Re: [css-images] Gamma errors in image downscaling

On Sun, Nov 9, 2014 at 12:06 PM, Tavmjong Bah <tav.w3c@gmail.com> wrote:

> On Sun, 2014-11-09 at 14:15 -0500, Lea Verou wrote:
> > A colleague brought this [1] to my attention, which seems to
> > demonstrate an image scaling bug in pretty much every browser I tried
> > the testcase [2] on. Furthermore, the results across UAs are
> > inconsistent, which might indicate a standardization opportunity. I,
> > myself, have noticed similar errors when scaling down certain patterns
> > in the past, so stumbling on these issues is not that rare.
> > Applying different values to the image-rendering property seems to
> > affect the results, but no value produces a correctly downscaled image
> > (including the various properietary values…).
> >
> >
> > Shouldn’t image-rendering: crisp-edges fix this? Is there any spec
> > violation here at all, or is that part of image scaling undefined? Any
> > thoughts?
>
> This has to do with using the srgb color space to do scaling. Scaling
> should be done in a linear color space. 'crisp-edges' does not fix this
> as it doesn't specify which color space to use.
>
> We briefly discussed this at the joint CSS/SVG meeting at TPAC last week
> although this part doesn't seem to be minuted. (I showed the bottom of
> [3] at the meeting.) High quality image processing software (Photoshop,
> Gimp, etc.) has already moved to linear colors spaces, the web should
> follow as resources permit.
>

I just installed GIMP and it doesn't seem to have this option. Instead, it
just subsamples as usual.
Do you know where to enable this in GIMP?


> At the moment there is no CSS property to hint that a linear color space
> should be used for scaling (actually, a linear color space should always
> be used so a property should not be needed). SVG does have the
> 'color-interpolation' property value 'linearRGB' to indicate
> interpolation (as in gradients, animations, and alpha compositing)
> should be done in a linear color space but nobody, that I know of,
> implements it. (The SVG 'color-interpolation-filters' property value
> 'linearRGB' is widely implemented as this is the default value for
> filters.)
>
> Tav
>
> > [1]: http://www.4p8.com/eric.brasseur/gamma.html
> > [2]: http://www.4p8.com/eric.brasseur/gamma_dalai_lama.html
>
> [3] http://tavmjong.free.fr/SVG/COLOR_INTERPOLATION/index.html
>
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