Re: Quickdraw Regions Explained

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From: info-mac@uw-beaver
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Subject: Re: Quickdraw Regions Explained
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Date: Thu, 31-Jan-85 20:21:08 EST
Article-I.D.: uw-beave.688
Posted: Thu Jan 31 20:21:08 1985
Date-Received: Sat, 2-Feb-85 11:15:29 EST
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Organization: U of Washington Computer Science
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From: Andy Stadler c/o

Congratulations to Jack Palevich for decoding the QuickDraw region
structure. This is the same region structure which Atkinson, Jobs, and
Apple had bragged on and on about, and are hoping to patent....

There is one thing I hope I can clarify. Close reading of the QuickDraw
manual reveals that it uses a non-standard row/column numbering system.
Instead of numbering each row and column of bits, the numbers are
assigned to the "infinitely thin" lines between them. So the example
region should have been drawn like this (top portion only):

x

1 2 2 2 2 2 2 2 2 2 2 3 3 3 3
9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
19 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | |
20 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+
y | |[]|[]| | | | | | | | |[]|[]| |
21 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | |[]|[]| | | | | | | |[]|[]| |
etc.

Note that this permits a cleaner definition of :

::=

You see, the two numbers are actually coordinates of the
"infinitely-thin" walls of a rectangle which *encloses* the bits of
interest. As far as the Y-coordinates, I suppose you could look at the
numbers as saying, "below this line, make the following changes...."

As far as the compression goes, you are right, and this is great for
rectangles. It fails on complex shapes (imagine encoding a circle!). I
wonder if there are any other encoding forms for other shapes? Anyone
want to try looking?

Andy Stadler
Arpanet: Andy Stadler c/o