landing "Convert internal vcl bitmap formats transparency->alpha (II)"

[Patrick Luby]

On 3/13/23 5:24 AM, Noel Grandin wrote:
> 
> However, there is one small glitch - there is a unit test which I have had
> to disable on macos, because neither myself Patrick Luby have been able to
> reproduce the failure locally on our macs, but it fails quite reliably on
> jenkins.
> 

Don't know if this is a solution for Jenkins builds, but I have recently 
switched to building on macOS with the following:

make && make check

For me, it seems that the unit tests fail far less frequently if make 
without check has run to completion first.

> I expect that at some point someone will log a regression bug which will
> lead more directly to the problem and then I can fix it.
> 

I haven't looked, but the first question that comes mind is if the 
failing test is drawing an image with an alpha mask.

On macOS, there is something unexpected happening when drawing 32 bit 
images with an alpha mask. In the attached screen snapshot running on a 
Retina display, it looks like the mask itself is being drawn and then 
the bitmap is drawn on top with the edges of the mask causing the edges 
of the image to not match up with the mask.

So maybe the test doesn't pass on Jenkins because it is using a 
non-Retina display to draw an image with an alpha mask?

Patrick
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