[FFmpeg-devel] [RFC] swscale RGB24->YUV420P

[John Cox]

Hi

The Pi has a use for a fast RGB24->YUV420P path for encoding camera
video. There is an existing BGR24 converter but if I build a RGB24
converter using the same logic (rearrange the conversion matrix and use
the same code) I get a fate fail on filter-fps-cfr (and possibly others)
which appears to decode a file to RGB24, convert to YUV420P and take the
CRC of that so almost any change to the conversion breaks this
(unrelated?) test.

My initial assumption was that if the code to conversion in
libswscale/rgb2rgb_template:bgr24toyv12_c was good enough for BGR24->YUV
then it should be good enough for RGB24->YUV too. However it breaks this
fate case - what is an acceptable way to resolve this?

A further question assuming that the above can be resolved - I have also
written aarch64 asm for this (RGB24/BGR24->YUV420P). It costs nothing in
the asm to round the output values to nearest rather than just rounding
down as the C template does and doing so improves the accuracy reported
by tests/swscale - however at that point the asm and the C don't produce
identical results. I assume that the x86 asm for BGR24 conversion does
match the C. What is the best thing to do here?

I've tested by hand with libswscale/test/swscale but fate integration
would be obviously better - I'm currently a bit lost in fate, where/how
should I do this?

Many thanks

John Cox