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Re: Gamma correction?




-----Original Message-----
From: Greg Sadowski <gregs at warsaw.engr.sgi.com>
To: telecine at alegria.com <telecine at alegria.com>
Date: den 28 februari 1998 09:15
Subject: Gamma correction?


>(I apologize for such a naive question ..)
>
Naive? Not at all, it is an issue which is going to have a major meaning now
that convergence is all around us. Non-CRT based displays are adding to this
too!

>The digital output from a telecine is in the RGB color space - right?


Yes, but non-linear colorspace, it could also be in YCrCb color space
Nevertheless, is should be handled with silk gloves.

>Is there a need for gamma correction to be done in a interface to
>a computer than? - or this is a part of the overall color adjustments?


GENERALLY computers do not understand gamma. Gamma in its fundamental sense
is used at the source to provide an inverse function of the non-linear
transfer function (gamma)  that the monitor electrongun creates such that
our preception becomes linearized. The path between, the transmission line,
shall not alter the transfer function.

The exception is of course when colorists adjust various gamma parameters
for best visual appearance.

Thus, if the computer is to be considered a transmission line then it ought
not alter anything but it must also display what has been input to it in the
way it was intended (coded). So you got a problem. The problem is aggravated
by the fact that most computer manufacturers have their own ways of
"adjusting gamma" by the use of lockup tables. It has been claimed that
these functions often are empirically generated. So,  these functions can be
very difficult to understand...well its messy and what you want to avoid is
having to recalculate non-linear parameters all the time. Errors add up
particularly if the functions are only approximated mathematically.

>Please direct me to the right literature on the subject.

An authority on this subject is Charles Poynton whose files I believe are in
the archives of the TIG.

/mike

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