How to separate a geometry having distinct objects ?

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On 23.01.2021 10:04, Gilbert Duval wrote:
> The main problem is that
> sometimes an intersection returns two or more separated object into the
> same geometry and my process need to split each of them separately in
> order to make correct cross pieces.
>
> Is it possible to split a geometry having separated objects so that each
> can be processed separately ?

Hello Gilbert,

You cannot automatically split it into separate objects using OpenSCAD,
but you can save the output from OpenSCAD and automatically split it
into separate objects using polyfix ( polyfix is a console application,
it comes with AngelCAD https://github.com/arnholm/angelcad/releases/ )

Using your example, let us call it 'separate.scad', you can save it to
e.g. 'separate.stl' containing everything. Then you run polyfix with the
-lumps option to extract separated objects (lump=separate object):

$ polyfix -lumps separate.stl -out=*.off

( You must select an output format different from STL, because polyfix
does not support -lumps in combination with STL output )

In this case the result is 32 files, each containing a separated object
from your original:

separate_0.off
.
.
.
separate_30.off
separate_31.off


Carsten Arnholm

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As Carsten said, there's no way to "split" geometry.

If, for instance, you wanted to take a sphere, cut the top half from the bottom half, and separate the two halves, the only way to do it is to create *two* spheres, cut away the top half of one, and cut away the bottom half of the other.

Now, because OpenSCAD is after all a programming environment, that doesn't mean that you have to type out both models.

Start with this module:

big = 1000;
module cut(d) {
	// First:  top half
	translate([0,0,d/2]) intersection() {
		children();
		translate([-big,-big,0]) cube([big*2,big*2,big]);
	}

	// Second:  bottom half
	translate([0,0,-d/2]) intersection() {
		children();
		translate([-big,-big,-big]) cube([big*2,big*2,big]);
	}
}

and here's how you use it:

cut(10) sphere(10);

and here's what you get:

The same principle applies to any other kind of cut:  for each "segment", generate the model and cut away what you don't want in that segment.

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I got the impression that he wanted to have a way to perform an intersection
and automatically get all the connected components of the intersection as
separate children.  This can't be done by your method because you don't know
what the connected components are in advance.  And there's no limit to how
many components there are.  

I don't understand what he's trying to do and *why* he needs the connected
components to be separate.  


JordanBrown wrote
> Discuss@.openscad

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>
> oobpgooekkgmbfan.png (4K)
> <http://forum.openscad.org/attachment/31692/0/oobpgooekkgmbfan.png>





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I got the impression that he wanted to have a way to perform an intersection
and automatically get all the connected components of the intersection as
separate children.  This can't be done by your method because you don't know
what the connected components are in advance.  And there's no limit to how
many components there are.

Right.  If, for instance, the model was a human body, and you wanted to cut it at the knees and end up with two separate legs, your cut processing would have to "just know" that there are two legs and to cut between them.  There's no way to derive that from an arbitrary model.


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I want to produce this kind of output :
<http://forum.openscad.org/file/t3103/Capture_d%E2%80%99%C3%A9cran_de_2020-12-01_15-28-26.png>

Making the x and y slices was easy, their intersection also. But then I was
unable to access those intersections one by one (because I need to split
each of them in one top half and one bottom half).
With OpenJSCAD I needed to write my own Split3d to split them, but such
function needs to read a polyhedron vertices and sort them and there's
nothing to do that in OpenSCAD.

here what I have for the moment :
<http://forum.openscad.org/file/t3103/Capture_d%E2%80%99%C3%A9cran_de_2021-01-23_09-23-41.png>



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Yes, it can be done by specifying manually each time an intersection contains
more than one piece, but it would make the script very unpractical as it is
intended to be used from customizer to create skeleton of whatever volume
the user want.
 
Here is the original OpenJSCAD script : see  here
<https://raw.githubusercontent.com/gilboonet/gilboonet.github.io/master/sq_edit/slice_2axis_example.js>  
<http://forum.openscad.org/file/t3103/Capture_d%E2%80%99%C3%A9cran_de_2021-01-24_13-13-26.png>


 



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Here is a part of my process
<http://forum.openscad.org/file/t3103/Capture_d%E2%80%99%C3%A9cran_de_2021-01-24_13-59-06.png>
I design a volume (save it as 3d file)
Then make its skeleton, here with 3 X slices and 3 Y slices
The skeleton is made by interlocking those slices.
When an intersection returns more than one geometry, each of those
geometries must be considered as one intersection

The interlocking is made by splitting  each intersection in 2 (top and
bottom)
And subtract all top to X slices and all bottom to Y slices.
This way one's have slices that interlock perfectly

I'm making a collection of designs to be built from cardboard and I'm using
part vanilla js and jscad 2 scripts to generate my plans. I was wondering
whether it could be possible to do the same with OpenSCAD.



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I wrote a jscad function that does such splitting, it may be possible to use
it to add this functionnality to OpenSCAD, here it is :
/
function sortNb (E){ // returns E numerically sorted and deduplicated
        return E.sort(function(a, b) {return a-b}).filter(
            function(item, pos, ary) {return !pos || item != ary[pos - 1]});
}
function scission3d (geom){
  var i, Pl, j, i1, j1, ok, ti, tj, z,
  zz = [], P, RScission, til, tjl, tii1, zzl, zzdl;
// construit table de correspondance entre Polygones (P)
// build polygons lookup table
  //P = geom.toPolygons();
  P = geom.polygons;
 
  RScission = [];
  Pl = P.length;
  for (i = 0; i < Pl; i++){
        ti = P[i].vertices;
        z = [];
        for (j = 0; j < Pl; j++){
      tj = P[j].vertices;
          ok = false;
          for (i1 = 0; i1 < ti.length; i1++){
            tii1 = ti[i1];
                for(j1 = 0; j1 < tj.length; j1++)
                  if (!ok)ok = vec3.distance(tii1, tj[j1]) < 0.01;
          }
          if (ok)z.push(parseInt(j));
        }
        z = sortNb(z);
        zz.push({e:0, d:z});
  }

// regroupe les correspondances des polygones se touchant
// boucle ne s'arrêtant que quand deux passages retournent le même nb de
polygones
// merge lookup data from linked polygons as long as possible
  ok = false;
  nElOk = 0;
  do {
    lnElOk = nElOk;
        nElOk = 0;
        for (i = 0; i < zz.length; i++){
          if (zz[i].e >= 0) {
            nElOk++;
                for (j = 0; j < zz[i].d.length; j++){
                  a = zz[i].d[j];
                  if (zz[a].e >= 0)
                    if (i != a) {
                          zz[i].d = sortNb(zz[i].d.concat(zz[a].d));
                          zz[a].e = -1;
                        }
                }
          }
        }
        ok = lnElOk == nElOk;
  }while (!ok);

// construit le tableau des CSG à retourner
// build array of CSG to return
  for (i = 0, zzl = zz.length; i < zzl; i++) {
    if (zz[i].e >= 0) {
                        z = [];
                        for (j = 0, zzdl = zz[i].d.length; j < zzdl; j++){
                                z.push(P[zz[i].d[j]]);
                        }
                        if(z.length > 0) {
                        RScission.push(geom3.create(z));
                        }
          }
  }

  return RScission;
}
/



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On 2021-01-24 14:35, nop head wrote:
> I understand your problem now. When a slice is broken by holes in the
> original shape you want to be able to access the individual pieces
> resulting from a single intersection. No you can't do that in
> OpenSCAD.

That is what I said in my initial reply. There is no way to do it in
OpenSCAD, the only way is to do it outside OpenSCAD. As noted, polyfix
will do it easily, but there is no way OpenSCAD can detect the number of
individual pieces generated because it can't evaluate the contents of a
file folder.

Carsten Arnholm

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On 2021-01-24 15:04, gilboonet wrote:
> I wrote a jscad function that does such splitting,

What you wrote is possibly similar to what polyfix does, except it never
evaluates the vertex coordinates, it relies exclusively on the
polyhedron topology (which may have been reconstructed if the source was
STL).

However, this or that algorithm is not likely to find its way into
OpenSCAD because the language does not allow returning the result of
such algorithms, so you will never be able to access it from OpenSCAD
code.

Carsten Arnholm


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The "transform3d" idea proposed in the "Passing function literals to builtin
modules" would make this possible.

So maybe it's less impossible than Carsten thinks.  


acwest wrote
> This is an example where having a function render which returns polyhedron
> would be very useful.
>
> On Sun, 24 Jan 2021, 10:45 , &lt;

> arnholm@

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If it becomes possible for a function to return polyhedron, I think I could
port my splitter and I have another script that I will be happy to port to
OpenSCAD, an unfolder that I use to make furniture clothe and that is
companion of the 2 axis slicer (I use both to make my designs plans).
The two are used here :
<http://forum.openscad.org/file/t3103/Capture_d%E2%80%99%C3%A9cran_de_2021-01-10_10-45-39.png>  

acwest wrote
> This is an example where having a function render which returns polyhedron
> would be very useful.





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Hello, I recently discovered Cascade Studio and with it I was able to make those separation I needed. Maybe the way Open Cascade manage geometries could be useful if you want to extend OpenSCAD possibilities to do the same. Cascade Studio is a young project but it is already very powerful in the hands of a basic user like me, I'm certain that 3d specialists will be able to make marvels with it. One really net of its possibilities is that it is super easy to share code, you simply share the url like this

 

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It is certainly possible to imagine a 3D design tool, even a programming language like OpenSCAD, that can do things like cut up an object and then manipulate the separate solids that result.

OpenSCAD can't do that without a major overhaul, because it doesn't do any of the geometry processing until after the program has finished running.

The immediate output from an OpenSCAD program is not geometry, not a list of triangles, et cetera.  It is a tree representation of shapes, transformations, and the operations to be done on those shapes.  Look at Design / Display CSG Tree to see.  If you write "intersection() { sphere(); cube(); }", you'll see that that's pretty much what the CSG tree shows.  All of the expressions have been evaluated, all of the loops unrolled, all of the conditionals considered, but *none* of the geometry work has been done.  At that point in the processing, OpenSCAD doesn't *know* anything interesting about the geometry.

It is only when the geometry work is done that it starts to be possible to determine things like bounding boxes, like how many solids result from a particular boolean operation, and so on - and that's too late for the OpenSCAD program to take any action, because it's already done.

Could OpenSCAD have been designed differently, so that it did the geometry as it was processing the program, as Cascade Studio apparently does?  Sure.  But it wasn't, and that's a pretty fundamental design decision and a big job to change.

BTW, this isn't something that's a problem with the OpenSCAD language per se.  Another environment could easily make the same design decision.  Carsten's AngelCAD has the same architecture - in fact, it does the geometry work in a totally separate program.  I don't know about Doug's Curv - Doug?


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Thank you for your reply JordanBrown, indeed I'm now doing those things with two designs tools, OpenJSCAD and Cascade Studio, and I'm pretty sure it is also possible with CadQuery and Blender Python. I tried to do it with FreeCAD Python, but it is still too buggy. Maybe ScriptCAD can do it too. This is a design tool that one of OpenJSCAD creators, René K Müller, made, and it computes a scene in two stages, first computing, then rendering. I did such thing with one my OpenJSCAD script that I use to unwrap 3d models that I use to make faceted statues or furniture clothe. First I run the OpenJSCAD script that unflatten a polyhedron and save polygon data (points, faces and neighboring), then I can use a nodeJS script on those data, one to render a multi-page PDF of the unflatten net as it is, another to render it as a puzzle (interlocking edges). I hope that someday OpenSCAD will enable such processes, but maybe it is not something users are intended to do with it.

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