On AHRM there's something on the topic but it seems more confused than I am:
Quote:
Warning:
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Conditions 2 and 3 can be contradictory if, for instance, you are
trying to move an image one pixel down and to the right. In this
case, we would want to use descending mode so our destination does
not overwrite our source before we use the source, but we would want
to use ascending mode for the right shift . In some situations, it is
possible to get around general guideline 2 above with clever masking .
But occasionally just masking the first or last word may not be
sufficient; it may be necessary to mask more than 16 bits on one or
the other end. In such a case, a mask can be built in memory for a
single raster row, and the A DMA channel enabled to explicitly fetch
this mask. By setting the A modulo value to the negative of the width
of the mask, the mask will be repeatedly fetched for each row.
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http://amigadev.elowar.com/read/ADCD.../node0121.html
If anyone could point to a working example maybe I can figure this out.