Combining different sweeps does not work
Wow,
I see that I make this mistake also with a lot of other files.
So there is work to do. I let you know the results.
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http://forum.openscad.org/file/n18963/sweep4.jpg After rewriting the code
it still doesn't work well.
And I don't understand why.
The image shows from top to bottom the lineair extrusion of the two 2D
layers. They look fine.
The lowest object is the sweep of the two 2D objects.
The 2D objects are build from 900 point and start on the left lower corner
follow the quarter circle to the bottom and then go counter clockwise back
to the start.
It is strange that there seem to appear crossing lines. They only appear at
the top and the bottom.
engelvleugel_v8_res.scad
http://forum.openscad.org/file/n18963/engelvleugel_v8_res.scad
B
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http://forum.openscad.org/file/n18964/sweep5.jpg
I printed all the points of the two layers and exported them to excel and
checked if the points in both layers correspond. And the do.
What I think that happens is that the top and the bottom of the sweeped
object are in fact open. Openscad closes this by drawing triangles between
the different points of the top and bottom layer. You cannot control the
points they choose to connect.
Can someone explain how Openscad closes the top and bottom layers of a
sweeped object?
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Maybe interesting to add the excel file sweep.xlsx
http://forum.openscad.org/file/n18965/sweep.xlsx
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To me your code looks fine and the sweep also works fine with 2016.04.06
using F5 and F6.
Sometimes it is a good idea to empty the cache or even to restart OpenSCAD
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Johan Jonker wrote
Can someone explain how Openscad closes the top and bottom layers of a
sweeped object?
Your question reduces to how Openscad triangulates the face of a polyhedron,
when it has more than 3 points. In an earlier version my sweep() did this
triangulation on its own. I used some sloppy algorithm that exploited some
symmetry of the point generator function I used for my airfoils. Later I
changed that and used the system's triangulation. For planar faces OpenSCAD
can do its own magic (I guess some ear cut algorithm or even a faster
routine). And - as you saw - for non-planar faces CGAL is used and tries
some alternate construction that can introduce curves and extra points to
the cap.
BTW, I never had problems with that, as long as the polygon was 'simple'
i.e. welldefined. That means: no self intersection, no double points.
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Here some nice and simple code allowing you to study non-planar triangulation
and the "minimal surfaces" the system generates. Seems to me like some
foward-backward ear-cut algorithm.
If you run the animation e.g. with 100 steps, you will see, that the
triangulation algorithm will work up to a certain slope but freaks out at
some point, making the result more or less unpredictable.
use
<Naca_sweep.scad>
N = 100;
t = 100;
T1 = -100*($t-.5);
T2 = 250*($t-.5);
sweep([gen(0, T1), gen(200, T2)]);
function gen(shift=0, T = 0) =
[for (i=[0:360/N:360]) [tsin(i), tcos(i), Tsin(2i)+shift]];
http://forum.openscad.org/file/n18970/jonker3.png
http://forum.openscad.org/file/n18970/jonker4.png
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Nice example.
I still see a remarkable difference between sweep and lineair extrusion with
the following code.
Do I use the wrong sweep version then? Or do you have the same results?
$fn=60;
use <Naca_sweep.scad>
punten =
[[0,0],[10,0],[10,8],[9,8],[9,1],[5,1],[5,9],[10,9],[10,10],[0,10]];
linear_extrude(height = 10, center = true, convexity = 10, twist = 0)
translate([10,10,10])
polygon(points= punten);
dat = vec3D(punten);
shape = [T_(0,0,0,dat),T_(0,0,10,dat)];
sweep(shape);
http://forum.openscad.org/file/n18983/sweep6.jpg
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Johan Jonker wrote
I still see a remarkable difference between sweep and lineair extrusion
with the following code.
Do I use the wrong sweep version then? Or do you have the same results?
The first problem is that your polygon is defined in reverse order for the
sweep you do (my code doesn't check for this). If you extrude shape =
[T_(0,0,0,dat),T_(0,0,-10,dat)]; to the reverse direction, or reverse your
definition, it should work (at least for the side parts of the extrusion).
The second problem is that the caps are not correctly triangulated by
polyhedron (i.e. the system). To be honest, I haven't yet encountered this
case yet and don't have an immediate solution for it. But cycling your
polygon definition around a bit, shows that it can work:
$fn=60;
use
<Naca_sweep.scad>
punten = [[9,8],[9,1],[5,1],[5,9], [10,9],[10,10],[0,10],
[0,0],[10,0],[10,8]];
dat = vec3D(punten);
shape = [T_(0,0,0,dat),T_(0,0,-10,dat)];
sweep(shape);
Therefore, I suspect polyhedron to employ a buggy triangulation. Unless
this doesn't get repaired, I'll implement my own triangulation when I find
time for it ... You can try to put this two constructions into a polyhedron
example
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#polyhedron
(sweep automatically constructs a polyhedron!) and announce the bug in a new
thread.
As workaround you always can try skin()
https://github.com/openscad/list-comprehension-demos instead of sweep().
You have to install scad-utils https://github.com/openscad/scad-utils
to settle its references. Then you can use it as you would use sweep(). It
does what you want - at least in this case.
Best Rudolf
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Parkinbot wrote
The second problem is that the caps are not correctly triangulated by
polyhedron (i.e. the system). To be honest, I haven't yet encountered this
case yet and don't have an immediate solution for it.
But
cycling your polygon definition around a bit, shows that it can work:
I must correct myself in this point. The caps are correctly triangulated by
the system. I seems, you didn't use the newest version of Naca_sweep.scad,
which dates from 07-02-16.
So, reversing your polygon definition or your extrusion direction or using
skin() will solve your problem.
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