Tolerance for Intersection?

Hi,
In WCS with Ortho on I draw two perpendicular lines. The end point of the first line is the startpoint of the second. Now, I move one of the lines away from the other 0.05 units. On the drawing you can clearly see that the lines are not intersecting any more. Although, when I use IntersectWith method it still shows as if lines are intersecting. The magic number os 0.07. If the distance between the lines is less than 0.07 they will be shown as intersecting. Is there a variable that distinguishes a tolerance for intersecting objects?

Here's the code I use to see if lines intersect:

Sub aa()
Dim returnObj1 As AcadObject, returnObj2 As AcadObject
Dim basePnt As Variant
ThisDrawing.Utility.GetEntity returnObj1, basePnt, "Select an object"
ThisDrawing.Utility.GetEntity returnObj2, basePnt, "Select an object"
intPoints = returnObj1.IntersectWith(returnObj2, acExtendNone)
MsgBox UBound(intPoints)
End Sub

Thanks,
Mike

 

Are those lines at large coordinate locations? You sample works fine near
0,0.

--
R. Robert Bell


Hi,
In WCS with Ortho on I draw two perpendicular lines. The end point of the
first line is the startpoint of the second. Now, I move one of the lines
away from the other 0.05 units. On the drawing you can clearly see that the
lines are not intersecting any more. Although, when I use IntersectWith
method it still shows as if lines are intersecting. The magic number os
0.07. If the distance between the lines is less than 0.07 they will be shown
as intersecting. Is there a variable that distinguishes a tolerance for
intersecting objects?

Here's the code I use to see if lines intersect:

Sub aa()
Dim returnObj1 As AcadObject, returnObj2 As AcadObject
Dim basePnt As Variant
ThisDrawing.Utility.GetEntity returnObj1, basePnt, "Select an object"
ThisDrawing.Utility.GetEntity returnObj2, basePnt, "Select an object"
intPoints = returnObj1.IntersectWith(returnObj2, acExtendNone)
MsgBox UBound(intPoints)
End Sub

Thanks,
Mike

 

Yes, the coordinates are pretty large, like 6 digits.

 

I'm not surprised. Remember that AutoCAD is accurate to only 14 significant
digits.

You could temporarily change the UCS origin to one of your pick points
before attempting to locate the intersection.

 

Thanks, I'll try that.

 

Robert, the intersection still returns the coordinates in WCS. That means that the problem cannot be solved by changing the UCS Origin.

 

Yes, it appears that method uses WCS to determine intersection, so the usual
workaround (for hatches and so on) doesn't help.

Sorry.

Another approach you could take when dealing with large coords is to copy
the objects to near WCS 0,0,0, determine intersection, then translate coords
back to original location. <blech>

--
R. Robert Bell


Robert, the intersection still returns the coordinates in WCS. That means
that the problem cannot be solved by changing the UCS Origin.

 

The problem you are seeing is fundamental to any floating
point coordinate system. Look at the coordinates of the
lines involved, and count the number of significant places
to the left of the decimal point, subtract from 8, and the
result is the number of usable digits to the right of the
decimal point.

Translating the geometry closer to the origin might seem
to help solve the computation problem but in fact, when
you translate the objects back to the original location they
may not be in the exact same location, and/or the calculated
intersection point that you must also translate in that case,
may not be accurate.

first line is the startpoint of the second. Now, I move one of the lines
away from the other 0.05 units. On the drawing you can clearly see that the
lines are not intersecting any more. Although, when I use IntersectWith
method it still shows as if lines are intersecting. The magic number os
0.07. If the distance between the lines is less than 0.07 they will be shown
as intersecting. Is there a variable that distinguishes a tolerance for
intersecting objects?