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From: ian@wcwvax.UUCP (Ian Kemmish)
Newsgroups: net.micro
Subject: Re: Re: What is "bitblt" ? (Really extending to colour)
Message-ID: <605@wcwvax.UUCP>
Date: Tue, 7-Jan-86 05:45:14 EST
Article-I.D.: wcwvax.605
Posted: Tue Jan  7 05:45:14 1986
Date-Received: Fri, 10-Jan-86 06:57:44 EST
Organization: Whitechapel Computer Works, London
Lines: 34

There are only 16 functions of two Boolean variables, so it's
easy to cater for all possible cases in monochrome.  In addition,
the fact that historically bitblt has often been done in hardware
has meant that there was no extra cost involved in providing all
sixteen.  By the time we get to mainly-software implementations,
history dictates what to implement!

In colour, with 8, 16 or 24 bits/pixel, it's clearly ludicrous to
try and cater for all eventualities.   A lot of people implement
their colour screens as multiple bitplanes, so they tend to just
allow monochrome bitblts on each plane.   This is why colour bitblt
often appears to be a no-no.   The trick is to define a sensible
(but small) set of "useful" operations.

Here at WCW, we have done:
PAINT (overlay), HIGHLIGHT (invertible), MIX and UNMIX (complementary).
Each of these rules can be applied direct between two rasters
(except for HIGHLIGHT, which is a unary operation) or via a stencil.

This set of rules seems to be enough to do most of the things people
need to do with bitmap displays - we have implemented a brush graphics
library with arbitrarily shaped, coloured and patterned brushes with
no real trouble.  We have also mapped these rules into the traditional
Boolean functions for operating on monochrome rasters.

Footnote:
I don't think the Mac actually uses bitblt very much at all.  I suspect
that Quickdraw calculates the region to fill with a pattern and then
fills that region.   The reason for my suspicion is that an ellispe
drawn with a rectangular brush actually comes out a slightly different
shape from a Quickdraw outline oval!


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