Hi Bryn
There are a couple of differences. surface::point_perp is lower level functionality. Given a point P and surface S, the local extrema of the distance function from P to S satisfy the "point perp" condition: specifically, the vector between P and a closest point on S forms a diameter of a sphere which is tangent to the surface.
There are two flavors of point_perp: each does less than api_entity_point_distance. Entity point distance finds a global minimum of the distance function and makes sure it is inside the face (not just on the surface of the face, it is inside the trimming curves).
surface::point_perp with a guess of the closest point does a relaxation (roughly like Newton's method) to find a local minimum of the distance function. point_perp without a guess checks all over the surface and finds a global minimum of the distance between the point and the surface. Neither point perp flavor knows about the trimming of the faces/edges, or anything at all about the body.
If you know something about an existing mesh, it might be possible to use that information along with one of the point perp functions. Given an arbitrary point in space, you could
- find the closest point on a triangle in your mesh
- compute the barycentric coordinates of the closest point
- use the barycentric coordinates and the uvs of the mesh points to get a reasonable guess for point perp.
If you are able to verify independantly that the foot (closest point on the surface) you find is in the face of interest, this can be much faster. e.g., if the foot is inside a triangle you can verify is in the face, you should be done.
The downside with using lower level functionality is that it offers less guarantees about the answer. Journalling isn't avialable either. But it provides another approach if the API doesn't do what you need. The lower level functionality does less but is faster than the API. I hope this helps.
Eric
