Method and apparatus for producing perception of high quality grayscale shading on digitally commanded displays

• US 5,185,602 A
• Filed: 04/10/1989
• Issued: 02/09/1993
• Est. Priority Date: 04/10/1989
• Status: Expired due to Term

• Current Assignee: NVIDIA Corporation
• Original Assignee: Cirrus Logic Incorporated
• Inventors: Reddy, Dayakar C., Richter, Bryan M., Bassetti, Chester F. Jr., Laiman, Ekaputra
• Primary Examiner(s): Brier, Jeffery A.
• Application Number:
  US07/335,622
• Time in Patent Office: 1,401 Days
• Field of Search: 340/703, 340/767, 340/781, 340/784, 340/793, 358/457, 358/458
• US Class Current: 345/89
• CPC Class Codes: G09G 2320/0247 Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• CPC Class Codes: G09G 2320/0261 in the context of movement of objects on the screen or movement of the observer relative to the screen
• CPC Class Codes: G09G 2320/0266 Reduction of sub-frame artefacts
• CPC Class Codes: G09G 2320/0276 for the purpose of adaptation to the characteristics of a display device, i.e. gamma correction
• CPC Class Codes: G09G 2360/18 Use of a frame buffer in a display terminal, inclusive of the display panel
• CPC Class Codes: G09G 3/20 for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
• CPC Class Codes: G09G 3/2003 Display of colours specific for liquid crystal displays G09G3/3607
• CPC Class Codes: G09G 3/2007 Display of intermediate tones
• CPC Class Codes: G09G 3/2025 the sub-frames having all the same time duration
• CPC Class Codes: G09G 3/2051 with use of a spatial dither pattern
• CPC Class Codes: G09G 3/2092 Details of a display terminals using a flat panel, the details relating to the control arrangement of the display terminal and to the interfaces thereto suitable for both CRT and flat panel G09G5/003; specific for a CRT G09G1/165
• CPC Class Codes: G09G 3/3611 Control of matrices with row and column drivers
• CPC Class Codes: H04N 3/127 using liquid crystals

Abstract

The perception of grayscale shading on a digitally commanded display is produced by commanding pixels of the display with brightness-setting signals of differing average duty cycles. Brightness-setting signals having one brightness level associated with them are phase shifted in relation to time and distributed to spaced apart pixel locations at which the one brightness level is to be produced. The energization of spatially adjacent pixels is scattered in time and pixels which are energized at the same time are selected to be spatially scattered so as to avoid the perception of visual disturbances such as flickering and surface streaming.

Claims

1. A method for producing the perception of images having four or more levels of distinguishable brightness on a digitally controlled display, comprising:

defining a set of brightness-setting waveforms each consisting of a number, D, of ON/OFF command bits, the number D satisfying the relationship

space="preserve" listing-type="equation">D=na-1

space="preserve" listing-type="equation">D=mb-1

space="preserve" listing-type="equation">a≠

b

where a, b, n, m and D are integers greater than zero, the integration of each of said D brightness-setting waveforms producing a different average brightness level over an integration period equal to D bit times;

defining a first square phase-placement matrix of dimensions D cells by D cells;

filling the cells of the phase-placement matrix with at least four different non-consecutive phase shifting values wherein for each brightness-setting waveform there are D possible phases, each phase corresponding to a different bit of the brightness setting waveform; and

each cell of the phase-placement matrix corresponding to a pixel in a matrix of D pixels by D pixels in the digitally controlled display; and

producing two or more brightness-setting signals having waveforms corresponding to two or more of the brightness-setting waveforms, the produced signals being applied to pixels of the matrix of pixels and shifted in accordance with different non-consecutive phase shifting values in the corresponding cells of the phase-placement matrix.

2. The method of claim 1 wherein the value D further satisfies the relationship

3. The method of claim 2, further including the step of selecting the numbers "a" and "b" to satisfy the relationship

4. The method of claim 1 wherein at least two matrix cells of the phase-placement matrix contain identical phase shifting values, and wherein matrix cells containing identical values are spaced apart from one another by at least one matrix cell not containing an identical value.

5. The method of claim 4, wherein the images being produced are formed of a series of pixels, each having a designated brightness level, further including the steps of

defining a second square phase-placement matrix of dimensions D cells by D cells;

filling the cells of the alternative phase-placement matrix with an alternative pattern of phase shifting values; and

comparing a level of brightness to be produced at a next-to-be-displayed pixel to a level of brightness produced at a previously-displayed pixel;

using the first phase-placement matrix with the next-to-be-displayed pixel when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are equal; and

using the second phase-placement matrix when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are unequal.

6. The method of claim 1, wherein the images being produced are formed of a series of pixels, each having a designated brightness level, and further wherein the number, D, of ON/OFF command bits of the brightness-setting waveform is defined by the relationship D=A² +B², A and B are integers, A≠

B, and A² +B² +1 possible brightness-setting waveforms are defined, and further including the steps of

discarding a brightness-setting waveform corresponding to a brightness level substantially equal to but not identical to the minimum brightness level available from the A² +B² +1 brightness-setting waveforms; and

selecting from the remaining brightness-setting waveforms for controlling the series of pixels.

7. The method of claim 6, further including the steps of

discarding a brightness-setting waveform corresponding to a brightness level substantially equal to but not identical to the maximum brightness level available from the A² +B² +1 brightness-setting waveforms;

and

selecting from the remaining brightness-setting waveforms for controlling the series of pixels.

8. A stippling method comprising the steps of:

defining a plurality of different brightness levels, each being relatively uniformly spaced in brightness level from the other;

defining a square display area of at lest four pixels;

commanding diagonally adjacent pairs of the pixels to a first brightness level; and

commanding horizontally adjacent pixels to different brightness levels, wherein the different brightness level selected is the next brightness level of the plurality of brightness levels which is above or below the first brightness level; and

further wherein the pixels are commanded to the first or different brightness levels by way of brightness setting waveforms, each having the same number of bits.

9. The method of claim 8, wherein the pixels are commanded to be at least six different brightness levels.

10. The method of claim 8, wherein the pixels are commanded to be at least sixteen different brightness levels.

11. A system for controlling a digitally commanded display unit having pixels arranged in rows and columns to produce, over a number of consecutive frames, the perception of an image having one or more levels of brightness which are selectable from a plurality of levels of brightness, comprising:

row addressing means for designating the row number of a desired pixel;

column addressing means for designating the column number of the desired pixel;

image memory means responsive to the row and column numbers for the desired pixel for producing brightness data indicating a desired level of brightness to be produced at the desired pixel;

waveform memory means for storing a predetermined set of brightness-setting waveforms, wherein each of the waveforms has a predetermined number of bits and produces a different brightness level when applied to a pixel, one bit per frame, over a number of consecutive frames, and for selecting one of the stored brightness-setting waveforms in response to the brightness data, the waveform memory means including bit position selecting means for selecting a designated bit from the selected brightness-setting waveform and outputting said selected bit as a display command bit for use by the display unit to control the state of the desired pixel during a particular frame being displayed by the display unit; and

phase-placement patterning means, responsive to the row and column numbers for the desired pixel, to the brightness data, and to the particular frame being displayed, for storing a set of predetermined phase-placement patterns and for designating to the waveform memory means, in accordance with a phase-placement pattern which is selected in response to the brightness data, the bit of the selected brightness-setting waveform to be output by the waveform memory means for the desired pixel and the particular frame being displayed, wherein each of the phase-placement patterns is associated with a different brightness level and a corresponding brightness-setting waveform, and each pattern designates different non-consecutive phase from pixel to pixel with which its corresponding brightness-setting waveform is to be applied to the pixels of the display unit, so that for a particular frame and a particular pixel, the phase-placement pattern means designates a bit of the selected brightness-setting waveform which is to be output.

12. The system according to claim 11 wherein the phase-placement patterns are squares of dimensions D pixels by D pixels;

the dimension D being selected in accordance with the relations

space="preserve" listing-type="equation">D=na-1

space="preserve" listing-type="equation">D=mb-1

space="preserve" listing-type="equation">a≠

b

where a, b, n, m and D are integers greater than zero.

13. The system of claim 12 wherein the dimensions D satisfies the relationship

14. The system of claim 12 wherein dimension D is a prime member.

15. The system of claim 11 wherein the phase-placement patterning means has a first set of predetermined phase-placement patterns and a second set of predetermined phase-placement patterns different from the first set, and further including means responsive to brightness data from the image memory means for comparing a desired level of brightness to be produced at a next-to-be-displayed pixel to a desired level of brightness produced at a previously-displayed pixel, and for controlling the phase-placement patterning means to use the first set of phase-placement patterns with the next-to-be-displayed pixel when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are equal, and to use the second set of phase-placement patterns when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are unequal.

16. The system of claim 12 wherein D equals 17.

17. The system of claim 11 wherein the phase-placement patterns comprise a matrix of P rows and Q columns, wherein each row of the matrix includes a wrap-around sequence which designates a different non-consecutive phase from pixel to pixel with which a corresponding brightness waveform is to be applied to the pixels of the display unit;

and further wherein the wrap-around sequence in each row is shifted in position by a predetermined number of pixels from one row to the next, with any portion of the wrap-around sequence which extends beyond the end of a row being wrapped around to the beginning of the row.

18. The system of claim 17, wherein P equals Q and the predetermined number of pixels by which the wrap-around sequence is offset within each row, from one row to the next, is three (3) or greater.

19. The system of claim 18 wherein P equals Q, and Q equals 17, the wrap-around sequence is 17 pixels long, each brightness-setting waveform is 17 bits long, and the war-around sequence is offset by four pixels from one row to the next in each phase-placement pattern.

20. The system of claim 11 wherein the bits of the brightness-setting waveform command a pixel to be OFF or to be ON, and further wherein, in each waveform, the bits which command a pixel to be ON are positioned, and the phases of the brightness-setting waveforms designated by the phase-placement patterns to be applied from pixel to pixel are selected, so that the pixels which are ON in the frame are substantially uniformly distributed in the rows and columns of the display unit.

21. The system of claim 19 wherein the bits of the brightness-setting waveform command a pixel to be OFF or to be ON, and further wherein, in each waveform, the bits which command a pixel to be ON are positioned, and the phase from pixel to pixel designated by the phase-placement patterns to be applied from pixel to pixel are selected, so that the pixels which are ON in the frame are substantially uniformly distributed in the rows and columns of the display unit.

22. The system of claim 11, wherein each brightness-setting waveform in the set of brightness-setting waveforms has N bits, where N=A² +B², A and B are integers, A≠

B, and A² +B² +1 possible brightness-setting waveforms are defined, and further wherein a brightness-setting waveform is discarded which corresponds to a brightness level substantially equal to but not identical to the minimum brightness level available from the A² +B² +1 brightness-setting waveforms, and the remaining brightness-setting waveforms are used for controlling the pixels of the display unit.

23. The system of claim 22, further wherein a brightness-setting waveform is discarded which corresponds to a brightness level substantially equal to but not identical to the maximum brightness level available from the A² +B² +1 brightness-setting waveforms, and the remaining brightness-setting waveforms are used for controlling the pixels of the display unit.

24. A method for controlling a digitally commanded display unit having pixels arranged in rows and columns to produce over a number of consecutive frames the perception of an image having a number of levels of brightness greater than six, comprising the steps of:

a. defining an irregular rectangle of A pixels in height and B pixels in length, where A and B are integers, and A≠

B, A² +B² >

6;

b. defining a set of brightness-setting waveforms having a number of bits equal in number to the sum N=A² B², wherein each brightness-setting waveform produces a different level of brightness when applied to a pixel over a period of N frames;

c. defining a set of wrap-around sequences of N pixels in length, wherein each wrap-around sequence corresponds to a different brightness-setting waveform, and each wrap-around sequence designates a different non-consecutive phase from pixel to pixel with which a corresponding brightness waveform is to be applied to the pixels of the display unit;

defining a first set of phase-placement patterns each pattern of which is associated with a different brightness level and a corresponding brightness-setting waveform, and each such pattern being constructed of a wrap-around sequence corresponding to the different brightness level, wherein in each phase-placement pattern the corresponding wrap-around sequence is used in each row, but offset every A-rows by B-pixels; and

e. applying the brightness-setting waveforms to the pixels of the display unit by utilizing the phase-placement patterns to define the non-consecutive phase from pixel to pixel with which the brightness-setting waveforms are applied to the pixels of the display unit.

25. The method of claim 24 wherein the brightness-setting waveforms have bit states of one or zero, and the distribution of the bit states within each brightness-setting waveform and the non-consecutive phases of the brightness-setting waveforms designated in the phase-placement patterns, are selected to substantially uniformly distribute the bits having a state of one over the rows and columns of each frame displayed by the display unit.

26. The method of claim 25, wherein A² +B² +1 possible brightness-setting waveforms are defined, the method further including the step of

f. discarding a brightness-setting waveform corresponding to a brightness level substantially equal to but not identical to the minimum brightness level available from the A² +B² +1 brightness-setting waveforms, and

g. selecting from the remaining brightness-setting waveforms for controlling the pixels of the display unit.

27. The method of claim 26, further including the steps of

g. discarding a brightness-setting waveform corresponding to a brightness level substantially equal to but not identical to the maximum brightness level available from the A² +B² +1 brightness-setting waveforms, and

i. selecting from the remaining brightness-setting waveforms for controlling the pixels of the display unit.

28. The method of claim 25, including the steps of selecting A equal to 1 and B equal to 4, to provide brightness-setting waveforms 17 bits in length, and 18 possible brightness-setting waveforms.

29. The method of claim 24 wherein the step "e." further includes the steps of

defining a second set of phase-placement patterns different from the first set;

k. comparing a level of brightness to be produced at a next-to-be-displayed pixel to a level of brightness produced at a previously-displayed pixel;

l. using the first set of phase-placement patterns with the next-to-be-displayed pixel when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are equal; and

m. using the second set of phase-placement placement patterns with the next-to-be displayed pixel when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are unequal.

30. The apparatus of claim 25, wherein A² +B² +1 possible brightness-setting waveforms are defined, and further wherein a brightness-setting waveform is discarded which corresponds to a brightness level substantially equal to but not identical to the minimum brightness level available from the A² +B² +1 brightness-setting waveforms, and the remaining brightness-setting waveforms are used for controlling the pixels of the display unit.

31. The apparatus of claim 30, further wherein a brightness-setting waveform is discarded which corresponds to a brightness level substantially equal to but not identical to the maximum brightness level available from the A² +B² +1 brightness-setting waveforms, and the remaining brightness-setting waveforms are used for controlling the pixels of the display unit.

32. A method for controlling a digitally commanded display unit having pixels arranged in rows and columns to produce over a number of consecutive frames the perception of an image having more than six levels of brightness, comprising the steps of:

a. defining a set of brightness-setting waveforms each having N bits, where N=A² B², A and B are integers, A≠

B, and wherein each brightness-setting waveform produces a different level of brightness when applied to a pixel of the display unit over a period of N frames, and each bit of brightness-setting waveform represents a different phase of the waveform;

b. applying the brightness-setting waveforms to pixels of the display unit in accordance with a set of warp-around sequences, wherein each wrap-around sequence controls the application of a corresponding brightness-setting waveform to pixels of the display unit in groups of N consecutive pixels located in a predetermined block of pixels, so that in each group of N consecutive pixels a different non-consecutive phases of the brightness-setting waveform is applied to each pixel from one pixel to the next, and the location of a group of N consecutive pixels in a particular row is controlled to be offset by B-pixels from the relative location of a group of N consecutive pixels A-rows away.

33. The method of claim 32 wherein step "a." further includes the step of

selecting A and B so that an irregular rectangle is defined of A pixels in height and B pixels in length.

34. The method of claim 33 wherein step "b." further includes the step of

d. sizing the predetermined block of pixels to be N pixels in length and N rows in height.

35. The method of claim 33 wherein step "b." further includes the step of

d. defining a set of phase-placement patterns, each pattern of which is associated with a different brightness level and a corresponding brightness-setting waveform, wherein each phase-placement pattern is constructed of a wrap-around sequence;

and

e. applying the brightness-setting waveforms to the pixels of the display unit in accordance with the set of phase-placement patterns.

36. The method of claim 35 wherein step "e." further includes the step of

g. defining a first set of predetermined phase-placement patterns and a second set of predetermined phase-placement patterns different from the first set, and

h. comparing a level of brightness to be produced at a next-to-be-displayed pixel to a level of brightness produced at a previously-displayed pixel;

i. using the first set of phase-placement patterns with the next-to-be-displayed pixel when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are equal; and

j. using the second set of phase-placement patterns when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are unequal.

37. The method of claim 32, wherein 18 levels of brightness are provided and step "a." includes the step of setting A equal to one and B equal to 4.

38. The method of claim 32, wherein the brightness-setting waveform of N bits defines A² +B² +1 possible brightness-setting waveforms, the method further including the steps of

g. discarding a brightness-setting waveform corresponding to a brightness level substantially equal to but not identical to the minimum brightness level available from the A² +B² +1 brightness-setting waveforms, and

h. selecting from the remaining brightness-setting waveforms for controlling the pixels of the display unit.

39. The method of claim 38, further including the steps of

i. discarding a brightness-setting waveform corresponding to a brightness level substantially equal to but not identical to the maximum brightness level available from the A² +B² +1 brightness-setting waveforms, and

j. selecting from the remaining brightness-setting waveforms for controlling the pixels of the display unit.

40. An apparatus for controlling a digitally commanded display unit having pixels arranged in rows and columns to produce over a number of consecutive frames the perception of an image having more than six levels of brightness, comprising:

means for defining a set of brightness-setting waveforms each having N bits, where N=A² B², A and B are integers, A≠

B, and wherein each brightness-setting waveform produces a different level of brightness when applied to a pixel of the display unit over a period of N frames, and each bit of brightness-setting waveform represents a different phase of the waveform;

means for applying the brightness-setting waveforms to pixels of the display unit in accordance with a set of warp-around sequences, wherein each wrap-around sequence controls the application of a corresponding brightness-setting waveform to pixels of the display unit in groups of N consecutive pixels located in a predetermined block of pixels, so that in each group of N consecutive pixels a different non-consecutive phases of the brightness-setting waveform is applied to each pixel from one pixel to the next, and the location of a group of N consecutive pixels in a particular row is controlled to be offset by B-pixels from the relative location of a group of N consecutive pixels A-rows away.

41. The apparatus of claim 40 wherein the defining means A and B are selected so that an irregular rectangle is defined of A pixels in height and B pixels in length.

42. The apparatus of claim 41 wherein the predetermined block of pixels in the applying means is sized to be N pixels in length and N rows in height.

43. The apparatus of claim 42 wherein the applying means further includes

means for defining a set of phase-placement patterns, each pattern of which is associated with a different brightness level and a corresponding brightness-setting waveform, wherein each phase-placement pattern is constructed of a wrap-around sequence;

and

means for applying the brightness-setting waveforms to the pixels of the display unit in accordance with the set of phase-placement patterns.

44. The apparatus of claim 40 wherein 18 levels of brightness are provided and A equals one and B equals 4.

45. The apparatus of claim 43 wherein the phase-placement patterning means has a first set of predetermined phase-placement patterns and a second set of predetermined phase-placement patterns different from the firsts set, and further including means for comparing a level of brightness to be produced at a next-to-be-displayed pixel to a level of brightness produced at a previously-displayed pixel, and for controlling the defining means to use the first set of phase-placement patterns with the next-to-be-displayed pixel when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are equal, and to use the second set of phase-placement patterns when the desired levels of brightness of the next-to-be-displayed and the previously-displayed pixels are unequal.