Grayscale (YUV400) support about libavif

[joedrago]

I see a lot more buzz for this in general than I expected. To do it right, I'll want to rework the plane flags to have a Y and a UV which will be a breaking change so I'll have to bump the major library version, but it is probably not a huge code change under the hood.

I might not get to it this week as I'm on vacation, but I'll add it soonish.

[q3cpma]

Thanks and nice reactivity. Makes AVIF experimentation easier.

[joedrago]

I got most of the way implementing this today, and then found out that rav1e isn't there yet, WRT monochrome support. I've opened a bug with them, and I'm going to postpone support for libavif monochrome until they support it. Hopefully it won't be too long.

xiph/rav1e#2190

[tdaede]

FWIW rav1e implemented monochrome support in our most recent prerelease, if you'd like to try it out.

[q3cpma]

Would it be possible for the JPEG/PNG reader to detect grayscale input? I currently have to use ffmpeg as intermediary.

[joedrago]

You can demand that any image is forced to be made grayscale via --yuv 400, but I'd be fine with altering any avifenc input code to detect that the YUV format should be 400 "by default". I'm pretty sure there is a "gray" color space in PNG so that might be easy, but I don't know about JPEG. Is there a grayscale flag somewhere in JPEG? (I'll have to look.)

[q3cpma]

On Thu, Jul 09, 2020 at 01:03:13PM -0700, Joe Drago wrote: You can demand that any image is forced to be made grayscale via `--yuv 400`, but I'd be fine with altering any avifenc input code to detect that the YUV format should be 400 "by default". I'm pretty sure there is a "gray" color space in PNG so that might be easy, but I don't know about JPEG. Is there a grayscale flag somewhere in JPEG? (I'll have to look.)

Indeed, sorry for my lack of attention, should've noticed it.

[vigneshvg]

libavif now supports monochrome. avifenc also uses yuv400 as the yuv format when dealing with grayscale PNG images:

libavif/apps/shared/avifpng.c

Line 321 in e58d3e0

avif->yuvFormat = AVIF_PIXEL_FORMAT_YUV400;