Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!bu-cs!tjh
From: tjh@bu-cs.BU.EDU (Timothy Hall)
Newsgroups: comp.sys.sgi
Subject: Re: Overlays, underlays
Message-ID: <37228@bu-cs.BU.EDU>
Date: 28 Aug 89 21:29:23 GMT
References: <20790@uflorida.cis.ufl.EDU> <41054@sgi.sgi.com>
Reply-To: tjh@buit5.bu.edu (Timothy Hall)
Followup-To: comp.sys.sgi
Distribution: usa
Organization: Boston University
Lines: 18

In article <41054@sgi.sgi.com> woo@sharona.csd.sgi.com (Mason Woo) writes:
>In article <20790@uflorida.cis.ufl.EDU>, pff@beach.cis.ufl.edu (Pablo Fernicola) writes:
>> Are overlays and underlays available on the Personal Iris?  
>> If they are not available, can I achieve the same results using writemasks?
>
>You cannot achieve the same results with writemasks with an RGB application.
>RGBwritemask would preserve bitplanes which would be directly used to 
>display colors.  You couldn't draw into these bitplanes and then overlay
>the other RGB bitplanes.
>
True, but if one is willing to give up the LSB of the RGB planes it is
possible to fake 3 more overlay planes.  So you could use 
RGBwritemask( 0xfe, 0xfe, 0xfe ) when writing to the RGB planes and
RGBwritemask( 1, 1, 1 ) when writing to the "overlays".  Use the gammaramp
function to set what your overlay colors will be.

-Tim
tjh@bu-pub.bu.edu