STR #3489

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STR #3489

Application:FLTK Library
Status:5 - New
Priority:3 - Moderate, e.g. unable to compile the software
Scope:3 - Applies to all machines and operating systems
Subsystem:Image Support
Summary:alpha blending does not work correctly in Fl_Image.cxx
Created By:meixner
Assigned To:Unassigned
Fix Version:Unassigned
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Name/Time/Date Text top right image
#1 meixner
05:44 Aug 20, 2018
The math behind the alpha blending is not correct, as a result colors may be slightly off.

Alpha is in the range 0..255 and the calculation is as follows:

*dstptr++ = (srcg * srca + dstr * dsta) >> 8;

assume srcg==255, srca==255 (->dsta=0)

*dstptr++=(255*255+dstr*0)>>8; // (=>254)

So white becomes light grey although using a completeley opaque color.

To fix the issue in alpha_blend():
- change srca and dsta from unsigned char to unsigned
- add "srca+=srca>>7" after "srca=*srcptr++"
- change "dsta=255-srca" to "dsta=256-srca"

The changes have to be performed for both grayscale + alpha and RGBA.
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