Why aren't both sides of a surface equally opaque?

• To: mathgroup at smc.vnet.net
• Subject: [mg80974] Why aren't both sides of a surface equally opaque?
• From: "David Park" <djmpark at comcast.net>
• Date: Thu, 6 Sep 2007 05:38:13 -0400 (EDT)

```The following illustrates a strange behavior of surfaces with respect to
opacity. The code draws a narrow cylinder with a red line on the inside of
the cylinder and a black line on the outside of the cylinder. The cylinder
is completely opaque. The inside red line does not show through the
cylinder, which is proper behavior. However, the black outside line does
show through the back face for a considerable range, which is certainly
improper behavior.

cylinder =
First[ParametricPlot3D[{5 Cos[\[Theta]1], 5 Sin[\[Theta]1],
z}, {\[Theta]1, 0, 2 \[Pi]}, {z, -1, 1},
PlotStyle -> {FaceForm[Orange, LightGray]},
PlotPoints -> {41, 5},
Mesh -> None]];

outsideline =
First[ParametricPlot3D[{5.1 Cos[\[Theta]1], 5.1 Sin[\[Theta]1],
0.2}, {\[Theta]1, 0, 2 \[Pi]},
PlotStyle -> {Black},
PlotPoints -> 41]];

insideline =
First[ParametricPlot3D[{4.9 Cos[\[Theta]1],
4.9 Sin[\[Theta]1], -0.2}, {\[Theta]1, 0, 2 \[Pi]},
PlotStyle -> {Red},
PlotPoints -> 41]];

Show[Graphics3D[
{cylinder,
outsideline,
insideline}],
Lighting -> "Neutral",
ViewPoint -> {1.718, -2.747, 0.975},
Boxed -> False

--
David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/

```

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