Someone recently asked about determining if a point resides within a polyline using VBA. There are a few ways to achieve this, some being more complex than others, each with their own advantages. However, there is one way that is particularly simple within the AutoCAD® environment, as we are able to use built in features such as the IntersectWith method.
The concept for finding out if a point is within a polyline is this – if we were to draw a line from the point in any direction to infinity, the number of intersections the line would have with the polyline would be an odd number. Think about it – if it was a square, it would cross the polyline once. For irregular shapes where the line crosses it many times, it firstly has to exit the shape – any re-entry to the shape must have a corresponding exit – so if the point is within the polyline, any line eminating from it to infinity must cross the boundary an odd number of times. And for the same reason, any point outside the boundary must cross the boundary an even number of times.
So, armed with this knowledge, we need a way to actually achieve this in AutoCAD®. And what better way of doing so is there than to simply draw a Ray in any direction, and find out how many intersections with the boundary there are? Well, that’s exactly what this code does:
Dim selPolyline As AcadLWPolyline
Dim selPoint As AcadPoint
Dim pnt As Variant
Randomize 'Initialise random number generator
ThisDrawing.Utility.GetEntity selPolyline, pnt, "Pick polyline"
ThisDrawing.Utility.GetEntity selPoint, pnt, "Pick point"
If isPointInPolyline(selPolyline, selPoint) Then
ThisDrawing.Utility.Prompt "The point resides within the polyline"
ThisDrawing.Utility.Prompt "The point resides outside the polyline"
Function isPointInPolyline(pl As AcadLWPolyline, pnt As AcadPoint) As Boolean
Dim p1 As Variant
Dim p2 As Variant
Dim ray As AcadRay
Dim arr As Variant
Dim upperbound As Long
Dim IntersectionCount As Long
p1 = pnt.Coordinates
p2 = p1
' edit on 03/12/2010 for increased reliability
' horizontal ray exchanged for a ray with a random direction
p2(0) = p2(0) + 1 - Rnd * 2 'offset x coordinate for secondary point in Ray by random amount
p2(1) = p2(1) + 1 - Rnd * 2 'offset y coordinate for secondary point in Ray by random amount
' end of edit
Set ray = thisSpace.AddRay(p1, p2)
arr = ray.IntersectWith(pl, acExtendNone)
upperbound = UBound(arr)
If upperbound = -1 Then
' No intersections - the point must not be inside the polyline
' Assumes no elevation
isPointInPolyline = False
IntersectionCount = (upperbound + 1) / 3
'number of elements in array is equal to the upperbound + 1 because of element zero
'we divide by 3 to find the number of individual intersections because each has
'3 coordinates - X, Y and Z
If IntersectionCount Mod 2 = 0 Then
'There are an even number of intersections - it cannot be inside the polyline
isPointInPolyline = False
'There are an odd number of intersections - it must be inside the polyline
isPointInPolyline = True
Function thisSpace() As AcadBlock
If ThisDrawing.ActiveSpace = acModelSpace Then
Set thisSpace = ThisDrawing.ModelSpace
Set thisSpace = ThisDrawing.PaperSpace
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