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MathGroup Archive 2000

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Re: Graphics Directives Lost In Certain Cases

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25652] Re: Graphics Directives Lost In Certain Cases
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Wed, 18 Oct 2000 02:52:25 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <8seamo$6rm@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

I'm not sure but you may try
Show[Graphics3D[{{Line[{{0, 0, 0}, {10, 10, 10}}]}, RGBColor[0, 0, 1], 
        AbsoluteThickness[
          2], {Line[{{0, 0, 0}, {10, 10, 10}/
                2}], {Polygon[{{5., 5., 5.}, {4., 4.306186217847897, 
                  3.6938137821521027}, {3.6938137821521027, 4., 
                  4.306186217847897}}]}}}], AspectRatio -> Automatic, 
    PlotRange -> All, Lighting -> False];

                      ^^^^^^^^^^^^^^^^^

to see at least a blue polygon. The point is, that the lighting
calculation
super seeds the other color calculations. 

I think there is some inconsitency in Mathematica regarding colors and
surface
colors. In a 3d graphic I would expect, that any color is a surface
color, but
lines and points have no normal vector to calculate the shading. 

You see the inconsistence when yoy are using Plot3D[] with a color. The
graphics
 
 ParametricPlot3D[{x, y, x^2 + y^2, Hue[(x^2 + y^2)/4]}, {x, -2, 2}, {y,
-2, 
    2}]

is colored but

ParametricPlot3D[{x, y, x^2 + y^2, Hue[(x^2 + y^2)/4]}, {x, -2, 2}, {y,
-2, 
    2}]

not and
ParametricPlot3D[{x, y, x^2 + y^2, Hue[(x^2 + y^2)/4]}, {x, -2, 2}, {y,
-2, 
    2}, Lighting -> False]

is again with colors, but a bit different from

ParametricPlot3D[{x, y, x^2 + y^2, SurfaceColor[Hue[(x^2 + y^2)/8]]},
{x, -2, 
    2}, {y, -2, 2}]


So -- always when a lighting calculation is done -- the colors are
dropped.

Hope that helps
  Jens

David Park wrote:
> 
> Dear MathGroup,
> 
> I have come across a strange behavior of 3D graphics where graphics
> directives are lost in certain cases. It has to do with a 3D arrow routine I
> am working on, but here is the bare bones essence of the problem.
> 
> The following plot gives a second thick blue line plotted on top half of the
> first thin black line.
> 
> Show[Graphics3D[
> {{Line[{{0, 0, 0}, {10, 10, 10}}]},
>  RGBColor[0, 0, 1],
>  AbsoluteThickness[2], {Line[{{0, 0, 0}, {10, 10, 10}/2}]}}],
>    AspectRatio -> Automatic, PlotRange -> All];
> 
> If I combine the second line with this polygon, the color and thickness
> directives are lost. Why?
> 
> Show[Graphics3D[{{Line[{{0, 0, 0}, {10, 10, 10}}]}, RGBColor[0, 0, 1],
>      AbsoluteThickness[2], {Line[{{0, 0, 0}, {10, 10, 10}/2}],
>       {Polygon[{{5., 5., 5.}, {4., 4.306186217847897, 3.6938137821521027},
>          {3.6938137821521027, 4., 4.306186217847897}}]}}}],
>    AspectRatio -> Automatic, PlotRange -> All];
> 
> But the directives are not lost if the second line is not right on top of
> the first line.
> 
> Show[Graphics3D[{{Line[{{0, 0, 0}, {10, 10, 10}}]}, RGBColor[0, 0, 1],
>      AbsoluteThickness[2], {Line[{{0, 0, 0}, {10, 10, 10.000001}/2}],
>       {Polygon[{{5., 5., 5.}, {4., 4.306186217847897, 3.6938137821521027},
>          {3.6938137821521027, 4., 4.306186217847897}}]}}}],
>    AspectRatio -> Automatic, PlotRange -> All];
> 
> Nor are the directives lost if a different polygon is used.
> 
> Show[Graphics3D[{{Line[{{0, 0, 0}, {10, 10, 10}}]}, RGBColor[0, 0, 1],
>      AbsoluteThickness[2], {Line[{{0, 0, 0}, {10, 10, 10}/2}],
>       {Polygon[{{5., 5., 5.}, {4., 4.306186217847897, 3.6938137821521027},
>          {4., 3.6938137821521027, 4.306186217847897}}]}}}],
>    AspectRatio -> Automatic, PlotRange -> All];
> 
> Needless to say, most of the time I lose the directives. Does anyone know
> the cause of this?
> 
> David Park
> djmp at earthlink.net
> http://home.earthlink.net/~djmp/


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