Re: [csswg-drafts] [css-color] [filter-effects] Should no-op filters produce different output from no filter? (#7100)

Certainly in 1998-2000 the SVG spec was created as sRGB-only. The [`color-interpolation-filters`](https://drafts.fxtf.org/filter-effects/#propdef-color-interpolation-filters) property has the values` auto` | `sRGB` | `linearRGB` where `linearRGB` is the initial value, and corresponds to the CSS Color 4 [`srgb-linear`](https://drafts.csswg.org/css-color-4/#predefined-sRGB-linear) colorspace which (like all the predefined RGB spaces there) is unbounded.

There is an open issue [SVG color-interpolation-filters to use more color spaces]( https://github.com/w3c/fxtf-drafts/issues/448) which could be solved by adding additional values to `color-interpolation-filters`. As you say, another option is to allow extended sRGB as a way to access WCG colors, which would then not require new values on that property but would require careful review of wording about clipping out-of-range values. Such as this one:

> The RGBA result from each [filter primitive](https://drafts.fxtf.org/filter-effects/#filter-primitive) **will be clamped into the allowable ranges for colors and opacity values**. Thus, for example, the result from a given filter primitive will have any negative color values or opacity values adjusted up to color/opacity of zero.
https://drafts.fxtf.org/filter-effects/#FilterPrimitivesOverviewIntro

Note that processing in unbounded `srgb-linear` should give the same result as processing in CIE XYZ, both are linear-light.

Notice too that [feColorMatrix](https://drafts.fxtf.org/filter-effects/#feColorMatrixElement) has the `sRGB-linear` to `XYX` conversin matrices baked in for saturate, hue rotate and luminance. So using unbounded `srgb-linear` as the filter working space does seem a promising approach.

The only worry is whether existing content is _depending on the clamping behavior_; if so, some form of opt-in might be needed. Perhaps a `linearRGBextended` value, or some such?

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Received on Friday, 4 March 2022 19:19:13 UTC