Changebasis giving odd results

Rhino Developer
1

Transform.changebasis seems to be giving me incorrect results, unless I’m misunderstanding somewhere here… I have two planes with a shared origin but different normals. I take a sphere with the same origin, and transform it via change basis from one plane to the next. I would expect the sphere to just rotate, but it translates too.

Any idea whats going on?
Thanks,
Sam

        'create the start plane
        Dim StartPlane = New Plane(New Point3d(50, 30, 20), New Vector3d(0, 0, 1))
        Dim StartPlaneSurface As New PlaneSurface(StartPlane, New Interval(-111, 111), New Interval(-111, 111))
        doc.Objects.AddBrep(StartPlaneSurface.ToBrep)

        'create the end plane
        Dim EndPlane = New Plane(New Point3d(50, 30, 20), New Vector3d(1, 1, 1))
        Dim EndPlaneSurface As New PlaneSurface(EndPlane, New Interval(-111, 111), New Interval(-111, 111))
        doc.Objects.AddBrep(EndPlaneSurface.ToBrep)

        'create the transform
        Dim xform = Transform.ChangeBasis(EndPlane, StartPlane)

        'create the initial sphere
        Dim spherestart As New Rhino.Geometry.Sphere(New Point3d(50, 30, 20), 50)
        doc.Objects.AddBrep(spherestart.ToBrep)

        'transform the sphere
        spherestart.Transform(xform)
        doc.Objects.AddBrep(spherestart.ToBrep)
2

I think you want Transform.PlaneToPlane().

3

Great, that works, thanks.

I’m still wondering though, does change basis only achieve orientation and not position? I thought the point was to change coordinate systems… I ask because I’ve used it previously and might have to go back and fix what I’ve done.

Thanks,
Sam

4

my understanding is change basis gives you back the transform that would give you the current location, but in the other coordinate system…so this “moves” the object when applied. I think that implies that a for a change basis to only create a rotation of the object, the origins(and orientations) would have to be different for the two planes(effectively the second plane rotated around the center of the sphere).

5

OK, I will use the plane method from now on.

Thanks!
Sam

6

There’s a good explanation of this over at the GH forum: http://www.grasshopper3d.com/xn/detail/2985220:Comment:656329