Using Surface.ClosestPoint to determine whether a point is on/inside a surface

Hi all,

I am trying to write a scripting component that determines whether a point is on a surface or outside. It works quite well using standard GH components, but I would like to pack it into a single scripting component.

The weird thing is: the Surface.ClosestPoint method seems to think any point inside the u,v domain is on the surface, so if a point is outside the surface, but inside the surface's u,v domain, the closest point is equal to the original point.

Grasshopper's "Surface CP" component seems to work quite well...

Does anyone have an idea what I am missing here?

My source code is:

private void RunScript(Point3d P, Surface S, double t, ref object PI, ref object PO, ref object ii)
{
    double u;
    double v;

    if (S.ClosestPoint(P, out u, out v))
    {
      Point3d tP = S.PointAt(u, v);
      double d = P.DistanceTo(tP);

      if (d <= t)
      {
        // distance is within tolerance
        PI = P;
        ii = true;
      }
      else
      {
        // distance is larger than tolerance
        PO = P;
        ii = false;
      }
    }
}

this is the GH defnition

and this is the Rhino output:

(there is an old discussion on the forum that seems to end without a real result..., see http://www.grasshopper3d.com/forum/topics/surface-closest-point-and...)

Attachments:

check-point-on-surface.gh, 23 KB

Replies to This Discussion

Permalink Reply by Andrew Heumann on March 14, 2013 at 8:40am

What Grasshopper calls a "trimmed surface" is actually a Brep with one face, not a "Surface" data type. Surfaces (in RhinoCommon) are always untrimmed, which would explain your result. Try Brep.ClosestPoint()

Permalink Reply by Florian Frank on March 14, 2013 at 2:30pm

Andrew is absolutely right. The Surface Datatype doesn't store trimm curves. Brep which means boundary representation does...it's a Surface with specified boundary also known as a trimmed Surface.

This means for your script, that in RhinoCommon all trimmed Surfaces should be casted as Brep type. See the difference in the pic below:

also the script works a little bit different, cause you have to run trough all faces of the brep...in your case obviously there is just one.

Cheers

Attachments:

checkpointonsurface.gh, 23 KB

Permalink Reply by pepe ballesteros on March 15, 2013 at 1:28am

hi,

could you explain the line code

double tolerance = (t == 0) ? doc.ModelAbsoluteTolerance : t;

Permalink Reply by Stefan on March 15, 2013 at 1:40am

it's shorthand for if-then-else,

see http://msdn.microsoft.com/en-us/library/ty67wk28%28v=vs.100%29.aspx

written out, it would be something like

double tolerance;
if (t==0)
tolerance=doc.ModelAbsoluteTolerance;
else
tolerance=t;

Permalink Reply by pepe ballesteros on March 15, 2013 at 4:26am

nice,

thanks

Permalink Reply by Stefan on March 15, 2013 at 2:21am

Excellent!
Thanks a lot.

Just for clarification: I was wondering a bit why you used BrepFace.DuplicateFace.

My a assumption is that you did this so you can use the Brep.ClosestPoint method. Is that right? Or is there something else behind it?

Best regards

Stefan

-------------------------------------------------------------

Here is the code including comments to illustrate what I mean:

private void RunScript(Brep B, Point3d P, double t, ref object Pi, ref object Po)
{
    double d = double.MaxValue;
    double tolerance = (t == 0) ? doc.ModelAbsoluteTolerance : t;

    foreach(BrepFace f in B.Faces)
    {
      // short version:
      double dist = f.DuplicateFace(false).ClosestPoint(P).DistanceTo(P);

      /*
      // long version fo the uninitiated:

      // cast BrepFace to Brep because BrepFace has only a method
      //   public bool ClosestPoint(Point3d testPoint, out double u, out double v)
      // which is somewhat cumbersome to use
      Brep currentFaceAsBrep = f.DuplicateFace(false);

      // with a Brep we can directly use a method
      //   public Point3d ClosestPoint(Point3d testPoint)
      Point3d clPoint = currentFaceAsBrep.ClosestPoint(P);

      // now compute the distance
      double dist = clPoint.DistanceTo(P);
      */

      if(dist < d)
        d = dist;
    }

    if(d <= tolerance)
      Pi = P;
    else
      Po = P;
}

Permalink Reply by Florian Frank on March 15, 2013 at 7:19am

Hi Stefan, actually i didn't have to DuplicateFace...just realized right now. The BrepFace had the same method...this is because i wrote it quickly an didn't take a look at the sdk...happens sometime ;-)

The comments are all right...you got every step of the script.

Have a nice day

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