Indexed color values in image coding

1. A method comprising:

obtaining an encoded stream;

decoding the encoded stream to generate a picture including a current pixel group and a neighbor pixel group proximate to the current pixel group;

for the neighbor pixel group, applying an indexed set of color values including a color value used within the neighbor pixel group; and

when an indexing signal is present in the encoded stream, employing the indexed set of color values for the current pixel group, the indexing signal conveying an output of an evaluation function of a bit length, and the indexing signal comprising an indication that employing the indexed set of color values for the current pixel group provides a compression advantage compared to coding the current pixel group without employing the indexed set of color values.

2. The method of claim 1 , further comprising selecting the neighbor pixel group using a predetermined position defined relative to the current pixel group.

3. The method of claim 2 , where the predetermined position comprises a position on a scan line above the current pixel group.

4. The method of claim 1 , where employing the indexed set of color values comprises assigning, during decoding, at least one of the color values to a pixel within the current pixel group responsive to an index value within the encoded stream.

5. The method of claim 1 , where:

the picture includes multiple contiguous pixel groups comprising the neighbor pixel group and the current pixel group; and

where decoding further comprises scanning the multiple contiguous pixel groups in lines.

6. A product comprising:

a machine-readable medium other than a transitory signal;

instructions stored on the machine-readable medium, the instructions configured to, when executed, cause a processor to:

obtain a current pixel group within an encoded stream;

obtain an indexed set of color values, the indexed set of color values including an entry referenced while decoding a neighboring pixel group adjacent to the current pixel group; and

when a value of the entry is within a difference threshold of a color value for the current pixel group, employ the entry while decoding the current pixel group.

7. The product of claim 6 , where the color value for the current pixel group is within the difference threshold only when the color value for the current pixel group is identical to the value of the entry.

8. The product of claim 6 , where the color value for the current pixel group is within the difference threshold when a component value of the color value for the current pixel group is within the difference threshold from a corresponding component value of the color value of the entry.

9. The product of claim 6 , where the neighbor pixel group includes a pixel in a line directly above the current pixel group.

10. The product of claim 6 , where the entry comprises three component color values.

11. The product of claim 6 , where the encoded stream comprises a picture including the current pixel group and neighbor pixel group, the picture including multiple contiguous pixel groups.

12. The product of claim 11 , where the instructions are configured to scan the multiple pixel groups in lines within the picture.

13. The product of claim 6 , where the instructions are further configured to determine whether to employ the entry to the current pixel group responsive to an escape code within the encoded stream.

14. The product of claim 6 , where the difference threshold comprises a quantization error threshold for the current pixel group.

15. A device comprising:

output interface circuitry configured to output a decoded picture including a current pixel group and a neighbor pixel group proximate to the current pixel group; and

coding circuitry in data communication with the output interface, the coding circuitry configured to:

obtain an encoded stream;

for the neighbor pixel group, employ an indexed set of color values including a color value used within the neighbor pixel group;

when an indexing signal is present in the encoded stream, employ the set of color values for the current pixel group, the indexing signal configured to convey a result of a determination that employing the indexed set of color values for the current pixel group provides a compression advantage over coding the current pixel group without employing the indexed set of color values; and

send the decoded picture to the output interface circuitry.

16. The device of claim 15 , where the coding circuitry is configured to employ the indexed set of color values by assigning at least one of the color values to a pixel in the current pixel group.

17. The device of claim 15 , where the coding circuitry is further configured to select the neighbor pixel group using a predetermined position defined relative to the current pixel group.

18. The device of claim 17 , where the predetermined position comprises a position on a scan line above the current pixel group.

19. The device of claim 15 , where the coding circuitry is configured to employ the indexed set of color values by assigning at least one of the color values to a pixel within the current pixel group responsive to an index value within the encoded stream.

20. The device of claim 15 , where the current pixel group comprises a square block of pixels.