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From: keithd@cadovax.UUCP (Keith Doyle)
Newsgroups: comp.sys.amiga
Subject: Re: Another Blitter Question
Message-ID: <1818@cadovax.UUCP>
Date: Mon, 19-Oct-87 17:11:43 EDT
Article-I.D.: cadovax.1818
Posted: Mon Oct 19 17:11:43 1987
Date-Received: Wed, 21-Oct-87 07:25:38 EDT
References: <1808@cadovax.UUCP> <678@rocky.STANFORD.EDU>
Reply-To: keithd@cadovax.UUCP (Keith Doyle)
Organization: Contel Business Systems, Torrance, CA
Lines: 37

In article <678@rocky.STANFORD.EDU> rokicki@rocky.UUCP (Tomas Rokicki) writes:
>> src words:  |___xxxxxxxxxxxxx|xxxxxxxxxxx_____|________________|
>> 
>> dst words:  |____________yyyy|yyyyyyyyyyyyyyyy|yyyy____________|
>
>In this case, you again set up a three-word transfer.  The source
>comes from B; the destination comes from C and goes to D.
>Here we use a shift of 10.  We need to mask the destination this
>time (since it spans the three words), so we set the A shift equal
>to 0, the FWM to 0x000f, and the LWM to 0xf000.  You blit again,
>and everything works.

This implies that if the A shift is 0, then the FWM and LWM are applied
after shifting?  No, more likely since the FWM and LWM seem to be applied
before shifting if you ARE shifting A, that you are assuming that A will
always be 0xffff, and the FWM, LWM will be generated by masking it at
the appropriate times.  Unfortunately, I am trying to use the A channel 
as a source mask plane which won't allow me to use it in that manner.
Looks like what I'm trying to do can't be done  (A=src mask plane, B=src,
C=dest, arbitrary rectangle blit)  The system routine that does this 
flashes horribly, must be because it has to do it in two passes or
something.

>Get BlitLab on one of the fish disks; it has full documentation on
>how the blitter works.  

That's where I started.  The stuff on the shifting and FWM/LWM was
a little to terse to glean such answers.


>Oh, and that use of the A channel for masking; that's free in terms of time.

But if I had other plans for it, it's not so free.  I have to take the time
that it takes to do two operations.  Oh well.

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