Overlays, underlays

[Timothy Hall]

In article <41054 at sgi.sgi.com> woo at sharona.csd.sgi.com (Mason Woo) writes:
>In article <20790 at uflorida.cis.ufl.EDU>, pff at 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 at bu-pub.bu.edu