WPF OpacityMask weird behavior

Question

I think I might have found a bug in WPF, or maybe I just don\'t fully understand how OpacityMasks work. As you can see on the first image, I have a diamond shaped path and a red

Answer 1

Clemens solved my issue

Set ViewportUnits="Absolute" on the VisualBrush, and then set its Viewport property to an appropriate rectangle, e.g. Viewport="0,0,100,100"