[filter-effects] Poor interop of `<feComposite operator="arithmetic">` with large k values

[smfr]

Mozilla's ref test analyzer uses SVG filters to highlight differences between two images. One filter primitive is:

<feComposite result="c" in="c1" in2="c2" operator="arithmetic" k2="255" k3="255"/>

The arithmetic operator has poor interop, possibly because of rounding issues. Test case:

        <filter id="filter" filterUnits="objectBoundingBox" x="0" y="0" width="100%" height="100%">
            <feFlood x="10" y="10" width="200" height="200" flood-color="rgb(128, 10, 0)" flood-opacity="0.5" result="first" />
            <feFlood x="100" y="100" width="200" height="200" flood-color="rgb(10, 128, 0)" flood-opacity="1" result="second" />
            <feComposite in="first" in2="second" operator="arithmetic" k2="255" k3="255" />
        </filter>

This gives different results in each major browser:

Chrome's results even change based on the screen color profile.

[karlcow]

feComposite is now being handled by the css wg.
https://drafts.fxtf.org/filter-effects/#feCompositeElement

operator="arithmetic"
https://drafts.csswg.org/filter-effects-1/#element-attrdef-fecomposite-operator

The compositing operation that is to be performed. All of the operator types except arithmetic match the corresponding operation as described in [COMPOSITING-1] with in representing the source and in2 representing the destination. The arithmetic operator is described above.

The initial value for operator is over.

The above definition is:

If the arithmetic operation is chosen, each result pixel is computed using the following formula:

result = k1*i1*i2 + k2*i1 + k3*i2 + k4
where:

i1 and i2 indicate the corresponding pixel channel values of the input image, which map to in and in2 respectively
k1, k2, k3 and k4 indicate the values of the attributes with the same name

@fantasai @smfr should this be handled in discussions by the CSS WG?

[smfr]

Yes, it should.