Is your original mask in picture1 or in picture1.mask?
(your “A mask is a picture” statement let me think your mask is drawn directly in picture1, not its mask )
example of the scenario:
Picture1 is a photo of an apple.
It has a mask which correctly masks the apple
The mask is (obviously??) a picture in its own right, although a mask doesnt have its own mask.
So we have Picture1 with both Picture and Mask which I can see in the debugger.
Picture2 is (for example) a face, of the same dimensions.
It has no mask.
I issue Picture2.mask = new picture( picture2.width, picture2.height, 32)
Now it has a mask
I issue picture2.mask.graphics.drawpicture picture1.mask,0,0 etc
The result is that in the debugger, picture2.mask is pure white.
(Sometimes I think I saw Picture2 become white also … almost as if the drawing went to the picture graphics, not the mask)
I haven’t tried ApplyMask yet, but from the documentation it also wants to be passed a picture, so I’m not hopeful.