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From: ads@canon.co.uk (Adam Billyard)
Newsgroups: comp.graphics,comp.lang.postscript
Subject: Re: How do I draw a depth-cued line in PostScript?
Message-ID: <1991Jun26.140611.1052@canon.co.uk>
Date: 26 Jun 91 14:06:11 GMT
References: <1991Jun26.054357.7870@cs.UAlberta.CA>
Reply-To: ads@canon.co.uk
Organization: Canon Research Europe, Guildford, UK
Lines: 35

lnds@sherlock.mmid.ualberta.ca (Mark Israel) writes:

> How do I draw a depth-cued line in PostScript?
> 

The PostScript imaging model has no notion of variable shading over the
area to be filled. The metaphor is of opaque paint being pushed through
a stencil. Display PostScript extends this a little by including
transparency, but in general if you want continuous shading, you must
write your own filler.

The standard way to create continuous shading (in PostScript) is to
overpaint the path again and again while altering the path (usually by
scaling) and altering the colour you're using.  Its very inefficient. 

e.g.

gsave
clippath pathbbox 4 2 roll translate scale	% 1,1 space
1 -0.1 0
{
  newpath .5 .5 2 index 0 360 arc 1 exch sub setgray fill
} for
grestore

Depth-cueing implies a 3rd dimension, so you'll have to keep the control
points you use as triples. Since PostScript is a complete language, its
quite possible to do this sort of thing.

You might look in Salmon and Slater "Computer Graphics".  They present a
complete 3D viewing model and wireframe rendering entirely written in
PostScript.  It might help demonstrate how you enrich the 2D paths with
a 3rd dimension.

	Adam.