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# Line Of Intersection Between Two Compenetrating Surfaces

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 Line Of Intersection Between Two Compenetrating Surfaces on Jul 16, 2012 at 12:59:49 pm

I need to be able to find (extract and use) the line of intersection between two compenetrating planes.
In other modelling programs, as boolean operations apply also to surfaces (planes), this is a very straightforward task. You apply an "intersection" operation between the two surfaces and you are left with the line of intersection.

Now, this method apparently does not work on Cinema 4D, as you cannot apply a boolean operation to surfaces.

Is there a way to solve this?

In my specific case, as the two surfaces are the result of extruding a curved line in plan (ZX plane) and a curved line in elevation (front view, YX plane), I found a way to solve the issue with a minimal margin of error.
Basically I extrude only one of the two splines and I project the other (Mesh/Spline/Project) onto the surface just created. Now, if the spline to project has very few points the margin of incoherence between the surface and the resulting spline will be quite visible. In this case, before projecting the spline, add as many points as you can (Mesh/Spline/Round) in order to reduce the margin of error.
Be aware though, that even if barely visible, you will still have incoherences between the projected spline and the plane. The result is really only an approximation of a true line of intersection between two planes.

Cheers.

 Re: Line Of Intersection Between Two Compenetrating Surfaceson Jul 16, 2012 at 4:49:58 pm

it's multi-step but you should be able to get it by doing a Boole - either A Subtract B or A Without B, make sure Select Intersections is checked, make the Boole editable then the resultant mesh should have your intersection selected (Edge mode) which you can extract with Edge to Spline

 Re: Line Of Intersection Between Two Compenetrating Surfaceson Jul 17, 2012 at 7:32:20 am

Thanks Brian, it actually worked! Your instructions were sufficient to get it done, but I would like to add some extra info.

I used the boolean operation with the following setting:
-A without B
-High Quality
-Hide new edges
-Create Phong breaks at intersections
-Select intersections

This resulted in a Boole which shows one of the surfaces with a cut. So I made the Boole editable.
If you look at the objects structure now, you will notice that the actual surface object is nested inside the editable Boole, which is in fact an empty object. I extracted the surface object from the editable Boole and I erased the Boole.
Then I selected Edge Mode, which immediately highlighted the cut, and then I went to Mesh/Commands/Edge to Spline.
This created a spline nested in the surface object, i.e. your line of intersection.