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From: Helge Blischke <h.blischke@acm.org>
Newsgroups: comp.lang.postscript
Subject: Re: setrgbcolor with lineto/curveto
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Date: Wed, 13 Jun 2012 16:16:37 +0200
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JohnF wrote:

> Apologies if this is simple, but I don't know postscript
> very well, and failed to google it...
> I'd like to draw a lineto/curveto with the color varying
> from one endpoint to the other. That is, it should start
> with  a b c setrgbcolor  at one endpoint and end up with
> a' b' c' setrgbcolor  at the other end. It's decorative,
> so a-->a', b-->b', c-->c' can vary linearly, or anything
> like that, along the line's length. Thanks,

You will have to (1) encapsulate your path to be drawn into gsave and 
grestore to prevent unwanted side effects and (2) define a shading pattern 
and then do the stroke.

For the details see the PLRM (Postscript Language Reference Manual).
Note that shading is a level3 feature.

Helge