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Windows 11 AlphaBlend Transparency Bug (AlphaFormat=0) - Solution: Use AC_SRC_ALPHA with Premultiplied Alpha
![](data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' height='64' class='font-weight-bold' style='font: 600 30.11764705882353px "SegoeUI", Arial' width='64'%3E%3Ccircle fill='hsl(243.2, 28.000000000000004%, 33%)' cx='32' cy='32' r='32' /%3E%3Ctext x='50%25' y='55%25' dominant-baseline='middle' text-anchor='middle' fill='%23FFF' %3EC%3C/text%3E%3C/svg%3E)
Celicom
0
Reputation points
Its a duplicate of my post on SO, but I hope it may help someone here too.
After some Windows 11 updates, I have recently ran into a problem where GDI’s AlphaBlend function stopped working correctly when BLENDFUNCTION parameters SourceConstantAlpha != 0 and AlphaFormat = 0. It sporadically produces incorrect transparency output. I have an "Intel Iris Xe" video card where I cannot disable the HUGS (Hardware-Accelerated GPU Scheduling) to test if it is the root-cause. I wonder if anybody has this problem and how it is resolved.
The issue disappeared in the "Safe Mode" or when adding a Sleep(20) around the AlphaBlend calls but that’s not a real fix. The working solution at the moment is to use only pre-multiplied ARGB source bitmap with SourceConstantAlpha = 255 and AlphaFormat = AC_SRC_ALPHA in the BLENDFUNCTION parameter - then AlphaBlend works fine.
Windows development | Windows API - Win32
{count} votes
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![](data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' height='64' class='font-weight-bold' style='font: 600 30.11764705882353px "SegoeUI", Arial' width='64'%3E%3Ccircle fill='hsl(262.40000000000003, 13%, 35%)' cx='32' cy='32' r='32' /%3E%3Ctext x='50%25' y='55%25' dominant-baseline='middle' text-anchor='middle' fill='%23FFF' %3EDR%3C/text%3E%3C/svg%3E)
Darran Rowe 2,156 Reputation points2025-06-30T12:46:45.5766667+00:00 Are you sure that this is Windows and not a video driver issue?
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Celicom 0 Reputation points
2025-06-30T15:26:38.98+00:00 I think it is caused by the Windows updates because the driver did not change (Intel Iris Xe Graphics, v31.0.101.4255 from 2023).
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Darran Rowe 2,156 Reputation points
2025-07-02T20:55:49.0633333+00:00 It is difficult because a long standing bug in the video drivers hasn't been ruled out.
As an example, if there was a race condition in the video drivers, then a Windows update could have changed timings enough that this issue almost always presents itself. It should be easy to verify, since if you can reproduce this on a system with a different video driver, preferably a different GPU manufacturer, then it is a Windows issue.
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