The described behavior doesn't occur when blending two 32 bit Forms.
Would you elaborate on your scenario? How do those lower-depth forms come to be these days?
We use Pen
, which is a subclass of BitBlt
. More specifically, we use Pen>>roundNib:
among others to create a circular brush for painting on a Form
. This method creates a Form
with depth 1
. One can then set the color of the brush with Pen>>color:
, which calls BitBlt>>fillColor:
which in turn sets its halftoneForm
. According to the class comment of BitBlt
, the pixel values of the halftoneForm
are ANDed with the sourceForm
pixels before the blending rule is applied.
A workaround for us might be to use Pens with 32 bit sourceForms, if that is possible. I only noticed the different behavior of 32 bit forms while trying to create a code snippet to reproduce the problem, so I haven't tried this workaround yet. Also, I think that the problem still occurs when using alpha values between 1
and 254
, even if those might not be as noticeable.
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