Features of base color index map mode for video and image coding and decoding
Innovations in the use of base color index map (“BCIM”) mode during encoding and/or decoding simplify implementation by reducing the number of modifications made to support BCIM mode and/or improve coding efficiency of BCIM mode. For example, some of the innovations involve reuse of a syntax structure that is adapted for transform coefficients to instead signal data for elements of an index map in BCIM mode. Other innovations relate to mapping of index values in BCIM mode or prediction of elements of an index map in BCIM mode. Still other innovations relate to handling of exception values in BCIM mode.
1. A computing device comprising one or more processing units and memory, wherein the computing device implements an encoder of video or images, the encoder being configured to perform operations comprising:
encoding data for a coding unit of a picture using intra-picture compression, the coding unit including one or more blocks, wherein the encoding uses a base color table and an index map, the base color table assigning one or more index values to one or more corresponding base colors, the index map including at least one of the one or more index values, and the index map further including one or more exception values, the one or more exception values not representing any of the one or more corresponding base colors in the base color table, the one or more exception values being designated using a highest possible index value among the one or more index values of the base color table, and wherein the encoding includes:
setting a syntax element that indicates whether prediction mode for the coding unit is inter or intra, the syntax element indicating that the prediction mode for the coding unit is intra;
setting a flag value that indicates whether or not to use a base color index map mode, the flag value indicating to use the base color index map mode; and
entropy coding the syntax element and the flag value; and
outputting the encoded data, wherein the encoded data includes the syntax element and the flag value.
2. The computing device of claim 1 wherein the encoding further includes, for one of the one or more exception values:
encoding a sample value using a quantization parameter.
3. The computing device of claim 1 wherein the one or more index values are packed index values or packed index residual values.
4. The computing device of claim 1 wherein a given index value, among the one or more index values of the index map, is predicted using a neighboring index value that depends on whether prediction direction is horizontal or vertical.
5. The computing device of claim 4 wherein the prediction direction is horizontal and the neighboring index value is left of the given index value in the index map.
6. The computing device of claim 4 wherein the prediction direction is vertical and the neighboring index value is above the given index value in the index map.
7. The computing device of claim 1 wherein the base color table is created during the encoding the data for the coding unit.
8. In a computing device with a video decoder or image decoder, a method comprising:
receiving encoded data for a coding unit of a picture, the coding unit having been encoded using intra-picture compression, the coding unit including one or more blocks, wherein the encoded data includes a syntax element and a flag value, the syntax element indicating whether prediction mode for the coding unit is inter or intra, and the flag value indicating whether or not to use a base color index map mode; and
decoding the encoded data, wherein the decoding uses a base color table and an index map, the base color table assigning one or more index values to one or more corresponding base colors, the index map including at least one of the one or more index values, and the index map further including one or more exception values, the one or more exception values not representing any of the one or more corresponding base colors in the base color table, the one or more exception values being designated using a highest possible index value among the one or more index values of the base color table, and wherein the decoding includes:
entropy decoding the syntax element and the flag value;
determining, based on the syntax element, that the prediction mode for the coding unit is intra; and
determining, based on the flag value, to use the base color index map mode.
9. The method of claim 8 wherein the decoding further includes, for one of the one or more exception values:
reconstructing a sample value using a quantization parameter.
10. The method of claim 8 wherein the one or more index values are packed index values or packed index residual values.
11. The method of claim 8 wherein a given index value, among the one or more index values of the index map, is predicted using a neighboring index value that depends on whether prediction direction is horizontal or vertical.
12. The method of claim 11 wherein the prediction direction is horizontal and the neighboring index value is left of the given index value in the index map.
13. The method of claim 11 wherein the prediction direction is vertical and the neighboring index value is above the given index value in the index map.
14. The method of claim 8 wherein the base color table is decoded during the decoding the encoded data for the coding unit.
15. One or more computer-readable memory or storage devices storing computer-executable instructions for causing a computing device, when programmed thereby, to perform operations comprising:
receiving encoded data for a coding unit of a picture, the coding unit having been encoded using intra-picture compression, the coding unit including one or more blocks, wherein the encoded data includes a syntax element and a flag value, the syntax element indicating whether prediction mode for the coding unit is inter or intra, and the flag value indicating whether or not to use a base color index map mode; and
decoding the encoded data, wherein the decoding uses a base color table and an index map, the base color table assigning one or more index values to one or more corresponding base colors, the index map including at least one of the one or more index values, and the index map further including one or more exception values, the one or more exception values not representing any of the one or more corresponding base colors in the base color table, the one or more exception values being designated using a highest possible index value among the one or more index values of the base color table, and wherein the decoding includes:
entropy decoding the syntax element and the flag value;
determining, based on the syntax element, that the prediction mode for the coding unit is intra; and
determining, based on the flag value, to use the base color index map mode.
16. The one or more computer-readable memory or storage devices of claim 15 wherein the decoding further includes, for one of the one or more exception values:
reconstructing a sample value using a quantization parameter.
17. The one or more computer-readable memory or storage devices of claim 15 wherein a given index value, among the one or more index values of the index map, is predicted using a neighboring index value that depends on whether prediction direction is horizontal or vertical.
18. The one or more computer-readable memory or storage devices of claim 17 wherein the prediction direction is horizontal and the neighboring index value is left of the given index value in the index map.
19. The one or more computer-readable memory or storage devices of claim 17 wherein the prediction direction is vertical and the neighboring index value is above the given index value in the index map.
20. The one or more computer-readable memory or storage devices of claim 15 wherein the base color table is decoded during the decoding the encoded data for the coding unit.