[filter-effects] Clarify mapping of Cs and Cb to in and in2 in feBlend

[BigBadaboom]

https://www.w3.org/TR/filter-effects/#element-attrdef-feblend-mode

The definition for the mode attribute of <feBlend> states:

One of the blend modes defined by “Compositing and Blending Level 1” [COMPOSITING-1]

However that specification defines the blend functions in terms of source (Cs) and backdrop (Cb) parameters. We should specify how in and in2 map to those two values.

From testing (Chrome and FF) it appears that Cs maps to in, and Cb maps to in2.

[AmeliaBR]

That's a very good point. I can never remember whether in or in2 should be "on top". (I always think the second input should go on top of the first).

PS, I checked and SVG 1.1 wasn't much clearer: the formulas use image A (top for normal blending) and image B (background), without explicitly saying which one is in versus in2. But that is presumably where the "second image is the background" interpretation comes from.

[dirkschulze]

I added a sentence for feBlend and feComposite that shall make it more obvious which input maps to source or destination/backdrop.