Re: Linear Colorspace

----- Sam Wilson <[email protected]> wrote:
> On 02/12/15 17:24, Bob Friesenhahn wrote
> > Use:
> >
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> >     image.gamma(0.454545);  // 1/2.2
> >     [ do something ]
> >     image.gamma(2.2);
> >
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Good enough for me for now. On a technical level, isn't this an 
> approximation since sRGB doesn't have a constant gamma?

Yes.  It's a pretty good approximation except in the deep shadows.

> > The above should accomplish what the article recommends.  Keep in mind
> > that whenever you apply a gamma correction change, the image is
> > subject to quantization error (new value is subject to the number of
> > discrete levels available).  The quantization error will be reduced by
> > using a Q:16 build.
> Right. I selected a quantum of 16 when I built the library for just this 
> reason.
> > There are factors other than gamma to consider such as the colorspace
> > used.  This is a reason why there is a note in the documentation for
> > -colors which says:
> >
> > "Empirical evidence suggests that distances in color spaces such as
> > YUV or YIQ correspond to perceptual color differences more closely
> > than do distances in RGB space.  These color spaces may give better
> > results when color reducing an image."
> >
> > The same issues apply to resize and any other operations which blend
> > pixels.
> Are you suggesting that resizing in YUV or YIQ would yield better (or at 
> least) the same results?

It's possible.  They might make a mess of the Dalai image, though, which
is designed to tell whether gamma was used.  I might do some experimenting
and report back, but not today.

Glenn

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