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From: doug@terak.UUCP (Doug Pardee)
Newsgroups: net.micro
Subject: Re: What is \"bitblt\" ?
Message-ID: <956@terak.UUCP>
Date: Tue, 31-Dec-85 12:15:10 EST
Article-I.D.: terak.956
Posted: Tue Dec 31 12:15:10 1985
Date-Received: Thu, 2-Jan-86 04:52:16 EST
References: <976@brl-tgr.ARPA>
Organization: Calcomp Display Products Division, Scottsdale, AZ, USA
Lines: 35

> Could somebody please tell me what "bitblt" is - I keep seeing it.

Bitblt is a concept used in many bit-mapped graphics systems.  It is
an operation which works on rectangular areas of bit-images.  The basic
concept is simple:

  destination_area = source_area           and
  destination_area = source_area_1  AND|OR|XOR [NOT] source_area_2
            (where black=0 and white=1)

There are a lot of extensions to the basic concept, but you get the
idea.

Bitblt is pretty much restricted to Macintosh-class displays; monochrome
with a resolution of 640x400 or so.  The AND|OR|XOR|NOT operators don't
have any particularly obvious meaning in color; what is the result of
(orange AND chartreuse)?  At higher resolutions the amount of data to
be processed becomes overwhelming, even with special-purpose hardware
performing the operation.  And the amount of memory needed to hold the
various operands also grows with color and high-resolution; to support
bitblt reasonably on a 1024x1024 8-plane color system would require
4 to 8 megabytes of storage just for image-diddling.  [A classic speed-
vs-memory tradeoff: if you have special-purpose bitblt hardware in order
to get around the speed problems, you have to use non-virtual memory to
hold the images].

Opinion:  bitblt is an endangered species.  The popularity of the bitblt
concept was because it was such a simple and powerful approach to the
problems of manipulating bit-images.  In color systems bitblt is neither
simple nor powerful; it has all the qualities of a ghastly kludge.  With
very few monochrome-only graphics systems being designed these days,
bitblt's days are numbered unless someone can come up with an extension
which will handle color as elegantly as bitblt handled black-and-white.
-- 
Doug Pardee -- CalComp -- {hardy,savax,seismo,decvax,ihnp4}!terak!doug