Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!dogie.macc.wisc.edu!indri!lll-winken!uunet!ficc!peter
From: peter@ficc.uu.net (Peter da Silva)
Newsgroups: comp.windows.x
Subject: Re: Fast, infinitely undoable graphics.
Message-ID: <4260@ficc.uu.net>
Date: 22 May 89 01:06:00 GMT
References: <2483@rice-chex.ai.mit.edu> <4243@ficc.uu.net> <2532@wheat-chex.ai.mit.edu>
Organization: Xenix Support
Lines: 36

In article <2532@wheat-chex.ai.mit.edu>, tmb@wheaties.ai.mit.edu (Thomas M. Breuel) writes:
> In article <4243@ficc.uu.net> peter@ficc.uu.net (Peter da Silva) writes:
> >  [keep a count of the number of times the bit has been set]

> >  This doesn't work if you ever draw anything with color 0 (i.e., erase).

> Obviously. However, if you have a system that is built on the paradigm
> of putting up individual "objects" on the screen such that you can
> remove them or manipulate them individually, it doesn't even really
> make sense just to draw a rectangle in the background color.

Sure it does. Let's say you want to draw a circle with a square hole in
it. How do you do this? You draw a circle and then you draw a square in
the background color.

So how do you undo the square?

> However, you can repeat the same trick of counting pixels for several
> "layers" or several "colors" (with the obvious semantics of one layer
> shadowing another).

Except that you lose ordering information, unless you do something like
adding a new layer every time you change colors. This would probably save
some space, but still wouldn't be able to deal with xor-mode drawing. That
would require each xor-mode operation to be cut up into a separate color-1
and color-1 (or whatever pair of colors you select) operation... which would
actually increase the memory required if you do a lot of xor-ing... not
to mention slowing drawing down quite a bit.

You might be able to save some space with a scheme like this. But not more,
I think, than something more conventional like run-length-encoded deltas.
-- 
Peter da Silva, Xenix Support, Ferranti International Controls Corporation.

Business: uunet.uu.net!ficc!peter, peter@ficc.uu.net, +1 713 274 5180.
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