Trapezoidal Pyramid

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Trapezoidal Pyramid

dalucero
Ok, I'm trying to render a pretty simple shape here.  A trapezoidal pyramid.  Using the polyhedron function, it compiles and shows me a preview fine, but it won't render.  Can anybody see what I've done wrong?

polyhedron(points=[[0,0,0],[17,0,0],[17,17,0],[0,17,0], [1.5,1.5,2],[15.5,1.5,2],[15.5,15.5,2],[1.5,15.5,2]],
faces =[[0,1,2,3],[0,1,5,4],[1,2,6,5],[2,3,7,6], [3,0,4,7],[4,5,6,7]]);
tp3
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Re: Trapezoidal Pyramid

tp3
Some of the faces are oriented the wrong way around which can be seen in the thrown-together view (they get a pinkish color when facing inwards).
( see http://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#polyhedron )

try:

polyhedron(points=[[0,0,0],[17,0,0],[17,17,0],[0,17,0],
[1.5,1.5,2],[15.5,1.5,2],[15.5,15.5,2],[1.5,15.5,2]],
faces =[[0,1,2,3],[4,5,1,0],[5,6,2,1],[6,7,3,2], [7,4,0,3],[7,6,5,4]]);

An alternative way to create this object would be:

linear_extrude(height = 2, scale = 14/17) square(17, center = true);
-- Torsten
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Re: Trapezoidal Pyramid

dalucero
AHA!  Left hand rule got me.  Thanks alot!  I ended up going with 4 faceted cylinders, lol.
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Re: Trapezoidal Pyramid

drxenocide
The two times I've had to do that I have done 2 seperate things (because I hate polyhedron()). The first time I did a hull of 2 parallel thin cubes. The second time I did a linear extrude of a cube.

On Sun, Nov 23, 2014 at 4:22 PM, dalucero <[hidden email]> wrote:
AHA!  Left hand rule got me.  Thanks alot!  I ended up going with 4 faceted
cylinders, lol.



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Re: Trapezoidal Pyramid

Bananapeel
In reply to this post by dalucero
$fn is rather handy (apart from adjusting quality), because you can use it to make pyramids and the like with the cylinder function. Then cut off the top:

difference()
{
    rotate([0,0,45])
    cylinder(d1=sqrt(pow(17,2)*2), d2=0, h=14.17, $fn=4);
   
    translate([0,0,2])
    linear_extrude(h=14)
    square([17,17], center=true);
}

:)
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Re: Trapezoidal Pyramid

Greg Frost
Wouldn't d2=_some non zero number_ be easier than difference()?

> On 25 Nov 2014, at 9:01 am, Bananapeel <[hidden email]> wrote:
>
> $fn is rather handy (apart from adjusting quality), because you can use it to
> make pyramids and the like with the cylinder function. Then cut off the top:
>
> difference()
> {
>    rotate([0,0,45])
>    cylinder(d1=sqrt(pow(17,2)*2), d2=0, h=14.17, $fn=4);
>
>    translate([0,0,2])
>    linear_extrude(h=14)
>    square([17,17], center=true);
> }
>
> :)
>
>
>
> --
> View this message in context: http://forum.openscad.org/Trapezoidal-Pyramid-tp10202p10215.html
> Sent from the OpenSCAD mailing list archive at Nabble.com.
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://rocklinux.net/mailman/listinfo/openscad
> http://openscad.org - https://flattr.com/thing/121566
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Re: Trapezoidal Pyramid

Bananapeel
That would just give you a pyramid, not a trapezoid.
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Re: Trapezoidal Pyramid

Bananapeel
Sorry, you're right. I can't find the precise value of d2, though, so I estimated it:

rotate([0,0,45])
cylinder(d1=sqrt(pow(17,2)*2), d2=sqrt(pow(14.6,2)*2), h=2, $fn=4);