Puzzling over face generation

[walterjonathanvdk]

I am attempting to generate new faces by adding crossing edges over existing faces. I am finding that inner loops which meet these crossing edges at a vertex are not easy to deal with. Some times they become empty loop appendages of the outer loop of an adjacent face rather than inner loops of the face that they are in. So I am trying to work out how Sketchup generates faces in the first place.

I have this code which creates a face with holes:

mod = Sketchup.active_model
ents = mod.entities
sel = mod.selection 

outer_pts = [[0, 0, 1], [3, 0 , 1], [3, 7, 1], [0, 7, 1]]
face = ents.add_face(outer_pts)

hole1_pts = [[1, 1, 1], [2, 1, 1], [1, 2, 1]]
hole1_face = ents.add_face(hole1_pts) 
hole1_face.erase!

hole2_pts = [1, 3, 1], [2, 4, 1], [1, 4, 1]
hole2_face = ents.add_face(hole2_pts) 
hole2_face.erase!

hole3_pts = [[1, 5, 1], [1, 6, 1], [2, 4, 1]]
hole3_face = ents.add_face(hole3_pts) 
hole3_face.erase!

Then I have this code which adds crossing lines in different orders:

a = [[2, 0, 1], [2, 1, 1]]
b = [[2, 1, 1], [2, 4, 1]]
c = [[2, 4, 1], [2, 7, 1]]

[a, b, c].each{|segment| ents.add_edges(segment)}
[a, c, b].each{|segment| ents.add_edges(segment)}

[b, c, a].each{|segment| ents.add_edges(segment)}
[b, a, c].each{|segment| ents.add_edges(segment)}

[c, b, a].each{|segment| ents.add_edges(segment)}
[c, a, b].each{|segment| ents.add_edges(segment)}

These are the results showing the order in which the edges where added and the right side face and its edges selected each time:

It might seem obvious, but what I conclude is that resulting organization of face geometry only depends on the location of the last or closing edge and not on the order in which the previous edges where added.

edit: actually that is not right. if so then version 3 and version 5 of the front face examples should end up with the same holes and they don’t…

The configuration I want is either the 5th version of the front face or the 1st version of the back face where all of the original inner loops are retained. So I tried to work out how the winding order of each faces edges might explain the results

I used this code to traverse the edges:

face = sel[0]
outerloop = face.outer_loop
face_edges = outerloop.edges

count = 0 
sel.add(face_edges[count])
count += 1

For the first version these are the fist 6 edges:

Here the upper triangle is an inner loop of the left face. What I presume is that, for front faces, Sketchup starts with the first least angle clockwise edge from the closer and then adds the next least angle anticlockwise edges until it returns to the first edge. Just using that logic though, the fifth edge should be the third edge. So, evidently or possibly, when a node is encountered that connects with a loop, that loop gets traversed as well, as a special condition.

Is this winding scenario correct?

This makes me wonder why the upper triangle was not included as an appended loop to the right face like the middle triangle was.

This is how the left face outer loop is wound:

Here the first edge is again the least clockwise angle edge from the closer and it seems to march around in the anticlockwise direction collecting each subsequent least anticlockwise edge from there.

Two of the inner loops are passed over which kinda makes sense since they would not be found using the anticlockwise least angle method.

But why is just the top triangle retained as an inner loop of the right face?

I’m not sure if Sketchup evaluates faces separately or consecutively, In either case, one of the faces is crossing the outer loop of another face.

I know that, in general, I can recreate the holes by adding them back as faces and then erasing those faces. In this case the result can be not helpful however. Say I use this code to try recreate the middle triangle:

hole2_pts = [1, 3, 1], [2, 4, 1], [1, 4, 1]
hole2_face = ents.add_face(hole2_pts) 
hole2_face.erase!

This is the result:

The hole is not created and it is another part of the face that ends up getting erased.

Will I end up having to erase every face, save its properties, and recreate every face in a situation like this?

[slbaumgartner]

I’m not where I can check right now, but it seems to me you may be encountering the long-standing “bow-tie” bug that can occur when a vertex of an inner loop lands on an edge of the outer loop. SketchUp’s loop-building logic somehow goes berserk and creates twisted loops that alternate between clockwise and anti-clockwise around their normal vector. Search the forum for bow-tie bug and you should find more discussion of the bug and possible workarounds. Good luck!

[walterjonathanvdk]

Yikes I get a lot of different results when playing around with this. Beserk huh, hmmm

[TIG]

Make all of the edges/faces in a group.
Use group.entities.intersect_with()

https://ruby.sketchup.com/Sketchup/Entities.html#intersect_with-instance_method

All if the edges should now split up the faces. Now collect the faces and if needed use face.reverse! to ensure the normal is what you want. New process the faces to find the ‘holes’ and then erase them by collecting those faces which have at least at lease an edge that has edges[1] [that is it’s an inner hole], then if so using face.erase! but first collecting all of the face.edges that have aface[1] then using erase! again on any ‘orphaned’ edges. A bit convoluted but it should work.

[walterjonathanvdk]

Thank you TIG. I doubt I would ever have stumbled on that solution. I found, in this specific situation, I did not have to reinvent the holes. I did it like this:

mod = Sketchup.active_model  
ents = mod.entities
sel = mod.selection 
g = ents.add_group

outer_pts = [[0, 0, 1], [4, 0 , 1], [4, 7, 1], [0, 7, 1]]
face = g.entities.add_face(outer_pts)

hole1_pts = [[1, 1, 1], [2, 1, 1], [1, 2, 1]]
hole1_face = g.entities.add_face(hole1_pts) 
hole1_face.erase!

hole2_pts = [1, 3, 1], [2, 4, 1], [1, 4, 1]
hole2_face = g.entities.add_face(hole2_pts) 
hole2_face.erase!

hole3_pts = [[1, 5, 1], [1, 6, 1], [2, 4, 1]]
hole3_face = g.entities.add_face(hole3_pts) 
hole3_face.erase!

hole4_pts = [[2, 4, 1], [3, 4, 1], [3, 3, 1]]
hole4_face = g.entities.add_face(hole4_pts) 
hole4_face.erase!
    
entities1 = g.entities

crossing_pts = [Geom::Point3d.new(2, 0, 1), Geom::Point3d.new(2, 1, 1), Geom::Point3d.new(2, 4, 1), Geom::Point3d.new(2, 7, 1)]
entities2 = ents.add_edges(crossing_pts)

transformation1 = IDENTITY
transformation2 = g.transformation

ents.intersect_with(true, transformation1, entities1, transformation2, true, entities2)
ents.erase_entities(entities2)
g.explode