Wrong definition of the Compound

[XeuTap]

The documentation says that Compound is "a collection of disconnected solids". But this is not the exact definition. If we refer to the OpenCascade documentation, we can find the following sentences:
"COMPOUND: A group of any of the shapes below."
TopAbs_ShapeEnum Description
"A TCompound is an all-purpose set of Shapes"
TopoDS_TCompound Description

This inaccuracy leads for the unexpected behavior.

Examples:

Compound's center of mass

sk = cadquery.Sketch().rect(50, 50).faces()
face1 = sk.val()
face2 = face1.copy().translate(Vector(100, 0, 0))
compound = cadquery.Compound.makeCompound([face1, face2])
compound_center = compound.Center()
Properties = GProp_GProps()
BRepGProp.SurfaceProperties_s(compound.wrapped, Properties)
expected_center = cadquery.Shape.CombinedCenter([face1, face2]) # Vector: (50, 0.0, 0.0)
actual_center = compound.Center() # Vector(0.0, 0.0, 0.0)

assert actual_center  == expected_center, "Incorrect center of mass of the compound, expected {}, got {}".format(expected_center, compound.Center())

The following code makes a compound consisting of 2 faces and calculates the center of mass. The problem is that shape_properties_LUT interpets TopAbs_COMPOUND as a volumetric shape and provides an incorrect result.

Shell

Similar behaviour occurs in the Shell.makeShell method

sk = cadquery.Sketch().rect(50, 50).faces()
face1 = sk.val()
face2 = face1.copy().translate(Vector(50, 0, 0))
face3 = face1.copy().translate(Vector(100, 0, 0))
shell_connected = cadquery.Shell.makeShell([face1, face2]) # TopoDS_Shell
assert isinstance(shell_connected.wrapped,TopoDS_Shell)
shell_disconnected = cadquery.Shell.makeShell([face1, face3]) # TopoDS_Compound
assert isinstance(shell_disconnected.wrapped, TopoDS_Shell), "Wrapped type mismatch, expected TopoDS_Shell, got {}".format(shell_disconnected.wrapped)

This code creates 2 shells: one from the connected faces, and another from disconnected. The type of the second shell is TopoDS_Compound, which corresponds to the OpenCascade definition. However, cadquery states that the wrapped attribute of the shell object should be TopoDS_Shell. If we call Center(), we'll get the same behaviour as in the previous example.

If this feedback is relevant, I’d be happy to open a PR with a solution that addresses the specific issue with Center(). However, I’m unsure whether other operations may also be affected by this underlying inconsistency.

[adam-urbanczyk]

Thanks, the Center() thing is definitely a bug. Regarding shell, I'd suggest using shell from the free function API. Parts of the cq.Shape API are rather dusty, and not really meant for end users.

Regarding PR, I'd rather reimplement Center() for cq.Compound.

[XeuTap]

Thank you, @adam-urbanczyk. I opened a PR where I reimplemented Center for cq.Compound. There is also a utility method that that inspects the Compound.
#1822

Regarding cq.Shell, in my opinion, this is not a major issue. Technically, it’s incorrect to attempt to create a shell from disconnected faces. A possible compromise from a UX perspective would be to handle this edge case and throw an exception.

[jmwright]

This should be fixed via #1822