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MathGroup Archive 2000

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RE: fade to mauve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg24424] RE: [mg24393] fade to mauve
  • From: "David Park" <djmp at earthlink.net>
  • Date: Tue, 18 Jul 2000 00:58:22 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Dr. Purcell,

I am not certain why you are scaling the Viewpoint by radius. The
coordinates for Viewpoint are not the natural coordinates for the plot, but
instead are in terms of the largest side of the bounding box. Therefore it
is not necessary to scale it to accommodate large objects.

If you don't scale Viewpoint, then the color shading remains the same for
all sphere radii.

Why does the fade to mauve occur when you do scale the Viewpoint? I am not
certain, but perhaps it is because you are viewing the entire sphere from so
far away that it is in a constant lighting region.

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/

> -----Original Message-----
> From: Christopher J. Purcell [mailto:purcell at drea.dnd.ca]
To: mathgroup at smc.vnet.net
>
> Using Vers 4.0 and i think all the way back to 2.2, if you ParametricPlot
> anything, say a hemisphere:
>
> radius = 1;
> ParametricPlot3D[
>   radius{Sin[theta]Cos[phi], Sin[theta]Sin[phi], Cos[theta]}, {theta, Pi,
>     2 Pi}, {phi, 0, Pi}, ViewPoint -> radius *{1.3, -2.4, 2.}]
>
> where the ViewPoint is close to the default {1.3, -2.4, 2.} you get the
> usual nice shaded image. But when you increase the magnitude of Viewpoint,
> the colors and shading start to fade. For example if you repeat the above
> with radius = 10,
> the shading has now faded to an ugly mauve (on a mac) even though the
> sphere has been scaled the same amount as ViewPoint. The same holds if you
> use Graphics3D to plot filled shaded polygons.
>
> When I draw big things and need to use big Viewpoints to get the
> perspective right, they lose their lighting.
>
> I don't see why it has to be this way.
>
> Dr. C. J. Purcell
> Group Leader/Transducers
> Defence Research Establishment Atlantic
> 9 Grove St., Dartmouth, NS Canada B2Y 3Z7
> Tel: 902-426-3100-x389
> Fax: 902-426-9654
>



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