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

• To: mathgroup at smc.vnet.net
• Subject: [mg80991] Re: Why aren't both sides of a surface equally opaque?
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Fri, 7 Sep 2007 02:06:02 -0400 (EDT)
• Organization: Uni Leipzig
• References: <fbojha\$j58\$1@smc.vnet.net>

```Hi,

that is a feature (!) because the Plot3D[] commands per default
show mesh lines. These meshlines are not more the outline of the
polygons. The mesh lines are separate lines. If you put a line
on a polygonal surface the result would be, that the line intersect
the polygonal surface approximation. To avoid that, the lines are
rendered with an offset in viewing direction.
What you see is just that offset, that is too huge for you example.

In principle, there should be an Graphics3D[] option the control the
amount of line offsets.

Regards
Jens

David Park wrote:
> 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
>
>

```

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