Correct implementation of "msAlphaBlendPM".#7358
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ThomasCusson wants to merge 1 commit intoMapServer:mainfrom
Open
Correct implementation of "msAlphaBlendPM".#7358ThomasCusson wants to merge 1 commit intoMapServer:mainfrom
ThomasCusson wants to merge 1 commit intoMapServer:mainfrom
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Hey ! |
ThomasCusson
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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 tomsAlphaBlend.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.