[Libreoffice-bugs] [Bug 156361] Skia regression: semi-transparent .svg in .odp fails to render

[bugzilla-daemon]

https://bugs.documentfoundation.org/show_bug.cgi?id=156361

--- Comment #6 from Patrick Luby <plub...@neooffice.org> ---
(In reply to Patrick Luby from comment #5)
> So, I think mpGraphics->BlendBitmap() needs to use the following blending
> algorithm:
> 
> r = 1 - ((1 - d) + ((1 - d) * (1 - s)))

Disclaimer: the above is a little too simplistic as the source grayscale can
still override the destination grayscale in certain cases. But it hopefully
illustrates what I think is happening.

I think the safest way to blend the alpha masks is to do the following:

1. In mpGraphics->BlendAlphaBitmap(), after drawing is finished, extract the
destination surface's alpha channel from its SkAlphaShader, invert it, and
overwrite the pSalAlphaBmp2 parameter.

2. Remove call to mpGraphics->BlendBitmap() since
mpGraphics->BlendAlphaBitmap() will pSalAlphaBmp2 has been overwritten.

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