Newsgroups: comp.graphics
Path: utzoo!utgpu!jarvis.csri.toronto.edu!dgp.toronto.edu!elf
From: elf@dgp.toronto.edu (Eugene Fiume)
Subject: Re: the word "bitmap"
Message-ID: <8901042308.AA12896@explorer.dgp.toronto.edu>
Organization: University of Toronto, CSRI
References: <8568@pasteur.Berkeley.EDU> <7549@watcgl.waterloo.edu>
Date: Wed, 4 Jan 89 18:08:18 EST

In my paper on the semantics of bit-mapped graphics (and other topics)
(computers&graphics 11, 2 (Apr. 1987)), I define bit-maps as Paul
Heckbert suggests, but just because I define something to be so doesn't
make it so.

Both Foley and van Dam, and Newman and Sproull are fairly cagey about
bit-mapping.  It's hard to get a straight answer as to whether or not
they intend bit-maps to be bi-level, since they are discussed in the
context of general raster graphics.  The point is that if the standard
texts don't define such concepts carefully, ambiguity is bound to
spread.

Example: The index entry for bit-mapped graphics in Foley and van Dam
reads
	Bit map, see Raster display, refresh buffer 

and the index entry for refresh buffer is oblivious to bit-maps.  Now,
if you chase down the page references for "refresh buffer", you will
see that p21 refers to bit-maps, and does not say that the colour space
must be bi-level.  In fact, in subsequent references, it becomes clear
that pixels within bit-maps do not need to be bi-level.  But then they
define boolean operations on bit-maps, which only make (proper) sense
when pixels are bi-level.  (Yes, yes, I know that one can try to make
sense of some "boolean" operations on n-bit pixels, n>1.)

It is clear that a word is needed for bi-level intensity functions
f: Z sup 2 -> {0,1} in which there is a one-one correspondence between
f and an image capable of displaying bi-level pixels.  For me, that word
is bit-map.  Note that I have distinguished between bit-maps and images
(e.g., for me a bit-map does not have a pixel aspect ratio or shape--
images do).
-- 
Eugene Fiume
Dynamic Graphics Project
University of Toronto
elf@dgp.toronto.edu