Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!usc!apple!sun-barr!oliveb!tymix!tardis!jms
From: jms@tardis.Tymnet.COM (Joe Smith)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Blit Wanted
Summary: First calculate the mask, then copy the source
Message-ID: <1113@tardis.Tymnet.COM>
Date: 29 May 90 03:21:46 GMT
References: <3074@goanna.cs.rmit.oz.au>
Reply-To: jms@tardis.Tymnet.COM (Joe Smith)
Organization: BT Tymnet, San Jose, CA
Lines: 20

In article <3074@goanna.cs.rmit.oz.au> phi@goanna.cs.rmit.oz.au (Philip Hingston) writes:
>I need a blit where the source overrides the destination except where the
>source has colour 0, when the destination show through. Is there a blit for
>this or must I make my own? Thanks.		phi

Tom Rokicki will probably provide more info on this subject.

Since the blitter operates as "dest=function_of(dest,source,mask)", you will
have to first calculate the mask which indicates where the color_0 pixels
are located in the source.  For instance:
	mask = all zeros
	for 2 bit-planes at a time do
	  mask = mask OR plane1 OR plane2
	endfor  (mask now has 0 where ever the source is color_0)
	dest = (dest AND NOT mask) or (source AND mask)
-- 
Joe Smith (408)922-6220 | SMTP: jms@tardis.tymnet.com or jms@gemini.tymnet.com
BT Tymnet Tech Services | UUCP: ...!{ames,pyramid}!oliveb!tymix!tardis!jms
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