Hello,

*$fa = 0.01;
$fs = 0.2;

difference(){
translate([0,0,10]) rotate([270,0,0])
rotate_extrude() polygon(points=[
[3,0],[3,20],
[6,20],[6,18],
[5,18],[4,16],
[4,0]]);
cylinder(20,8,8);}*

And below is what i'm seeing using this code and the preview mode:
http://forum.openscad.org/file/n14369/151111025932137720.png

I'm seeing "through" the oject at certain view angles:
Is this a known problem ? Does it come from my old graphic card drivers ?

I guess the other way to do this is by using some primitives solids but I
really like the rotate_extrude and polygon commands.

Thanks for your help.

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This issue is solved by increasing the convexity parameter on the
rotate_extrude call

just increasing it to 3 works for me, on your example.  More complex objects
may require higher values.  Higher values will take longer to preview

rotate_extrude(convexity=3) polygon(points=[
...

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Thank you thehans !

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