After some further thought and discussion on the FFmpeg and Xiph.org IRC channels, the current behavior seems reasonable. To my knowledge, PAL8 is guaranteed to have exactly 256 colors. This is why, somewhat surprisingly, GIFs aren't decoded as PAL8, they're decoded as BGRA. This means that they get a 0 written. In addition, it was suggested that 0 could mean that the number of colors is unknown, which would be exactly the case in this scenario. These fields don't seem to be considered especially important, either. For example, the reference FLAC encoder always writes 24-bit color for GIF, despite having a comment acknowledging that this may be incorrect.
On Sun, Jan 31, 2021 at 3:41 PM leo60228 <l...@60228.dev> wrote: > That's true, but as far as I can tell FFmpeg assumes that all paletted > images have 256 colors, and thus there isn't a way to get the exact number > of colors in a GIF. > I also doubt that this will cause problems in practice, it's somewhat > strange that this field is even there. > If I'm wrong on any of this, please let me know. > > On Sun, Jan 31, 2021 at 3:07 PM Derek Buitenhuis < > derek.buitenh...@gmail.com> wrote: > >> On 29/01/2021 16:12, leo60228 wrote: >> > + if (st->codecpar->format == AV_PIX_FMT_PAL8) >> > + avio_wb32(pb, AVPALETTE_COUNT); >> > + else >> > + avio_wb32(pb, 0); >> >> Is this correct, though? >> >> GIFs encoded by things that are not libavcodec may have less than >> AVPALETTE_COUNT entries/colors. >> >> >> - Derek >> _______________________________________________ >> ffmpeg-devel mailing list >> ffmpeg-devel@ffmpeg.org >> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel >> >> To unsubscribe, visit link above, or email >> ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe". > > _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
