[V8i VBA] ProjectPointOnPerpendicular

Hi Jon, Could you provide your test case ? I tested this method under a 2D model, it works fine. This method only needs one point and one element, such as line.

My test code is as below:

Sub TestProjectPoint()
   Dim oLine As LineElement
   Dim testPt As Point3d, outPt As Point3d
   Dim rMatrix As Matrix3d
  
   Set oLine = ActiveModelReference.GetElementByID(DLongFromLong(15763))
   testPt = Point3dFromXY(209.2955, 410.2692)
   rMatrix = Matrix3dIdentity
   outPt = oLine.ProjectPointOnPerpendicular(testPt, rMatrix)
   MsgBox outPt.X & "," & outPt.Y
End Sub

Regards, YongAn

Unknown said:

I tested this method under a 2D model, it works fine

That's the problem: it always works fine.  The method has no way of reporting a condition when it's impossible to create a perpendicular point.  See diagram in my edited first post.

In your second case, this method will return a projected point on the extended part of the line. Please see my pic (it is urgly than yours. :)).

There are another two methods (ProjectPointOnPerpendicular1 and ProjectPointOnPerpendicular2) to suit other cases.

HTH, YongAn

Unknown said:

This method [ProjectPointOnPerpendicular] will return a projected point on the extended part of the line

VBA help for ProjectPointOnPerpendicular doesn't mention that it projects the point onto the extended element.  On the other hand, VBA help for ProjectPointOnPerpendicular2 says: ProjectPointOnPerpendicular2 projects to the end point of the element if the elements do not intersect.

In fact, ProjectPointOnPerpendicular2 and ProjectPointOnPerpendicular appear to behave the same way: if there is no perpendicular point, the projection ends up on the end-point of the line..

They are different. The below pic shows my test result.

HTH, YongAn

Unknown said:

They are different

With a complex line string, they are the same  8-(

But thanks for your help!

Unknown said:

They are different

With a complex line string, they are the same  8-(

But thanks for your help!

I realise I am fashionably late this discussion (only 3 years late...) however I had precisely this issue today; I needed to verify that a returned coordinate from ProjectPointOnPerpendicular was in fact on the chosen element and not out with. I first thought I had a solution by the following:

• construct a temporary line using the returned coordinate as the start point and the same coordinate as the end point however adding +1 master unit to the coordinates .Z value
• With the chosen element, use  GetIntersectionPoints along with the temporary line and if the returned list of points is -1 then I know we don't have an intersection.
  
  

  Unknown said:

  Yongan.Fu

  They are different

  With a complex line string, they are the same  8-(

  
  Just like your findings above, the same irregular behaviour is found with GetIntersectionPoints, works fine for a line but not for a complex element i.e. constructing a test case with a line and a coordinate that can be graphically proven as not perpendicular to the line, GetIntersectionPoints return an upper boundary of -1 however switch the line for a complex chain ensuring the coordinate is still non-perpendicular, it returns 1 and the returned coordinate is the end point of the complex chain.

So after concluding GetIntersectionPoints wouldn't give me the result I needed I tried another approach:

• Extract a tangent vector from complex element
• Construct a vector from the returned 'supposed' perpendicular intersection coordinate and the source coordinate.
• Measure the angle between the vector's, if its not 90 then we know its not perpendicular (though for B-Spline Curves I've had to cater for floating point accuracy and allow for minor angle differences either side of perpendicular (between 89.995° and 90.005° I'm happy to consider perpendicular)