[Cin] Two links on ICC in MP4

[Georgy Salnikov]

On Fri, 11 Apr 2025, Andrew Randrianasulu wrote:

> > As far as I understand in this case idea was to embed display profile into
> > media. But few apps outside of Final Cut Pro/quicktime can do anything
> > useful about this info?
> >
> > It states that instead of more flexible color management video world used
> > few fixed mode monitor color spaces, and whole process was a bit manual if
> > your input image/video was not in one of those.

CMS for photography & publishing is logically coherent. There is input
profile, image profile, output profile. The input profiles are that without
which it is hardly possible to process RAW input. The profiles embedded into
images say nothing about displays, they just are to specify in which
colorspace the image data are if they are not in sRGB (and even if it is
sRGB, it is common now to embed sRGB into it explicitly). The output
profiles are just for the particular output devices (for displays, for
printing), they make no assumptions about the nature of the data for their
output. Concerning displays, in X11 there are mechanisms long ago to make
some color correction, such as xgamma, now it can be done by colord or
Argyll, but for the best accuracy the photo processing program itself should
do it. Anyway, it is not a big deal: photo processing does not demand real
time picture refresh, the math needs not be extremely fast, everything is
optimized for highest quality, not for highest speed.

The first problem of CMS for video is the need for the real time
visualization, the whole math must be fast. The second is a real zoo of
display formats, hardware accelerators, etc.

For example, let's imagine, some external tool like xgamma or colord has the
necessary calibration and can, in principle, correct our video on the
display.

But what if the user has attached not a DVI monitor, but a real TV to the
composite video output, for example, what signal will be generated there by
the video card?

These gamma (LUT) corrections are applied through the X11 VidModeExtension.
But will they be applied if rendering is done via the Xv extension, for
example, or via OpenGL?

Let's imagine, we have implemented some kind of CMS in our program. Then we
like, for better efficiency, make use of hardware accelerated video decoding
like VDPAU, LIBVA or something like these. Now decoding is done not under
control of our implementation, but by some (proprietary) driver/firmware on
the video card. And we do not know exactly what happens there.

May be, the safest CMS for video would be to output some good standard
picture on the monitor, to adjust it manually for the most precise colors,
and then use these adjustments untouched. Like many years ago I ran some
tools for adjustments of xgamma, but actually adjusted not xgamma
parameters, but the CRT monitor settings themselves.

Perhaps, I am a pessimist...
_______________________________________________________________________________

Georgy Salnikov
NMR Group
Novosibirsk Institute of Organic Chemistry
Lavrentjeva, 9, 630090 Novosibirsk, Russia
Phone   +7-383-3307864
Email   sge at nmr.nioch.nsc.ru
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