[Plplot-devel] How to support gradients within PLplot

[Alan W. I. <ir...@be...>]

What started me thinking about how to support gradients within PLplot is the
large PLplot logo at the top of our webpage which is smoothly darkened on
the left.  I don't know how Werner created that effect, but it is unlikely
to be within PLplot because we currently don't support gradients.  Instead,
what we do (e.g., example 16) is approximate gradients by a series of
(normally rather wide) steps. Thus, in my attempt to mimic that logo within
PLplot, I plan to approximate the darkening effect, but I may have to use a
lot of narrow steps to get a good-looking smooth effect.  This seems a
misuse of SVG which natively supports gradients so I may just hand-edit the
SVG code for the rectangle involved to apply a smooth gradient to it.

I have done a quick google survey,
and it appears most/all of our modern devices (e.g., svg), and at least one
of our older devices (e.g., ps) have native gradient capability. This
represents an opportunity for PLplot to also support RGB (and RGBA)
gradients for those drivers with that capability.  That would obviously
help for such
specialized applications as generating our logo, but I think it would also
be a big improvement for all our plot styles that represent surfaces. To
take a concrete example, look at
http://plplot.sourceforge.net/examples-data/demo08/x08.01.png where there is
room for improvement in how the bottom of the banana-shaped valley is
represented even for the relatively fine grid used for that example.
Typically, we represent those surfaces as a series of filled triangles.  The
three points of the triangle allow you to approximate the local gradient and
apply it to the RGB and alpha of the colour of the triangle.  That implies
much smoother looking surface results and/or larger triangles/smaller plot
files to represent the same surface for those devices that have native
gradient support.

Note, the question of PLplot support for gradients has internal core library
implications (e.g., triangle representation for surfaces), API implications
(what is the most convenient way to let PLplot users specify gradients?),
and implications for each driver with potential native gradient capability
(how to translate from PLplot's representation of gradients to the driver's
representation).  Furthermore, we will probably want to do a core library
fallback if the device driver doesn't have native gradient capability. Note,
so far I have only been talking about linear gradients, and my preliminary
feeling is we should stick just to those, but I should at least mention that
there are other kinds of gradient styles as well, see, e.g., the radial
gradient that is supported in the SVG specification 
(http://www.w3.org/TR/SVG/pservers.html).

So there is a lot to consider here. Nevertheless, gradient capability is
obviously important for graphics, and I therefore think it is time we
started discussing how we should support gradients with PLplot.

So please give your thoughts on this topic especially if you can think of
better ways to implement it than my preliminary thoughts on that matter
above.

Alan
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Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state implementation
for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
Linux Links project (loll.sf.net); and the Linux Brochure Project
(lbproject.sf.net).
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Linux-powered Science
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