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Re: Gamma correction on G4 PowerBook

On 2003-09-29 13:06:59 +0200, Michel Dänzer wrote:
> No idea, but both the hardware and the XVidMode extension allow for
> more general colour correction than just gamma curves.

Gamma curves should be sufficient (they are on my Risc PC under RISC OS).

> > This is strange because with xgamma, only the image (displayed by
> > Mozilla) seems to be affected, whereas with the -gamma option of
> > XFree86, the whole screen was affected (but the results on the image
> > were similar to what xgamma gives).
> Weird, they should produce the same result given the same value,
> otherwise it's probably a bug.

This may be a feature: pixels that have a known gamma values are
not affected. But this shows that the gamma correction is not a
hardware one, thus giving bad results. In other words, here what
seems to happen: Each RGB color is indexed by 256 values. Instead
of doing a continuous gamma correction on these 256 values (as
output of the graphic card), it seems that the X server does a
mapping 256 values -> 256 values for some pixels, i.e. as if the
pixel values were different. But as in the 256 values (without
hardware gamma correction), most of them are very dark and only
a few ones are bright, bright gradations look very bad.

Vincent Lefèvre <vincent@vinc17.org> - Web: <http://www.vinc17.org/> - 100%
validated (X)HTML - Acorn Risc PC, Yellow Pig 17, Championnat International
des Jeux Mathématiques et Logiques, TETRHEX, etc.
Work: CR INRIA - computer arithmetic / SPACES project at LORIA

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