"Namely, I want to combine the above scrolling image with one
which has the inverse of the above alpha channel applied on the
same image but against a different background."
Correction: Its not the inverse, but the complement that I
think I mean.
if x is the original alpha for a pixel, then 255-x is the
value in the new alpha channel (instead of ~x). This may explain
why the visual results with BlendMode.Erase are not exactly
correct, since it merely inverts the alpha channel, but as I said
previously the real problem with it is the speed.