[FFmpeg-trac] #979(swscale:open): Unexpected Color Conversion (bgr->yuv vs rgb->yuv); first is bad, bmp affected (was: Unexpected Color Conversion (bgr->yuv vs rgb->yuv))

[FFmpeg]

#979: Unexpected Color Conversion (bgr->yuv vs rgb->yuv); first is bad, bmp
affected
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             Reporter:  natt        |                    Owner:
                 Type:  defect      |                   Status:  open
             Priority:  normal      |                Component:  swscale
              Version:  git-master  |               Resolution:
             Keywords:  bmp         |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |
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Changes (by Balling):

 * keywords:   => bmp


Comment:

 This is fixed by -vf scale=flags=accurate_rnd (https://ffmpeg.org/ffmpeg-
 all.html#Scaler-Options), or more accurately accurate_rnd+whatever
 algorithm you want. WRONG documentation, BTW, you can use them together.

 255, 255, 255 BGR value is converted to (as can be checked by ffplay -i
 file -vf extractplanes=y) YCbCr values 234, 127, 127 (instead of good 235,
 128, 128). So it is just an off-by-one somewhere. And no, it is not out-
 of-gamut. I suppose the bug is indeed somewhere in second line about
 special "unscaled" converter, whatever that means (you can print those
 with +print_info):

 {{{
 [swscaler @ 00000275e7aa00c0] bicubic scaler, from bgr24 to yuv420p using
 MMXEXT
 [swscaler @ 00000275e7aa00c0] using unscaled bgr24 -> yuv420p special
 converter
 }}}
 as there is no this second line with print_info+accurate_rnd.

 What I am concerned with are JPEGs: they are also affected (?), need to
 check RGB one.

 Also see duplicate #8056: bmp is affected too, since it uses BGR.

 I also clarified what is the bad one: bgr, not rgb.

 P.S. -vf scale=out_range=pc is not affected.
 P.P.S. If accurate_rnd is bit perfect, we just need to switch as default
 for '''only''' bgr formats. Simple as that.

--
Ticket URL: <https://trac.ffmpeg.org/ticket/979#comment:12>
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