Re: representing patterns and structures

phayes@cs.uiuc.edu (Pat Hayes)
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Date: Mon, 21 Nov 1994 16:28:23 -0600
To: "Peter Clark" <pclark@cs.utexas.edu>,
        brayman@zuben.ca.boeing.com (Bill Brayman)
From: phayes@cs.uiuc.edu (Pat Hayes)
Subject: Re: representing patterns and structures
Cc: srkb@cs.umbc.edu
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Heres an interesting issue for this discussion. Years ago, complicated
interactive CAD systems allowed notions of 'grouping', so you could specify
eg. a pattern of holes all of a certain diameter* a certain spacing* apart
in a hexagonal* grid filling a certain area* on a sheet of metal, just by
specifying the starred parameters. Then it was possible to say that they
should be all moved a leeeetle bit closer together, and have the system
draw the result for you on the screen. Very pretty, until you wanted to
fill in just one of these holes. This was impossible if the thing was
specified this way, since in a real sense that particular hole-token didnt
actually exist in the internal representation.

How could we allow for such pattern-exceptions? One idea is to have them
added as post-defaults, so that this piece of surface starts being there,
then removed by the hole-grid, then replaced by the exception. But this
fails when someone changes the grid-parameters: its essential to tie the
repatching to the place where that particular hole is, and this thing - one
particular hole in the pattern described as a pattern - just isnt *there*
in the representation, to have anything tied to it.

The interesting thing about this kind of example is that people seem to
have no intuitive trouble at all understanding what is meant, which
suggests that people's representations are somehow more complex or
sophisticated than the CAD system's parameter-storing scheme. Any ideas
about how it might work?

Pat Hayes


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