Palette coding modes and palette flipping

A method for coding a video signal using a color palette, the method comprising:

• determining a first cost associated with coding at least a first pixel of the video signal using index mode, wherein the index mode is enabled for the first pixel;
• determining a second cost associated with coding at least the first pixel using copy-above mode, wherein the copy-above mode is enabled for the first pixel;
• determining a third cost associated with coding a first run of pixels using the index mode and a second run of pixels using the copy-above mode, wherein the first run starts with the first pixel, the second run starts immediately after the first run, and the third cost is a minimum cost among a plurality of combinations of run-lengths for the first and second runs;
• and
• selecting a palette coding mode based on the first, second and third costs.

A video coding device for coding a video signal using a color palette, the video coding device comprising:

• a processor configured to:
• determine a first cost associated with coding at least a first pixel of the video signal using index mode, wherein the index mode is enabled for the first pixel;
• determine a second cost associated with coding at least the first pixel using copy-above mode, wherein the copy-above mode is enabled for the first pixel;
• determine a third cost associated with coding a first run of pixels using the index mode and a second run of pixels using the copy-above mode, wherein the first run starts with the first pixel, the second run starts immediately after the first run, and the third per-pixel average bit cost is a minimum cost among a plurality of combinations of run-lengths for the first and second runs;
• and
• select a palette coding mode based on the first second and third costs.

A method for coding a plurality of pixels using a color palette, the method comprising:

• determining a first combination mode coding cost associated with coding a first run of pixels in an index mode and a second run of pixels in a copy-above mode, the first run of pixels starting with a first pixel and having a first index mode run-length, the second run of pixels starting at a second pixel that is immediately after the first run of pixels, the second run of pixels having a run-length equal to a maximum copy-above mode run-length associated with the second pixel;
• determining a second combination mode coding cost associated with coding a first alternative run of pixels in the index mode and a second alternative run of pixels in the copy-above mode, the first alternative run of pixels starting with the first pixel and having an alternative index mode run-length that is different than the first index mode run-length, the second alternative run of pixels starting at a third pixel that is immediately after the first alternative run of pixels, the second alternative run of pixels having a run-length equal to a maximum copy-above mode run-length at the third pixel;
• and
• selecting a palette coding mode based on the first combination mode coding cost, the second combination mode coding cost, and a threshold coding cost, wherein:
• on a condition that the first combination mode coding cost is smaller than the second combination mode coding cost and the threshold coding cost, the palette coding mode is selected so that the first run of pixels is coded in the index mode for the first index mode run-length and the second run of pixels is coded in the copy-above mode for the maximum copy-above mode run-length associated with the second pixel;
• and
• on a condition that the second combination mode coding cost is smaller than the first combination mode coding cost and the threshold coding cost, the palette coding mode is so that the first alternative run of pixels is coded in the index mode for the first alternative index mode run-length and the second run of pixels is coded in the copy-above mode for the maximum copy-above mode run-length at the third pixel.