Stitch Inpaint takes an RGB patch over an RGBA picture and keeps the
picture's own alpha, instead of stopping on a channel mismatch. The halo
fix spreads colours only.
Image Compare keeps alpha in a preview that has see-through parts and
flags it, so the panel shows them over a checkerboard (Slide, A/B and
Erase). The Erase mix lays the top picture over the other, the saved PNG
is RGBA, and the PSD carries each layer's transparency plus a merged
alpha, its flat colours stored over white as Photoshop does. A see-through
B is resized premultiplied. Align measures over gray and warps all four
channels. Solid pictures give byte-identical files.
Refs #232
ausboss
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