Dispersion-based technique for performing spacial dithering for a digital display system

• US 6,081,252 A
• Filed: 07/11/1997
• Issued: 06/27/2000
• Est. Priority Date: 07/11/1997
• Status: Expired due to Term

• Current Assignee: Cabot Corporation, National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Reddy, Dayakar Chandram
• Primary Examiner(s): Shalwala, Bipin
• Assistant Examiner(s): Kovalick, Vincent E.
• Application Number:
  US08/891,469
• Time in Patent Office: 1,082 Days
• Field of Search: 345/149, 345/152, 345/153, 345/155, 358/457
• US Class Current: 345/605
• CPC Class Codes: G09G 3/2014 by modulation of the duration of a single pulse during which the logic level remains constant
• CPC Class Codes: G09G 3/2051 with use of a spatial dither pattern
• CPC Class Codes: G09G 5/022 using memory planes

Abstract

A technique for performing spacial dithering for a digital display system. A display panel is portioned into eight-by-eight arrays of 64 pixels, each pixel including three primary color sub-pixels. A predetermined number of bits represent a color level for each sub-pixel according to image data. If the display system can accept only fewer bits than the predetermined number, bits must be discarded. The discarded bits are used to modify the remaining bits before the remaining bits are passed to the display system. A memory stores 64 tables, each table including an entry for each pixel within an eight-by-eight array. The entry for a pixel determines whether or not the color level for the corresponding sub-pixel is to be decremented by one. A first of the sixty-four tables does not result in decrementing any color levels. Each successive table decrements the color level for one more pixel than the previous table. The values of discarded bits select among the stored tables. whereas, the values of bits that are retained are selectively decremented according to entries in the selected table. Entries in the tables are dispersed so as to avoid decrementing color levels in a pattern that would otherwise tend to result in image artifacts. This is accomplished by avoiding vertical, horizontal or diagonal lines, and checker-board patterns of pixels for which the color level is decremented. As a result, artifacts induced by the dithering process are reduced in comparison to prior dithering techniques.

Claims

1. A method of performing spacial dithering upon a stream of data values representative of an image to be displayed on a display panel for forming a stream of dithered values, wherein each dithered value includes a number of valid bits that is less than a number of bits included in each data value, the method comprising steps of:

a. removing a number of least significant bits from each of the stream of data values for forming a stream of truncated values, wherein the stream of truncated values includes a number of valid bits; and

b. selectively decrementing each of the stream of truncated values according to a corresponding entry in a selected table for forming the stream of dithered values wherein the selected table is selected from a group of stored tables according to the bits removed from each truncated value.

2. The method according to claim 1 further comprising a step of modulating pixels of the display panel according to the stream of dithered values.

3. The method according to claim 1 wherein the selected table includes an entry corresponding to each of an array of pixels.

4. The method according to claim 3 wherein the array of pixels includes eight rows of pixels arranged in eight columns.

5. The method according to claim 3 wherein entries indicative of decrementing the corresponding truncated value are arranged other than in vertical, horizontal or diagonal lines of corresponding pixels.

6. A method of performing spacial dithering upon a stream of data values representative of an image to be displayed on a display panel for forming a stream of dithered values, wherein each dithered value includes a number of valid bits that is less than a number of bits included in each data value, the method comprising steps of:

a. removing a number of least significant bits from each of the stream of data values for forming a stream of truncated values, wherein the stream of truncated values includes a number of valid bits;

b. selecting a table from a group of tables wherein the table is selected according to the bits removed from each truncated value and wherein the table includes an entry corresponding to each of an array of pixels, each entry indicative of whether or not a corresponding truncated value is to be decremented and wherein a ratio of entries indicative of decrementing the corresponding truncated value to entries not indicative of decrementing the corresponding truncated value is related to a value of the removed bits; and

c. selectively decrementing each of the stream of truncated values according to the corresponding entry in the selected table for forming the stream of dithered values.

7. The method according to claim 6 wherein the entries indicative of decrementing the corresponding truncated value are arranged other than in vertical, horizontal or diagonal lines of corresponding pixels.

8. The method according to claim 6 further comprising a step of modulating pixels of the display panel according to the stream of dithered values.

9. The method according to claim 6 wherein the array of pixels includes sixty-four pixels.

10. The method according to claim 9 wherein the array of pixels includes eight rows of pixels arranged in eight columns.

11. An apparatus for selectively performing spacial dithering upon a stream of data values representative of an image to be displayed on a display panel for forming a stream of dithered values wherein each dithered value includes a number of valid bits that is equal to or less than a number of bits included in each data value, the apparatus comprising:

a. a circuit for selectively removing a number of least significant bits from each of the stream of data values for forming a stream of truncated values wherein the stream of truncated values includes a number of valid bits; and

b. a circuit for selectively decrementing each of the stream of truncated values according to a corresponding entry in a selected table for forming the stream of dithered values wherein the selected table is selected from a group of tables according to the bits removed from each truncated value.

12. The apparatus according to claim 11 further comprising a circuit for modulating pixels of the display panel according to the stream of dithered values.

13. The apparatus according to claim 11 wherein the table includes an entry corresponding to each of an array of pixels, each entry indicative of whether or not a corresponding truncated value is to be decremented.

14. The apparatus according to claim 13 wherein the entries indicative of decrementing the corresponding truncated value are arranged other than in vertical, horizontal or diagonal lines of corresponding pixels.

15. The apparatus according to claim 13 wherein a ratio of entries indicative of decrementing the corresponding truncated value to entries not indicative of decrementing the corresponding truncated value is related to a value of the removed bits.

16. The apparatus according to claim 13 wherein the array of pixels includes sixty-four pixels.

17. The apparatus according to claim 16 wherein the array of pixels includes eight rows of pixels arranged in eight columns.

18. An apparatus for selectively dithering a data value representative of an intensity for a pixel in a display panel for forming a dithered value, wherein the dithered value includes a number of valid bits that is equal to or less than a number of bits included in the data value, the apparatus comprising:

a. a decrement circuit having a data input, a control input and an output, wherein the data input of the decrement circuit is coupled to receive a selected number of most significant bits of the data value, the decrement circuit for selectively decrementing a value of the selected most significant bits of the data value according to a level of the control input of the decrement circuit and wherein the output of the decrement circuit forms the dithered value;

b. a multiplexer having a plurality of data inputs, a control input and an output, wherein the output of the multiplexer is coupled to the control input of the decrement circuit and wherein the control input of the multiplexer is coupled to receive a value representative of a value of a selected number of least significant bits of the data value; and

c. plurality of stored tables, each table including a plurality of entries, each entry corresponding to a position in an array of pixels and each entry indicative of whether or not a value of the selected most significant bits of the data value is to be decremented by the decrement circuit, wherein each table entry corresponding to the position of the pixel in the array is coupled to a corresponding one of the data inputs of the multiplexer.

19. The apparatus according to claim 18 further comprising a circuit for modulating pixels of the display panel according to the stream of dithered values.

20. The apparatus according to claim 18 wherein the entries indicative of decrementing the corresponding truncated value are arranged other than in vertical, horizontal or diagonal lines of corresponding pixels.

21. The apparatus according to claim 18 wherein for each table, a ratio of entries indicative of decrementing the value of the selected most significant bits to entries not indicative of decrementing the value of the selected most significant bits is related to the selected number of least significant bits.

22. The apparatus according to claim 18 wherein the pixel is a sub-pixel corresponding to a primary color.

23. The apparatus according to claim 18 wherein the array of pixels includes sixty-four pixels.

24. The apparatus according to claim 23 wherein the array of pixels includes eight rows of pixels arranged in eight columns.