[FFmpeg-trac] #2394(swscale:new): BGRA downscaling shift colors

[FFmpeg]

#2394: BGRA downscaling shift colors
--------------------------------------+---------------------------------
               Reporter:  Cigaes      |                  Owner:
                   Type:  defect      |                 Status:  new
               Priority:  normal      |              Component:  swscale
                Version:  git-master  |               Keywords:  bgra
             Blocked By:              |               Blocking:
Reproduced by developer:  0           |  Analyzed by developer:  0
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 Downscaling in BGRA produce a color shift.

 How to reproduce:
 {{{
 ./ffmpeg_g -lavfi color=c=blue:s=256x256,format=bgra,scale=4:4 \
   -vframes 1 -f rawvideo - | xxd
 }}}
 {{{
 ffmpeg version N-51202-gf8217da Copyright (c) 2000-2013 the FFmpeg
 developers
   built on Mar 21 2013 20:43:35 with gcc 4.7 (Debian 4.7.2-5)
   configuration: --enable-shared --disable-static --enable-gpl --enable-
 libx264 --enable-libass --enable-libfreetype --enable-libopus --assert-
 level=2
   libavutil      52. 22.100 / 52. 22.100
   libavcodec     55.  1.100 / 55.  1.100
   libavformat    55.  0.100 / 55.  0.100
   libavdevice    55.  0.100 / 55.  0.100
   libavfilter     3. 48.100 /  3. 48.100
   libswscale      2.  2.100 /  2.  2.100
   libswresample   0. 17.102 /  0. 17.102
   libpostproc    52.  2.100 / 52.  2.100
 Output #0, rawvideo, to 'pipe:':
   Metadata:
     encoder         : Lavf55.0.100
     Stream #0:0: Video: rawvideo (BGRA / 0x41524742), bgra, 4x4 [SAR 1:1
 DAR 1:1], q=2-31, 200 kb/s, 90k tbn, 25 tbc
 Stream mapping:
   scale -> Stream #0:0 (rawvideo)
 Press [q] to stop, [?] for help
 frame=    1 fps=0.0 q=0.0 Lsize=       0kB time=00:00:00.04 bitrate=
 12.8kbits/s
 video:0kB audio:0kB subtitle:0 global headers:0kB muxing overhead
 0.000000%

 0000000: ed00 00f6 ed00 00f6 ed00 00f6 ed00 00f6  ................
 0000010: e700 00f4 e700 00f4 e700 00f4 e700 00f4  ................
 0000020: fd00 00ff fd00 00ff fd00 00ff fd00 00ff  ................
 0000030: fd00 00ff fd00 00ff fd00 00ff fd00 00ff  ................
 }}}

 The same thing with RGBA instead of BGRA produces uniform {{{0000 fdff}}},
 which seems correct.

 The phenomenon increases with the downscaling factor. The output size,
 OTOH, does not matter much. Only the last two lines are exempt.

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2394>
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