Interfacing OpenCascade with NetGen

blobfish

CAD practitioner
Is there something you used from OSG? @blobfish
yes undercut

One of the things I wonder about, is the 2 vertical walls that are orthogonal to test direction. These 2 are accessible but no 'hits', so how to determine whether an orthogonal face is accessible? Maybe for those faces, your approach is used where the ray start on the face and project in the opposite direction?
 

Quaoar

Administrator
Staff member
One of the things I wonder about, is the 2 vertical walls that are orthogonal to test direction. These 2 are accessible but no 'hits', so how to determine whether an orthogonal face is accessible? Maybe for those faces, your approach is used where the ray start on the face and project in the opposite direction?

Here is an excerpt from the spec:

The [face accessibility] algorithm is outlined as follows:
  • A part is tessellated with CAD-oriented meshing algorithm (BRepMesh of OpenCascade or NetGen) to obtain a piecewise approximation of the initial curved geometry. The values of the linear and angular deflections required by the tessellation algorithm are selected automatically.

  • [Optional] The mesh is refined to achieve better skewness (aspect ratio or scaled jacobian) criteria and size threshold. The refinement is necessary to achieve the better accuracy of the accessibility analysis. Some simple techniques can be used, e.g., edge collapsing, edge split, edge swapping. Also, midpoint refinement and triangle subdivision can be employed at this stage.

  • The BVH structure is constructed for fast ray-triangle intersections. One important modification to BVH traversal procedure available in Analysis Situs consists in avoiding heap memory usage.

  • In each BVH facet, the index of the corresponding CAD face is preserved. The indices of facets could be preserved, but they should not address the corresponding elements from BVH as those indices are reordered dynamically (that is how BVH works). To keep track of the occluded triangles, we can store them directly with the coordinates, although that is not a memory-efficient solution.

  • The algorithm accepts a tool direction (axis) and associates with each triangle the reversed direction as a ray (bundle) to emit. The ray source is slightly shifted in the direction of the local normal vector at the corresponding facet. Therefore, if a facet is located next to a negative feature (e.g., hole) in the direction of inspection, an intersection point will not be detected.

  • For each facet’s ray, the intersection test is conducted. If a facet happens to be occluded, it is recursively subdivided with the limited subdivision depth. The recursive subdivision technique allows to slightly refine the large facets contributing to the occluded regions without modification of the initial mesh:
jI6vuWF8YSSZmMHm3-Bl6AdUTV21eI-lZkzNgdnqRMnWIBnrxOA4pSY655mG0QtNSalv9JUENeiEJ1yLX1VFCv5YQD9IYpmDop7vCqjbvgQfBPMTaOYrHOgMLqQ2mA=s0

  • For all faces, the total “score” of tests, hits and void intersections is counted. If the number of intersections by the total number of tests is high enough (e.g. 95%), the face is considered non-accessible. At this stage, all faces can be classified as Fully Accessible, Partially Accessible or Inaccessible.

  • The results of analysis are exported to WebGL-compatible file format (glTF).
The algorithm is performed for all the input directions that are supposed to be the principal machining directions. If a single face F is detected as an accessible face for at least one direction, it is deduced to be accessible in general, so it goes to the result.

So, yes, the rays are emitted from the facets in the direction opposite to the tool axis. Note also that there's a slight shift of the origin point aimed at avoiding false-positive intersections.
 

blobfish

CAD practitioner
So, yes, the rays are emitted from the facets in the direction opposite to the tool axis. Note also that there's a slight shift of the origin point aimed at avoiding false-positive intersections.

I was contrasting our approaches. I am creating a plane on the bounding sphere with the size of the bounding sphere radius. On this plane I create a uniform grid of points to fire rays into the mesh. With that method, the 'vertical' walls of mesh are indeterminate. With your method, the vertical walls are determinate. With my method a non-uniform BRepMesh will work fine. With your method you have to have a uniform mesh(thus netgen) or create multiple points on the bigger triangle faces. I still feel like this is a solved problem with raytracing. Have you looked into occt newer visual ray tracing?
 

Quaoar

Administrator
Staff member
With your method you have to have a uniform mesh(thus netgen) or create multiple points on the bigger triangle faces.
The customer wanted to have colored meshes in the output glTF. Or it was me who proposed that and then the customer started to want this :D

I still feel like this is a solved problem with raytracing. Have you looked into occt newer visual ray tracing?
It kind of is. I'm not that aware of any details of the OpenCascade's ray tracing algorithm, but I'm using their BVH data structures, which are super-fast to construct. It's no surprise as this stuff was developed by a couple of very advanced folks, one of them having a Ph.D. in ray tracing. They now run this Light Tracer company: https://lighttracer.org/
 

blobfish

CAD practitioner
The customer wanted to have colored meshes in the output glTF. Or it was me who proposed that and then the customer started to want this :D


It kind of is. I'm not that aware of any details of the OpenCascade's ray tracing algorithm, but I'm using their BVH data structures, which are super-fast to construct. It's no surprise as this stuff was developed by a couple of very advanced folks, one of them having a Ph.D. in ray tracing. They now run this Light Tracer company: https://lighttracer.org/
I got the cadrays program working on my debian box, but the graphics of the entire application was FUBAR. That was probably just my system. I don't buy graphics cards and run the prop. drivers on them. I also tried to create a test program that used occt viewer with ray tracing, but like most things occt, it turned into a huge time sink and I moved on.

Light tracer has some really impressive renders!

I had never heard of glTF. Blender does support it, so I made a glTF of my example. online glTF viewer. I have spent some time in blender and that has given me some intuition of how 3d graphics work.
 

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Quaoar

Administrator
Staff member
I had never heard of glTF.
I use it because it's quite suitable for www and literally all my clients want to visualize shapes using three.js library that supports glTF natively. Other than that, glTF is quite horrible to me. Maybe it's good for low-level rendering using OpenGL or WebGL though. Btw, CAD Assistant opens your model nicely:

1633541369385.png

If I remember well, glTF appeared in OpenCascade 7.5 first. Then we (me and Julia @JSlyadne) copied and pasted it to Analysis Situs to add support of colored feature faces, edges, and customized scene hierarchy. OpenCascade tends to evolve into a weird hybrid of visualization engine plus modeling, so sometimes copying and pasting stuff is the only way to make use of interesting functionality :D
 

Quaoar

Administrator
Staff member
Today I noticed that if NetGen crashes, it might be helpful to generate OpenCascade facets with BRepMesh beforehand. No idea why and how, but this unconscious cheat fixed the NetGen crash on one of the cases I have. 🤖
 

Quaoar

Administrator
Staff member
Is that the guy in the YouTube video?
Yes. But all these experiments with NetGen did not go further that getting a sufficiently nice surface triangulation. For many computer graphics (CAD) algorithms, a perfect fit would be the commercial package Express Mesh shipped by OCC for quite some bucks. It's a classical quadtree-based mesher, and if I could I'd rather use it instead of NetGen. Especially for the algorithms like what we're discussing in this thread. FOSS world cries for a good faceter! :D

As for FEM, that seems to be quite a popular subject in the community. Is that because there's no good enough open-source FEM package? How about Salome?
 

DanB

Active CAD practitioner
Yes. But all these experiments with NetGen did not go further that getting a sufficiently nice surface triangulation. For many computer graphics (CAD) algorithms, a perfect fit would be the commercial package Express Mesh shipped by OCC for quite some bucks. It's a classical quadtree-based mesher, and if I could I'd rather use it instead of NetGen. Especially for the algorithms like what we're discussing in this thread. FOSS world cries for a good faceter! :D

As for FEM, that seems to be quite a popular subject in the community. Is that because there's no good enough open-source FEM package? How about Salome?
Tbh, I was completely unaware of Salome. I have tried to do topology optimization with FreeCad and the results were, let's say, underwhelming.

The reason could be that many of us are working in engineering companies that spend a shit ton on these types of software coming from asshole companies🤷‍♀️
 

Quaoar

Administrator
Staff member
The reason could be that many of us are working in engineering companies that spend a shit ton on these types of software coming from asshole companies🤷‍♀️
I'm pretty skeptical about the capacity of a single developer to build up a decent software package. You need to be an engineer, a programmer and know your niche perfectly well to get to the point where you can develop something meaningful. And after that, you'll have to spend a hell of a time making the product. Getting to this condition would cost years, and you'll become older, establish your family and lose the courage to work sleepless nights. That's why I think asshole companies will dominate.

Sorry for this complete offtopic, just hard to get up to speed in the morning, so I'm procrastinating here :D
 
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