Impact of colorspace on filters

[svgeesus]

originally noted here

gs to audit to ensure that the platform has good support throughout for out-of-sRGB colors:

• the <feColorMatrix> and <feComponentTransfer> filter primitives, to ensure that there are sensible ways to do those operations on colors outside of the sRGB gamut

[svgeesus]

It is clear, for example from the luminanceToAlpha definition, that the expected inputs are sRGB only:

Note that the default value for color-interpolation-filters is linearRGB which is a linear-light (not gamma-corrected) sRGB space. So this matrix calculates the relative Y value of a linear sRGB input.

[palemieux]

See parent issue at #4