SWSCALE: Quantization Errors in Y Studio Range

[troy_s]

Summary of the bug:

It appears FFMPEG has some +/- single unit quantization errors in the YCbCr transform.

How to reproduce:

Generate a raw YCbCr image and diff against theoretically correct values.

./ffmpeg -i ~/Documents/FFMPEG\ Color\ Research/ARIB_STD-B28.png -pix_fmt yuv444p -vcodec rawvideo -f rawvideo -vf scale="in_range=full:in_color_matrix=bt709:out_range=full:out_color_matrix=bt709" test.yuv
ffmpeg version N-64815-gb53bdae Copyright (c) 2000-2014 the FFmpeg developers
  built on Jul 19 2014 16:33:49 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
  configuration: --enable-gpl --enable-libx264 --disable-asm
  libavutil      52. 92.101 / 52. 92.101
  libavcodec     55. 69.100 / 55. 69.100
  libavformat    55. 48.101 / 55. 48.101
  libavdevice    55. 13.102 / 55. 13.102
  libavfilter     4. 11.102 /  4. 11.102
  libswscale      2.  6.100 /  2.  6.100
  libswresample   0. 19.100 /  0. 19.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, image2, from '/home/aphorism/Documents/FFMPEG Color Research/ARIB_STD-B28.png':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: png, rgb24, 1920x1080 [SAR 2835:2835 DAR 16:9], 25 tbr, 25 tbn, 25 tbc
Output #0, rawvideo, to 'test.yuv':
  Metadata:
    encoder         : Lavf55.48.101
    Stream #0:0: Video: rawvideo (444P / 0x50343434), yuv444p, 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc55.69.100 rawvideo
Stream mapping:
  Stream #0:0 -> #0:0 (png (native) -> rawvideo (native))
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=0.0 Lsize=    6075kB time=00:00:00.04 bitrate=1244160.0kbits/s    
video:6075kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%

Diff'd versions of the theoretically ideal values versus FFMPEG's output. Lowest red stripe is due to theoretical version preserving the -2% patch, and not relevant.