Hello,

Generating a polyhedron from two polygons is not too difficult when the
polygon has a limited number of vertices. However I have a polygon with
several tens in order to represent curves correctly.

Now my idea is to generate a second polygon from the original one, in a
plane parallel to the first one, scale and translate it a little bit and
then generate a polyhedron from the original and the new polygon.

As I want to achieve this repeatedly and possibly with different polygons,
do you think it would be possible to create a short algorithm to generate a
suitable path whatever the number of vertices? Which data
structures/strategies would you use then?

Thank you in advance.

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Was it difficulte to code such an algorithm? Any advice?

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This sort of thing has been done many times by many people.

There is a library called list-comprehension-demos that is perhaps one of
the first to do it and it displays a method for doing it.  And dotSCAD also
has code for this.  https://github.com/JustinSDK/dotSCAD

In BOSL2 (a library which is still in development):

https://github.com/revarbat/BOSL2/wiki/skin.scad

you can see code that does this, and the wiki shows examples of the results.
If you understand how to use list comprehensions then it is not difficult to
construct a polyhedron by connecting two polygons with the same number of
vertices, located anywhere in 3-space, though if you generate
self-intersecting sides the resulting polyhedron will be invalid.  You
simply have to construct the list of vertices and then write a list
comprehension that assembles the appropriate face list.

I wrote a skin() routine to connect a sequence of polygons with different
numbers of vertices using three different methods.  The most sophisticated
is a quadratic programming method to find a minimum edge length vertex
alignment for creating triangular faces between the two polygons.  I would
say that was a fairly tricky business.

The sweep() operation of taking a given fixed shape and applying
transformations is straight forward if you specify the transformations
completely but tricky when you try to sweep along a path, because a path
doesn't provide enough information to fully define the transformations for
each polygon.

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Thank you @acwest and @adrianv! I'll have a look to all of this.

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I'm not entirely clear on what you want to do, but could a hull() work?

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So far I've got the best results with BOSL's convex_hull (see example below).

BOSL2's skin would be perfect for that purpose but I couldn't even run the
first example without error messages. Is BOSL2 too early in its development
or is it me?

The basic hull won't do it for sure.

use <BOSL/convex_hull.scad>

steps = 100;
height = 10;

startshape = [for(i=[0:steps]) [cos(i/steps * 360) * 2 + 3, sin(i/steps *
360) * 4 + 7, 0]];
endshape = [for(i=[0:steps]) [cos(i/steps * 360) * 10 - 15, sin(i/steps *
360) * 5 - 12, height]];

allpoints = concat(startshape, endshape);

polyhedron(points = allpoints, faces = convex_hull3d(allpoints));

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I managed to use BOSL2's skin function. The examples were lacking the
required use<> code.

So, here's a basic example of what I want to do.

use <BOSL2/skin.scad>
use <BOSL2/rounding.scad>

$fn=32;
polygon_steps = 100;
random_range = 1;

startshape = [for(i=[0:polygon_steps]) [cos(i/polygon_steps * 360) * 2 + 3,
sin(i/polygon_steps * 360) * 4 + 7]];
endshape = [for(i=[0:polygon_steps]) [cos(i/polygon_steps * 360) * 10 - 15 +
rands(-random_range, random_range,1)[0], sin(i/polygon_steps * 360) * 5 - 12

• rands(-random_range, random_range,1)[0]]];

skin([startshape, endshape], z=[0,10], slices=10);

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