How do you intersect two faces?

CAUL

I want to intersect two groups ( g0, g1) and put the intersection edges in a third group. Since intersect_with is very slow on complex geometry it should pay to find pairs of faces from g0 and g1 that might intersect and then intersect those pairs separately. An example, I have two groups with roughly 11000 faces each. This requires 121 million face/face comparisons, however, with some space hashing this number drops to ~400.000 from which ~4000 pairs of potential face intersections are extracted (ie. face pairs whose bounding boxes intersect). The whole procedure takes about a second. (To intersect the groups from the UI takes 16 minutes..)

Now, is there a way in sketchup to intersect just two faces? I'm at loss on how to use intersect_with for this and the Geom class contains only plane/plane intersection.

jolran

Join the party...

intersect_with is slow, but probably necessary evil.

That's quite a large collection you are having.

I found it ever so slightly faster in 2Dboolean to intersect with 1 of the groups containing edges only, to cut out the desired frame of desired geometry. But in your case that might not aid.
I reported a strange behavior from that that never got answered, or maybe it's so well known it did not deserve an answer..

"Edge collection still remembers face" - error, sort of thing..(scroll to my post)
http://sketchucation.com/forums/viewtopic.php?f=180%26amp;t=55238%26amp;hilit=+intersect

There was some post long time ago about using linesweeps instead, in the Cartesian plane.
And perhaps break edges on "hit". I don't think it's trivial to build something like that..
Maybe if it could be ported to C doing the calculations, however the cost..

TIG's many plugins use intersections quite often. You should dig in there..

If you would come up with something revolutionary you'd be a hero..

slbaumgartner

The heart of your problem is that intersect_with is an instance method of the Entities collection class. So, you need to create a temporary Entities collection and copy one of the Faces into it. For instance (assuming that the Faces are drawn in model coordinates - otherwise you will need to use something other than the identity Transformation):


temp_ents = Sketchup.active_model.entities.add_group.entities # can reuse this if purge between pairs of Faces
temp_ents.add_face(face1.edges)
ident = Geom;;Transformation.new
intersect_arr = temp_ents.intersect_with(false, ident, temp_ents, ident, false, face2)

Because it is all Ruby, I don't know whether this will actually be any faster than letting the built-in function handle the whole thing though.

Steve

jolran

This is what I do (sort of) in 2dboolean. But with 11000 faces ...

@unknownuser said:

Because it is all Ruby, I don't know whether this will actually be any faster than letting the built-in function handle the whole thing though.

20 seconds is better than 25 every optimization counts or are worth trying

CAUL

Thanks for the replies!

slbaumgartner, splendid advice. It worked much smoother than I thought it would. The code is attached below. Here are some results from intersecting the two columns in the example file:

Intersect from UI: 983s
Outer shell (*****): 161s
g0.intersect_with g1 (******): 104s
Intersect face pairs: 4s

() Outer shell is probably dominated by the intersect
(*) In order to produce all edges for coplanar cuts two calls to intersect_with has to be made with the groups reversed. The result above is for a single call.

cuttest.zip

CAUL_CutTest0.3.rb