Compare the time of converting STEP to glTF using program and open STEP in CAD Assistant

[wsw0108]

The code, meshing parameters, and test step file, are described in #688 (comment)

• The code takes about 8 minutes(mostly on meshing shapes) to convert 3066.step to gltf
• CAD Assistant 1.6.0.0 takes about 8 seconds to open the step file

@dpasukhi, could you share the parameters CAD Assistant used to convert/render step file?

[dpasukhi]

Hello. The main difference - CAD Assistant is used the compound of all roots as an input. You can check the discussion here:
https://dev.opencascade.org/content/how-accelerate-triangular-mesh-speed-occt-mainly-used-visualization
and
https://dev.opencascade.org/content/cad-assistant-brepmeshincrementalmesh-parameters

CAD Assistant is based on OCCT 7.6 which can have some difference in the behaviors.

[wsw0108]

Thank you very much for your reply.

After read articles you mentioned, I found that it seems that the key point is to use the compound shape.
And the default parameters for BRepMesh_IncrementalMesh from log is:

• linear deflection: 0.001, relative
• angle deflection: 20 degree(in rad: 0.34906585)

So I update the code as below, but it still take about 5 minutes to process the step file(linear deflection is 0.1. If linear deflection is 0.001 as in code, it will take much longer time).

int main() {
    OSD_Parallel::SetUseOcctThreads(false); // false to use tbb
    BRepMesh_IncrementalMesh::SetParallelDefault(Standard_True);

    Handle(XCAFApp_Application) app = XCAFApp_Application::GetApplication();
    Handle(TDocStd_Document) doc;
    app->NewDocument("MDTV-XCAF", doc);

    STEPControl_Reader reader;

    auto status = reader.ReadFile("3066.step");
    if (status != IFSelect_RetDone) {
        return 1;
    }

    reader.TransferRoots();

    TopoDS_Shape shape = reader.OneShape();

    {
        BRepMesh_IncrementalMesh mesher;

        mesher.SetShape(shape);

        auto& params = mesher.ChangeParameters();

        params.Deflection = 0.001;
        params.Relative = true;
        params.Angle = 0.34906585; // 20 degree
        // params.MinSize = 0.1;
        params.InParallel = Standard_True;
        params.InternalVerticesMode = Standard_True;
        params.ControlSurfaceDeflection = Standard_True;

        mesher.Perform();
    }

    Handle(XCAFDoc_ShapeTool) shapeTool = XCAFDoc_DocumentTool::ShapeTool(doc->Main());
    shapeTool->AddShape(shape, true);

    {
        RWGltf_CafWriter gltfWriter("output.gltf", false);

        if (!gltfWriter.Perform(doc, TColStd_IndexedDataMapOfStringString(), Message_ProgressRange())) {
            return 1;
        }
    }

    return 0;
}

[gkv311]

        params.Deflection = 0.001;
        params.Relative = true;

Just to point out that IMeshTools_Parameters::Relative option of BRepMesh_IncrementalMesh is not exactly the same thing as Aspect_TOD_RELATIVE tessellation option of Prs3d_Drawer::TypeOfDeflection() used in visualization. It has similar intention, but use different formulas - thus cannot be trivially interchanged. In practice, I haven't seen much of IMeshTools_Parameters::Relative to be used in real application, so cannot say if its behavior is close enough or not.

In case of a visualization, the 'relative' option calculates bounding box of a shape and uses math in StdPrs_ToolTriangulatedShape::GetDeflection() to compute absolute deflection parameter to be passed to BRepMesh_IncrementalMesh (with IMeshTools_Parameters::Relative = false).

I'm not saying that this is exactly problem in your case, just another hint to check.

See the options in CAD Assistant log - it says that it has computed mesh with absolute deflection parameter equal to 1.06.

[wsw0108]

@gkv311 Thanks for the hint.

I just tried set deflection = 1.06 and relative = false, and the above program take about 3 seconds to process the model.

So rel.: 0.001 in the log does not means below statements?

params.Deflection = 0.001;
params.Relative = true;

[gkv311]

Apparently yes - the deflection is compute something like this:

    IMeshTools_Parameters aMeshParams;
    Handle(Prs3d_Drawer) aDrawer = new Prs3d_Drawer();
    // deflection, relative to bounding box extent
    aDrawer->SetDeviationCoefficient (0.001);
    // absolute deflection, computed as a fraction of shape's bounding box
    aMeshParams.Deflection = StdPrs_ToolTriangulatedShape::GetDeflection (aShape, aDrawer);

    //aMeshParams.Angle = aDrawer->DeviationAngle();

[wsw0108]

Thanks. Finally I use Prs3d::GetDeflection(aBox, 0.001, 0.0001) to calculate the absolute linear deflection.