Shape properties using VolumeProperties and VolumePropertiesGK

[markkulait]

There are two different functions to calculate the volume properties in the BRepGProp (GProp_GProps, GProp_PrincipalProps) class:

• VolumeProperties
• VolumePropertiesGK

Both calculate the shape volume correctly. However, when calculating the inertia properties of the system, it seems that the former produces the correct results but the latter does not. For a box and a cylinder: Principal moments of inertia Cylinder Volume: 196349.54084936203 Ixx, Iyy, Izz: (194317337.4428578, 194291128.82151097, 61359231.51543226) Cylinder GK Volume: 196349.5408493644 Ixx, Iyy, Izz: (6.122366410532748e-40, -2.2351741790771484e-07, -3.5762786865234375e-07) Box Volume: 124999.99999999999 Ixx, Iyy, Izz: (52083333.33333338, 52083333.33333331, 52083333.333333276) Box GK Volume: 124999.99999999999 Ixx, Iyy, Izz: (1.6326310428087332e-40, -8.940696716308594e-08, -8.940696716308594e-08)

[tpaviot]

see https://dev.opencascade.org/content/difference-between-brepgpropvolumeproperties-and-brepgpropvolumepropertiesgk

Both methods use different algorithms. GK means Gauss-Kronrod integration. To be rigourous, each algorithm results should be compared to the theoretical solution for volume/Ixx/Iyy/Izz.

[markkulait]

For a cylinder (r = 25, h = 100), Izz = 0.5mr^2, and m = Pir^2h (density = 1). This gives Izz = 61359231.5154 which is the same as the result given by the VolumeProperties function. The result given by the VolumePropertiesGK function is way off, probably it gives something close to zero.

[tpaviot]

>>> print(brepgprop.VolumePropertiesGK.__doc__)


        Parameters
        ----------
        S: TopoDS_Shape
        VProps: GProp_GProps
        Eps: float (optional, default to 0.001)
        OnlyClosed: bool (optional, default to Standard_False)
        IsUseSpan: bool (optional, default to Standard_False)
        CGFlag: bool (optional, default to Standard_False)
        IFlag: bool (optional, default to Standard_False)
        SkipShared: bool (optional, default to Standard_False)

        Return
        -------
        float

Did you try to tune the parameters for this function?

[markkulait]

No, both algorithms were run with eps = 1e-3 and OnlyClosed = True.