Correct implementation of "msAlphaBlendPM". - #7358
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…tiplied, no need to multiply them again. Compliance with the optimized approach of msAlphaBlend (using 256 instead of 255)
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thanks @ThomasCusson . Please also include a test for this as part of your pull request (you can see a few other COMPOP tests in /msautotest/renderers/). Find more info about msautotest at https://mapserver.org/development/tests/autotest.html |
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At our company we noticed, using the composite "src-over" blending mode (default), that some darkening is happening when opacity is not 0% or 100%. This is a typical problem of pre-multiplied alpha, so I looked into it.
It seems that the msAlphaBlendPM (PM stands for Pre-Multiplied) acts as if the bands were not pre-multiplied already, in a fashion similar to msAlphaBlend.
It results in a double multiplication by alpha channel, producing colors that are too dark on half-transparent areas. This issue probably arise only when using a change of projection, as it is only called in mapresample.c.
I also used "256" values instead of 255, to match the implementation from msAlphaBlend, but maybe it's not useful, I'll leave that up to your judgement.