Summary:
WARNING: Normalized tree is growing past 200000 elements. Aborting
normalization.

 WARNING: CSG normalization resulted in an empty tree

Dear OpenSCAD folks,

I have a 3d scan of a cat. I want to make it post-processable by
converting the surface area to a volume.

Here is the idea:

OpenSCAD has a polyhedron(). Polyhedron describes 2d surfaces in a 3d
volume. If you are very "lucky", those surfaces provide a closed volume.

Example

      +----------+f
     /g         /|
    /          / |
   /          /  |
  /          /   |
 +----------+    |
 |a         |b   |
 |          |    |
 |          |    |
 |          |    |
 |          |    |
 |          |    +e
 |          |   /
 |          |  /
 |          | /
 |c         |/
 +----------+d

 points=[
  [a],
  [b],
  [c],
  [d],
  [e],
  [f]
 ];
 faces=[
   [a,b,c,d],
   [b,d,e,f],
   [a,b,f,g]
 ]
 polyhedron(points, faces);

The STL file format has basically the same idea. Provide a list of
coordinates that form a surface, provide a list of surfaces.

Minor implementation detail: Instead of providing (multi point) polygon
surfaces, you may just provide triangles. Polyhedron() implements this as
polyhedron(triangles=) and STL implements this as STL-binary-file-format.

My personal opinion is that people hurt themselfs with polyhedron() and
STL. You may create something that appears to be a cube, but has just 5
sides. Its preview will be fine, but it cannot be rendered, cannot be
exported and cannot be printed.

It happend recently on this mailing list that people provided non-closed
polyhedra. The first one was a scan of a pet - a surface in 3D space, the
second example was a spare part for a car - a surface in 3D space.

I think there is a generic case that can be helped:

Instead of using polyhedron() to draw surfaces into 3d space, we can use
hull() to create volumes in 3d space.

An implementation looks like this

 module hullhedron( vertice_list, surface_list){
   for( a_surface = surface_list) {
     hull(){
       for( a_coordinate_index = a_surface) {
         translate( vertice_list[ a_coordinate_index ]) cube(1);
       }
     }
   }
 }

To prove my idea a am trying to post process the 3d scan of a cat. I got a
list of vertices and surfaces extracted from a STL. There are 3 million
vertices and a similar number of surfaces/volumes. I am exceeding OpenSCAD
caches. What do I need to set to fix

 WARNING: Normalized tree is growing past 200000 elements. Aborting

normalization.

 WARNING: CSG normalization resulted in an empty tree

?
Any other recommendation how to overcome limits for a resource wasteful
prototyp?
OS: Linux
OpenSCAD version: OpenSCAD-2023.07.09.ai15799-x86_64.AppImage, but I may
work with any of the pre compiled binaries

Outlook: If the prototyp works, we can start creating closed volumes from
3d scans by providing a common center per model that each volume needs to
connect to.

Bonus: I promised you a CAT-Scan; Picture is attached :-)

Bonus: hullhedron() example attached

Bonus: If you are interested in processing 60MByte of binary STL yourself,
I will provide raw data.

Cheers,
Marcus

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Minor implementation detail: Instead of providing (multi point) polygon surfaces, you may just provide triangles. Polyhedron() implements this as polyhedron(triangles=) and STL implements this as STL-binary-file-format.

My personal opinion is that people hurt themselfs with polyhedron() and STL. You may create something that appears to be a cube, but has just 5 sides. Its preview will be fine, but it cannot be rendered, cannot be exported and cannot be printed.

Instead of using polyhedron() to draw surfaces into 3d space, we can use hull() to create volumes in 3d space.

On 7/14/2024 12:28 AM, Marcus Poller via Discuss wrote:

Minor implementation detail: Instead of providing (multi point) polygon surfaces, you may just provide triangles. Polyhedron() implements this as polyhedron(triangles=) and STL implements this as STL-binary-file-format.

The "triangles" parameter is deprecated.  The "faces" parameter is more
general, and a direct replacement.  "triangles" is treated as an alias for
"faces".

My personal opinion is that people hurt themselfs with polyhedron() and STL. You may create something that appears to be a cube, but has just 5 sides. Its preview will be fine, but it cannot be rendered, cannot be exported and cannot be printed.

It is certainly easy to get bad results with polyhedron().  (A five-sided
cube is pretty obvious in preview, though.)  It is even easier with STLs
because you have less control over them.

Instead of using polyhedron() to draw surfaces into 3d space, we can use hull() to create volumes in 3d space.

Yes, though hull() is only useful with convex shapes.  Your cat is not a
convex shape.  You can decompose your general shape into a number of convex
shapes, but that's hard.

Also, polyhedron is fast.  Hulling a thousand tiny cubes takes about 10s
on my system.  polyhedron() is nearly instantaneous, once you have the list
of points.  (Less than 1s, including generating the list of points.)

One trick is that hull() doesn't care whether its argument is a valid
polyhedron.  It works to give it a cloud of points that are arranged into a
single "face".  That's a lot faster than hulling cubes. But again, the
result is always convex.

points = [
for(i=[0:999]) rands(0,100,3)
];
faces = [ [ for (i=[0:999]) i ] ];

hull() polyhedron(points=points, faces=faces);

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I believe it is significantly faster to give a face list of triangles
(and one quadrilateral or pentagon) in instead of a single face
containing all the points.

I think the solution to problems making polyhedra  is to write
supporting code to ensure that they are valid.

Due to the convexity requirement hull is rarely the answer.

On 7/14/2024 5:43 PM, Adrian Mariano via Discuss wrote:

I believe it is significantly faster to give a face list of triangles (and
one quadrilateral or pentagon) in instead of a single face containing all
the points.

Don't know.  But in the question posed, we don't really know all of the
faces... or we could just use them.

I think the solution to problems making polyhedra  is to write supporting
code to ensure that they are valid.

Yep.

Due to the convexity requirement hull is rarely the answer.

Yes, agree.  The "polyhedron point cloud" scheme is an improvement over
creating numerous cubes and hulling them, but still has big problems.

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