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MathGroup Archive 2001

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Lost Line Directives in 3D Graphics

  • To: mathgroup at smc.vnet.net
  • Subject: [mg30121] Lost Line Directives in 3D Graphics
  • From: "David Park" <djmp at earthlink.net>
  • Date: Sun, 29 Jul 2001 21:26:14 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Dear MathGroup,

Here is a case where it appears that Mathematica incorrectly renders the
postscript code for Graphics3D. I wonder if other people obtain the same
effect that I obtain.

Needs["Graphics`Animation`"]

grafs = {Line[{{0, 0, 0}, {2, 0, 0}}], RGBColor[1.`, 0.`, 0.`],
      AbsoluteThickness[
        4], {Line[{{0, 0, 0}, {0.999`, 0,
              0}}], {Polygon[{{1.`, 0.`, 0.`}, {0.6666666666666667`, 0.`,
                0.16666666666666666`}, {0.6666666666666667`, \
-0.14433756729740643`, -0.08333333333333333`}}],
          Polygon[{{1.`, 0.`,
                0.`}, {0.6666666666666667`, -0.14433756729740643`, \
-0.08333333333333333`}, {0.6666666666666667`,
                0.14433756729740643`, -0.08333333333333333`}}],
          Polygon[{{1.`, 0.`, 0.`}, {0.6666666666666667`,
                0.14433756729740643`, -0.08333333333333333`}, \
{0.6666666666666667`, 0.`, 0.16666666666666666`}}]}}};

plot1 = Show[Graphics3D[grafs], PlotRange ->
     {{-1, 1}, {-1, 1}, {-1, 1}}*2, ImageSize -> 500, Boxed -> False];


The plot consists of a long thin black line, and a shorter thick red line on
top of it, with a three polygon cone at the end, but slightly detached from
the short line. The line directives for the short line are lost, so that it
also appears thin and black.

But if we SpinShow the plot we see that in 4 out of 24 frames the line
directives are not lost. I don't understand postscript rendering of
graphics, but if we select the good frames and bad frames and use
Shift-Ctrl-E to look at the postscript code, we can see that there is a
definite difference between the frames. The good frames have some extra
code, including Mabswid commands.

SpinShow[plot1];

If the first line is moved just a little off the direction of the short
line, then the problem does not occur. The second line has to be on top of
the first line, and the polygons must also be present for the error to
occur.

grafs2 = {Line[{{0, 0, 0}, {2, 0, 0}}], RGBColor[1.`, 0.`, 0.`],
      AbsoluteThickness[4], {Line[{{0, 0, 0}, {0.999`, 0, 0}}]}};

plot2 = Show[Graphics3D[grafs2], PlotRange ->
     {{-1, 1}, {-1, 1}, {-1, 1}}*2, ImageSize -> 500, Boxed -> False];

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



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