I am experiencing a strange issue where:

1. My “fast” renders are slow
2. My “slow” renders are instant but error free
3. I get an error only when I try to export a particular object.

When I remove the minkowski function it works well. When I add it I only get an error when trying to export the object.

Is this a bug or (more likely) is there something I am missing?

Code:

ball_height = 56;
top_radius = 20;
head_minkow = 3;
threw_hole = 11;

minkowski(){
difference(){
sphere(r=top_radius);
cylinder(r=9, h=top_radius);
for(i=[0:72:360]){
rotate([0,60,i])
cylinder(r=9, h=top_radius);
// rotate([0,120,i])
// cylinder(r=6, h=ball_height/2);
}
}
sphere(r=head_minkow);
}

Output:

Parsing design (AST generation)...
Compiling design (CSG Tree generation)...
Rendering Polygon Mesh using CGAL...
Geometries in cache: 16
Geometry cache size in bytes: 1563728
CGAL Polyhedrons in cache: 2
CGAL cache size in bytes: 2523920
Total rendering time: 0 hours, 0 minutes, 0 seconds
Top level object is a 3D object:
Facets: 20088
Rendering finished.

ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion violation! Expr: e->incident_sface() != SFace_const_handle() File: /Users/kintel/code/OpenSCAD/libraries/install/include/CGAL/Nef_S2/SM_const_decorator.h Line: 330
AMF export finished: /Users/caitlin/Desktop/test.amf

Thanks for the help.

~Caitlin

Ahh!

I was not at all aware of this grid system. Do you know if any docs on it?

As for the union of cylinders how would you recommend I resolve this? It’s sort of frustrating that there is arbitrary geometry that does not work.

Caitlin

On Jan 4, 2020, at 2:53 AM, nop head nop.head@gmail.com wrote:


The reason it is slow to preview is because OpenCSG used for preview can't simulate Minkowski, so it has to be computed geometrically with CGAL. It is then cached so render is fast.

I think the reason it goes wrong is that the union of the cylinders has vertices that are too close for OpenSCAD's grid.

<image.png>

On Sat, 4 Jan 2020 at 07:58, Caitlin Campbell <caitlin@cissols.commailto:caitlin@cissols.com> wrote:
I am experiencing a strange issue where:

1. My “fast” renders are slow
2. My “slow” renders are instant but error free
3. I get an error only when I try to export a particular object.

When I remove the minkowski function it works well. When I add it I only get an error when trying to export the object.

Is this a bug or (more likely) is there something I am missing?

Code:

ball_height = 56;
top_radius = 20;
head_minkow = 3;
threw_hole = 11;

minkowski(){
difference(){
sphere(r=top_radius);
cylinder(r=9, h=top_radius);
for(i=[0:72:360]){
rotate([0,60,i])
cylinder(r=9, h=top_radius);
// rotate([0,120,i])
// cylinder(r=6, h=ball_height/2);
}
}
sphere(r=head_minkow);
}

Output:

Parsing design (AST generation)...
Compiling design (CSG Tree generation)...
Rendering Polygon Mesh using CGAL...
Geometries in cache: 16
Geometry cache size in bytes: 1563728
CGAL Polyhedrons in cache: 2
CGAL cache size in bytes: 2523920
Total rendering time: 0 hours, 0 minutes, 0 seconds
Top level object is a 3D object:
Facets: 20088
Rendering finished.

ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion violation! Expr: e->incident_sface() != SFace_const_handle() File: /Users/kintel/code/OpenSCAD/libraries/install/include/CGAL/Nef_S2/SM_const_decorator.h Line: 330
AMF export finished: /Users/caitlin/Desktop/test.amf

Thanks for the help.

~Caitlin

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It seems like this issue, about coordinate errors, gets raised a lot to
explain models that aren't working.  And yet, it seems like there's always,
in the end, a different explanation.  I'm not sure I've seen an example of a
model where this was really the problem.  And when I took a look at this
model, my observation is that the result looks very wrong at a large scale.
(Note that when I rendered and exported the model I did not get an error.)

The facets around the holes should be smooth but instead they are kind of
zig-zagging back and forth at a scale that is a billion times bigger than
where rationals are going to round to each other.  (Note for this run I had
$fn=16 in the last call to sphere.)

http://forum.openscad.org/file/t2477/weirdmodel.png

I tried commenting out the for loop, so there is just one cylinder
subtraction, and I got this weird behavior at the top edge (still with
$fn=16 on the last sphere).  I'm having a hard time believing that this has
something to do with rounding errors.

http://forum.openscad.org/file/t2477/weird2.png

I tried increasing $fn overall to 128 and got this, which doesn't have any
obvious artifacts:

http://forum.openscad.org/file/t2477/weird3.png

And then I tried decreasing overall $fn to 32 and increasing the last sphere
to $fn=32 as well.  That gave me this, which also looks good:

http://forum.openscad.org/file/t2477/weird4.png

I'm trying to run the full model now with $fn=32.

--
Sent from: http://forum.openscad.org/

What's wrong with the union of cylinders?  I exported it, loaded it into
PrusaSlicer, sliced it.  Everything appears to be OK---except that it looks
ugly where they meet, with a little artifact there. That artifact is just a
result of the cylinders being polyhedra instead of actually round, so it's
not wrong.

My run with $fn=32 completed in half an hour, and the rounding of the
circular holes still looks wonky, with weird zig-zags in the facets.

http://forum.openscad.org/file/t2477/weird5.png

--
Sent from: http://forum.openscad.org/