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List:       xfree-render
Subject:    [Render] Re: Experimental Xft + gamma correction patch
From:       Owen Taylor <otaylor () redhat ! com>
Date:       2002-07-06 21:52:58
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Tor Andersson wrote:

> Owen Taylor <otaylor@redhat.com> wrote:
> 
> > I've put up some screenshots at:
> >
> > http://people.redhat.com/otaylor/fonts/gamma-screenshots/
> 
> Hmm. On my 12" iBook with its default screen settings, the original,
> non-gamma-corrected image looks much better than the others.
> The gamma corrected version has too thick vertical and horizontal
 >stems and the diagonals all disappear (and the reverse is true on the
> white-on-black).

Note that the effective Macintosh gamma is 1.8, not 2.2, so this
may be why things look bad for you. (Recent HTML specs are *supposed*
to fix this for web pages, but I'm not sure if that's true in practice.)

Unfortunately, the results are clearly fairly dependent on monitor;
it's possible that this could be adjusted for either with a 
preprocessing step on the glyph grayscale data or by deliberately
undershooting the full gamma correction.

> but I think that might be because of the hinting. Do you have an image
> with the unhinted fonts to compare with? Both non-gamma corrected and
> gamma corrected?

Sorry, I don't have the gamma-correcting Xft hack compiled currently
to generate such screenshots. Hinting problems are certainly
very much part of the uneveness - but they should about equally
make diagonal stems too thin and too thin. On my monitor at least, for
uncorrected text all the diagnonals look too thick for black on white 
and most look too thin for white on black.

> IMO, all compositing and rendering should be done in (which?) linear space,
> and only be corrected for output media in a final conversion. Preferably by
 >setting a transfer function in the X server.

Setting your output gamma to be 1.0 is "just wrong" because the
human eye doesn't have a linear response. If you store images
with linear intensities, then you are throwing away much of your
possible color resolution. (References can be found with a 
web search; try "gamma faq" on google.)

Regards,
                                        Owen
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