A box's own pixels had to be opaque, but the high-pass that measures its
grain reads 3 px around it, and a transparent pixel there is black once
unpremultiplied. A blur earlier in the stack pads the layer with
transparent pixels, so boxes along that border measured the edge as
strong, smooth colour grain, and Match Grain drew 12 px colour cells
from a clean source on the frames that picked them. Boxes now need their
3 px surround opaque too (a summed-area table keeps the check O(1) per
candidate). Remove Grain picks its boxes the same way.
JonRC
c6ab02b83a