[Bug?] pointOnFace in rare cases returns a point at a corner of the face

nnikbin — Thu, 01 Jul 2021 04:12:33 GMT

BRepFace.pointOnFace in some rare cases returns a point at a corner of the face. For example it returns (40, 40, 0) for the following face. The related file is attached.

It seems to be in contrast to what is promised in the relevant document:

Returns a sample point guaranteed to lie on the face's surface, within the face's boundaries, and not on a boundary edge.

It may not seem a big deal but for an add-in which depends on the accuracy of this property it could mean the potential for wrong functionality.

Re: [Bug?] pointOnFace in rare cases returns a point at a corner of the face

kandennti — Thu, 01 Jul 2021 14:41:59 GMT

HI @nnikbin .

I'm sorry, but I don't see this as a bug.

At least the "Shape Manager" seems to consider a point on an edge to be on a surface.
You can check this by using the isParameterOnFace method.

https://help.autodesk.com/view/fusion360/ENU/?guid=GUID-e6682eb7-48f8-472d-9872-71da3c5ba8c8

# Fusion360API Python script import adsk.core, adsk.fusion, traceback def run(context): ui = adsk.core.UserInterface.cast(None) try: app :adsk.fusion.Application = adsk.core.Application.get() ui = app.userInterface des :adsk.fusion.Design = app.activeProduct root :adsk.fusion.Component = des.rootComponent face :adsk.fusion.BRepFace = root.bRepBodies[0].faces[0] pointOnFace :adsk.core.Point3D = face.pointOnFace eva :adsk.core.SurfaceEvaluator = face.evaluator _, prm = eva.getParameterAtPoint(pointOnFace) ui.messageBox(f'{eva.isParameterOnFace(prm)}') except: if ui: ui.messageBox('Failed:\n{}'.format(traceback.format_exc()))

If it is not convenient on the edge, I think there is no other way but to create such a mechanism, so I made one.

# Fusion360API Python script import adsk.core, adsk.fusion, traceback import random def run(context): ui = adsk.core.UserInterface.cast(None) try: app :adsk.fusion.Application = adsk.core.Application.get() ui = app.userInterface des :adsk.fusion.Design = app.activeProduct root :adsk.fusion.Component = des.rootComponent # Point On Face face :adsk.fusion.BRepFace = root.bRepBodies[0].faces[0] pointOnFace :adsk.core.Point3D = face.pointOnFace dumpPoint(pointOnFace, root, 'Point On Face') # Point On Face Exclude On The Edge for _ in range(10): excludeOnEdge = getPointOnFace_ExcludeOnEdge(face) dumpPoint(excludeOnEdge, root, 'Exclude On The Edge') except: if ui: ui.messageBox('Failed:\n{}'.format(traceback.format_exc())) def getPointOnFace_ExcludeOnEdge( face :adsk.fusion.BRepFace) -> adsk.core.Point3D: def getUniqueParameter( minPrm :adsk.core.Point2D, maxPrm :adsk.core.Point2D) -> adsk.core.Point2D: return adsk.core.Point2D.create( random.uniform(minPrm.x, maxPrm.x), random.uniform(minPrm.y, maxPrm.y)) def isPointOnEdges( pnt :adsk.core.Point3D, toleranceEdgeDistance :float = 0.1) -> bool: for edge in face.edges: if isPointOnEdge(pnt, edge, toleranceEdgeDistance): return True return False def isPointOnEdge( pnt :adsk.core.Point3D, edge :adsk.fusion.BRepEdge, toleranceEdgeDistance :float) -> bool: eva :adsk.core.CurveEvaluator3D = edge.evaluator _, prm = eva.getParameterAtPoint(pnt) _, prmPnt = eva.getPointAtParameter(prm) return pnt.isEqualToByTolerance(prmPnt, toleranceEdgeDistance) # *** pnt :adsk.core.Point3D = face.pointOnFace if not isPointOnEdges(pnt): return pnt surfEva :adsk.core.SurfaceEvaluator = face.evaluator bound :adsk.core.BoundingBox2D = surfEva.parametricRange() while True: prm = getUniqueParameter(bound.minPoint, bound.maxPoint) if not surfEva.isParameterOnFace(prm): continue _, pnt = surfEva.getPointAtParameter(prm) if not isPointOnEdges(pnt): return pnt def dumpPoint( pnt :adsk.core.Point3D, comp :adsk.fusion.Component, name :str): skt :adsk.fusion.Sketch = comp.sketches.add(comp.xYConstructionPlane) skt.name = name skt.sketchPoints.add(pnt)

Since it can be done in a random position, the result will change every time you run it.
There may be a better algorithm.

Re: [Bug?] pointOnFace in rare cases returns a point at a corner of the face

nnikbin — Fri, 02 Jul 2021 19:32:20 GMT

Hi @kandennti , Thank you for your reply and your code. Actually after finding the rare case issue, I used the almost same method in my code based on this post without even using face.pointOnFace. I think based on its random nature it is almost impossible that this method to return a point on an edge.

In my opinion it is a bug. I think the whole point of face.pointOnFace is to return a point which is guaranteed to be within the face's boundaries (not on the boundary) because the boundary conditions causes problems in many algorithms (for example offsetting the edges inward or outward). If we want to find a point on a face and we do not care if it is on an edge or not, we can just use a point on edge or corner and easily find it.

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[Bug?] pointOnFace in rare cases returns a point at a corner of the face

Re: [Bug?] pointOnFace in rare cases returns a point at a corner of the face

Re: [Bug?] pointOnFace in rare cases returns a point at a corner of the face