Bad documentation on chroma-derived-nc and friends

[Mingye Wang]

Summary of the bug:
The texinfo documentation on chroma-derived-{nc,c} is bad. In fact, it's worse than what we have in pixfmt.h. We should replace these all with pixfmt.h documentation.

How to reproduce:
Navigate to:

• https://ffmpeg.org/ffmpeg-codecs.html#Codec-Options, which has one instance of these options. Here we see chroma-derived-nc explained as "Chroma-derived NCL", which is very unhelpful compared to ​what pixfmt says. In general, the descriptions for color_primaries, color_trc, colorspace are of lower quality than their pixfmt.h version, so I recommend that they all be replaced.
  • While we are at it, the texinfo description for color_range is straight up wrong. pixfmt again has it right.
• https://ffmpeg.org/ffmpeg-filters.html#setparams-1, which has another instance. This one is just blank.

[Balling]

chroma-derived-nc is just a matrix that depends on primaries and white point. Every combination of primaries and white point leads to special matrix, like how CIE D65 + BT.709 is BT.709 matrix or Illuminant C + BT.470 primaries make BT.601 matrix.

Nc means it is using NC, non constant transfer function. There is also CL, where transfer works differently, like in BT.2020CL.

[Mingye Wang]

Yes, I get that. But the thing is, the user-facing documentation in the texinfo pages is worse than the programmer-facing document in pixfmt.h. (Have you clicked the link "what pixfmt says"? Because that goes right to the line for AVCOL_SPC_CHROMA_DERIVED_NCL in pixfmt.h.)

For chroma-derived-nc, texinfo says "Chroma-derived NCL", while pixfmt says "Chromaticity-derived non-constant luminance system". That's much better.

For color_range mpeg, texinfo says MPEG (219*2^(n-8)), while pixfmt very carefully tells you (219 * E + 16) * 2^(n-8) for luma and (224 * E + 128) * 2^(n-8) for chroma. That's not only better, but also more correct.

[Balling]

is worse than the programmer-facing document in pixfmt.h.

That is because I fixed the text in header file.