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From: keithd@cadovax.UUCP (Keith Doyle)
Newsgroups: comp.sys.amiga
Subject: Another Blitter Question
Message-ID: <1808@cadovax.UUCP>
Date: Fri, 16-Oct-87 13:04:25 EDT
Article-I.D.: cadovax.1808
Posted: Fri Oct 16 13:04:25 1987
Date-Received: Sun, 18-Oct-87 08:32:28 EDT
Reply-To: keithd@cadovax.UUCP (Keith Doyle)
Organization: Contel Business Systems, Torrance, CA
Lines: 31


Here's another one for the blitter gurus out there: 

Ok.  With the blitter, if you are doing a 2 word transfer out of a
source image, to a destination image which becomes a 3 word transfer
because of the alignment has changed because the image is being
moved, you have to setup to do a 3 word transfer.  Problem:  How
do you correctly apply the last-word-mask, which you normally
apply to the source, when the word you really want to mask from
the source is the next-to-last-word.  The last word from the
source needs to be completely masked out, along with some trailing
bits in the previous word.

Here'a a visual example:

src words:  |____xxxxxxxxxxxx|xxxxxxxxxxxx____|________________|

dst words:  |____________yyyy|yyyyyyyyyyyyyyyy|yyyy____________|

If you are blitting xxxxx to yyyyy, (using shift etc. to re-align
the bits) you can see that the 3rd word of the source contains no
useful information, and the last-word-mask wants to be applied
to the second source word, not the last.  How do you do this?
Do you have to use up one of your precious blitter sources to
do this with a mask plane?  If so, it seems that the last-word-mask
concept is pretty useless for arbitrary transfers.  Or is there some 
way to accomplish this more directly?


Keith Doyle
#  {ucbvax,decvax}!trwrb!cadovax!keithd  Contel Business Systems 213-323-8170