wiki articles on Wrong levels and colors upon playback

[Wilbert]

In the past few weeks i added two articles on wiki:

* Luminance levels (http://avisynth.org/mediawiki/Luminance_levels)
* Colorimetry (http://avisynth.org/mediawiki/Colorimetry)

The first one is the one from clsid (http://forum.doom9.org/showthread.php?t=143689) with some modifications and additions. The second one is from myself.

Please read them through and tell me whether they are helpful for understanding the issues, clear, correct, complete, etc ...

[Heaud]

I wish this was made several months back when I had no clue on how to fix my luma playback problem. This will be great for people who need the information, I'll use this to inform my friends if they fall into the VMR9 luma problem.

Thanks!

[hanfrunz]

please have a look at:
"YUV and luminance considered harmful: A plea for precise terminology in video" (http://klee.usr.dico.unimi.it/~dan/grafica/doc/articoli/YUV_and_luminance.pdf)

[Wilbert]

Yes, i'm aware of that. This holds for RGB as well (which is actually non-linear R'G'B').

The ideal way to accomplish these goals would be to form a luminance signal by matrixing RGB, then subjecting luminance to a nonlinear transfer function similar to the L* function.

There are practical reasons in video to perform these operations in the opposite order. First a nonlinear transfer function - gamma correction - is applied to each of the linear R, G and B. Then a weighted sum of the nonlinear components is computed to form a signal representative of luminance. The resulting component is related to brightness but is not CIE luminance. Many video engineers call it luma and give it the symbol Y'. It is often carelessly called luminance and given the symbol Y. You must be careful to determine whether a particular author assigns a linear or nonlinear interpretation to the term luminance and the symbol Y.
R'G'B', Y'CbCr and Luma. The documentation (not only the above two pages, but all documentation) should be cleaned up to avoid confusion. I have to think about how to do that properly ...