteen different grey scale shades. Moreover, the same look-up table can be applied to all of the display neighborhoods within an image field.

Figure 4 shows an example of a pixel transition order within a display neighborhood. This example is best understood by considering the case wherein a display neighborhood is to be uniformly shaded with shade #1. In this case, the look-up table of Figure 3 indicates that the single pixel location "b" is illuminated during a first frame of the sixteen-frame sequence; that the pixel location "h" is illuminated during the second frame; that the pixel location "o" is illuminated during the third frame; and so forth. The same pixel transition order can be seen in Figure 4, and, in fact, that diagram was used as the basis for constructing the look-up table in Figure 3.

In Figure 4, the consecutively illuminated pixel locations are connected by linear vectors v₁, v₂, and so forth. Thus, vector v₁ extends from pixel locations "b" to pixel locations "h"; vector v₂ extends from pixel locations "h" to pixel locations "o"; and so forth. Although the directions of the vectors change from frame to frame, all of the vectors have generally the same length. Accordingly, the distances separating consecutively-illuminated pixel locations are generally equal. This concept of providing generally equal separation distance during transitions is important to taking advantage of the visual averaging property. Employing the pixel transition order shown in Figure 4 to construct the look-up table in Figure 3 results in state transitions occurring substantially uniformly in space and time within each display neighborhood throughout an array of picture elements during a multi-frame display sequence.

It should be understood that the illumination conditions described in the preceding paragraph can be accomplished by simultaneously illuminating all three illumination elements (i.e., the red, green and blue illumination elements) at each of the pixel locations. Also, the conditions described in the preceding paragraph can be accomplished by selecting only one of the illumination elements for illumination, as long as the same color element is always selected. For instance, if an entire display neighborhood is to have shade "green #3", the green illumination elements at the three pixel locations "b", "h" and "o" are illuminated during the first frame of the sixteen-frame sequence; then, the green illumination elements at the three pixel locations "g", "i" and "p" are illuminated during the second frame; next, the green illumination elements at the pixel locations "a", "c" and "j" are illuminated during the third frame; and so forth. An entirely different -- and probably unwanted -- effect would result from, for instance, illuminating the green illumination elements at the three pixel locations "b", "h" and "o" during the first frame of the sixteen-frame sequence and then illuminating the yellow illumination elements at the three pixel locations "g", "i" and "p" during the second frame.

As will now be described, the above-described process can be employed such that any display neighborhood can have any one of 4096 different colors shades. To appreciate the process for arriving at this broad choice of colors, it should be first understood that each illumination element at each pixel location can have one of two states (i.e., either "on" or "off"). Thus, each pixel location can have any one of eight colors (i.e., 2³ colors). Furthermore, each color can be controlled, as described above, to have one of sixteen different shades. (A seventeenth shade is either all black or all white.) Thus, in the case wherein the illumination elements have colors red, green and blue, there are choices for any display neighborhood of sixteen shades of red, sixteen shades of green, and sixteen shades of blue of each of the eight colors. Any one of the sixteen red shades can be combined with any one of the sixteen green shades -- for a total of 16², or 256 shades. Furthermore, any one of those 256 shades can be combined with any one of the sixteen blue shades -- for a total of 4096 shades.

In normal practice, however, a given display neighborhood is not usually uniformly shaded, but, instead, shading is to be varied from pixel-to-pixel within the display neighborhood. Nevertheless, the look-up table of Figure 3 also determines how pixel illumination sequences are selected when the shading at a given pixel location changes -- that is, when the shading at a given pixel location is to be made lighter or darker. As a concrete example, assume that pixel location "p" has shade #1 and that a transition to shade #2 is to occur at the beginning of the second frame sequence where each sequence comprises sixteen frames. In that case, when producing shade #1, pixel location "p" is illuminated only in the sixth frame of the first frame sequence. In making the transition to shade #2, pixel location "p" is not illuminated again until the third frame of the second frame sequence; then, that pixel location is illuminated again in the eleventh frame, and so forth.

In the preceding example, it was assumed that the transition from one shade to another occurred at the beginning of the first frame of a sixteen-frame sequence. In practice, depending upon the image which is to be presented, it may be desired to change the shade of a given pixel location at any frame within a sixteen-frame sequence.

It can now be understood that the present invention provides a method and an apparatus for producing multi-color shaded images in successive frames of video information on opto-electronic display devices such as flat-panel LCDs (liquid crystal diodes) and similar display devices that do not intrinsically provide display shades. In the use of the present invention, no individual state transitions, which by themselves constitute only display noise, are perceived; instead, a coherent pattern of state transitions blending is seen that effectively simulates multi-color image displays.

The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as being illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.

Claims

A display method for an array of pixels, comprising the steps of:

subdividing said array of pixels into uniformly sized NxM display neighborhoods, where N, M ≥ 4;

defining patterns according to which pixels of the neighborhoods should be illuminated in successive frames;

characterized in that it comprises the step of defining a first sequence of NxM patterns corresponding to a neighborhood, wherein each pattern identifies one different pixel of the neighborhood to be illuminated such that the distance between two pixels identified by any two consecutive patterns of the first sequence is substantially equal to the distance between the two pixels identified by the first and last patterns of the sequence.
The display method according to claim 1, characterized in that it comprises the steps of:

defining a number of sequences of patterns equal to the number of shades to be represented, each shade characterized by an integer number of pixels to be illuminated in each pattern of a corresponding sequence, and

selecting the pixels to be identified by a particular pattern from a number of consecutive patterns of said first sequence equal to said integer number.
The display method according to claim 1, characterized in that it comprises, when a predetermined pixel of a neighborhood is to be illuminated once in every NxM frames for producing a first shade, the step of using a consecutive pattern of the first sequence at each frame and illuminating the predetermined pixel each time it is identified by a pattern of the first sequence.
The display method according to claim 2, characterized in that it comprises, when a predetermined pixel of a neighborhood is to be illuminated K times in every NxM frames for producing a K-th shade, the step of using a consecutive pattern of the K-th sequence at each frame and illuminating the predetermined pixel each time it is identified by a pattern of the K-th sequence.
The display method according to claim 1 or 2, characterized in that each pixel location includes a plurality of illumination elements each of a different color, a pixel illumination consisting in illuminating at least one of its illumination elements.
The display method according to any of claims 1 to 5, characterized in that the distance between two pixels identified by any two consecutive patterns of the first sequence is greater than the distance between adjacent pixels.
The display method according to any of claims 1 to 6, characterized in that the number of frames separating two consecutive illuminations of a predetermined pixel of a neighborhood is substantially constant.
The display method of claim 5, characterized in that it comprises the step of coordinating the illumination of the plurality of illumination elements of a pixel to produce an overall color for that pixel.
The display method according to any of claims 1 to 8, characterized in that M = N = 4, and in that two pixels identified by any two consecutive patterns of the first sequence are separated by a single row or column of pixels of the corresponding neighborhood.
A display device for an array of pixels, comprising:

uniformly sized display neighborhoods of NxM pixels of the array, where N, M ≥ 4;

a look-up table including patterns according to which pixels of the neighborhoods should be illuminated in successive frames;

characterized in that the look-up table includes a first sequence of NxM patterns corresponding to a neighborhood, wherein each pattern identifies one different pixel of the neighborhood to be illuminated such that the distance between two pixels identified by any two consecutive patterns of the first sequence is substantially equal to the distance between the two pixels identified by the first and last patterns of the sequence.
The display device according to claim 10, characterized in that the look-up table includes a number of sequences of patterns equal to the number of shades to be represented, each shade characterized by an integer number of pixels to be illuminated in each pattern of a corresponding sequence, and wherein the pixels to be identified by a particular pattern are selected from a number of consecutive patterns of said first sequence equal to said integer number.
The display device according to claim 10, characterized in that it comprises means for, when a predetermined pixel of a neighborhood is to be illuminated once in every NxM frames for producing a first shade, using a consecutive pattern of the first sequence at each frame and illuminating the predetermined pixel each time it is identified by a pattern of the first sequence.
The display device according to claim 12, characterized in that said means are operative, when a predetermined pixel of a neighborhood is to be illuminated K times in every NxM frames for producing a K-th shade, to use a consecutive pattern of the K-th sequence at each frame and illuminate the predetermined pixel each time it is identified by a pattern of the K-th sequence.
The display device according to claim 10 or 11, characterized in that each pixel location includes a plurality of illumination elements each of a different color, a pixel being illuminated when at least one of its illumination elements is illuminated.
The display device according to any of claims 10 to 14, characterized in that the distance between two pixels identified by any two consecutive patterns of the first sequence is greater than the distance between adjacent pixels.
The display device according to any of claims 10 to 15, characterized in that the sequences of the look-up table are such that the number of patterns separating two consecutive identifications of a predetermined pixel of a neighborhood is substantially constant.
The display device of claim 14, characterized in that it comprises means for coordinating the illumination of the plurality of illumination elements of a pixel to produce an overall color for that pixel.
The display device according to any of claims 10 to 17, characterized in that M = N = 4, and in that two pixels identified by any two consecutive patterns of the first sequence are separated by a single row or column of pixels of the corresponding neighborhood.