Loop.convex? question

[DavidBoulder]

I was experimenting with loop.convex? and found that extra co-linear points make a surface fail this test. As a result for example if I had a rectangle that I extrude it up, and then draw a rectangle on the bottom edge of one of the side surfaces that represents a door; the floor which would have otherwise passed the convex test, now fails it. I made a variation on the sample code in the API documentation to demonstrate it. Does anyone know an easy way to catch these cases so they don't fail?

 depth = 100
 width = 100
 model = Sketchup.active_model
 entities = model.active_entities
 pts = []
 pts[0] = [0, 0, 0]
 pts[1] = [width, 0, 0]
 pts[2] = [width, depth, 0]
 pts[3] = [0, depth, 0]

 # Add the face to the entities in the model
 face = entities.add_face pts
 loop = face.outer_loop
 status = loop.convex?
 if (status)
   UI.messagebox "Loop is Convex"
 else
   UI.messagebox "Loop is not Convex"
 end

 ptsB = []
 ptsB[0] = [200+0, 0, 0]
 ptsB[1] = [200+width/2, 0, 0]
 ptsB[2] = [200+width, 0, 0]
 ptsB[3] = [200+width, depth, 0]
 ptsB[4] = [200+0, depth, 0]

 # Add the face to the entities in the model
 face = entities.add_face ptsB
 loop = face.outer_loop
 status = loop.convex?
 if (status)
   UI.messagebox "Loop is Convex"
 else
   UI.messagebox "Loop is not Convex"
 end

[DavidBoulder]

Kind of a related question. I have a source data file that has clean versions of for example the floor that has only 4 verticies vs 6 (2 extra for the door). An alternative approach is to bring in those verticies, and do a loop.convex? test on them. But it appears I have to make a face before I can make a loop? Any way to do that without actually drawing a face. Otherwise, I can draw and erase a face, but seems like it would slow things down.

[kyyu]

You could get the vertices and get rid of the ones that form coliner edges. Make a new temporary face and go from there. Not pretty, but works. Maybe some has a better idea?

Oh, anyone got a simple/elequent way of getting the new vertices set, instead of what I did?

depth = 100
width = 100
model = Sketchup.active_model
entities = model.active_entities

ptsB = []
ptsB[0] = [200+0, 0, 0]
ptsB[1] = [200+width/2, 0, 0]
ptsB[2] = [200+width, 0, 0]
ptsB[3] = [200+width, depth, 0]
ptsB[4] = [200+0, depth, 0]

# Add the face to the entities in the model
face = entities.add_face ptsB

# Only keep non-colinear vertices
v = []
ver = face.outer_loop.vertices
n = (ver.length - 2)
    v1 = ver[0].position    
    v2 = ver[1].position
    u1 = [(v2.x - v1.x), (v2.y - v1.y), (v2.z - v1.z)].normalize!
    u1_saved = u1
for i in 1..n do
    v1 = ver[i].position    
    v2 = ver[i+1].position       
    u2 = [(v2.x - v1.x), (v2.y - v1.y), (v2.z - v1.z)].normalize!        
    if u1 != u2 then v.push ver[i] end
    u1 = u2      
end
    v1 = ver[(ver.length - 1)].position    
    v2 = ver[0].position       
    u2 = [(v2.x - v1.x), (v2.y - v1.y), (v2.z - v1.z)].normalize!        
    if u1 != u2 then v.push ver[(ver.length - 1)] end
    u1 = u2 
if u1 != u1_saved then v.push ver[0] end

# make new face & loop
group = entities.add_group
face2 = group.entities.add_face v
loop = face2.outer_loop
status = loop.convex?
if (status)
   UI.messagebox "Loop is Convex"
else
   UI.messagebox "Loop is not Convex"
end
group.erase!

[thomthom]

You could do your own convex function.

I got a method for getting only the corner vertices of a face:
http://www.thomthom.net/software/sketchup/tt_lib2/doc/TT/Face.html#corners-class_method
(Click View Source)

In my old TT_Lib1 library I had a method for getting the convext shape of a set of points in 2d space. http://forums.sketchucation.com/viewtopic.php?f=180&t=34306&start=15#p302805
Didier made one for 3d I think. (See later that thread)
Maybe you can cobble together something from this to your own convex? method?

[DavidBoulder]

Thanks for the feedback. The data file that is drawing my geometry in the first place already has a smaller list of verticies that is clean. So I guess that is the best option, going back to that, drawing a temporary face to test, and throwing it away. I don't know how much of a performance hit that will be over testing an existing face. But what I can do is pre-test with the existing face, and then only create a temporary face to test on the ones that initially fail.

David

Below is the code I ended up with. My SketchUp objects are drawn from data in an external file. I could have drawn temporary faces for convex test for every surface but decided to do the initial test off of the existing SketchUp faces, and then do a second test on faces that fail the convex test to confirm that they really are non-convex. This second test goes back to the data file, draws a temporary surface, tests it, and throws it away. The data file is already fee of un-necessary stray points.

               if @hash['NON_CONVEX_SURFACES']
               face = interior_partition_surfaces[index].entity
               loop = face.outer_loop
               status = loop.convex?
                 # if face is convex skip over it
                 if status
                   next
                 else
                   # failed first convex test, run second one
                   pointstemp = interior_partition_surfaces[index].model_object_polygon.points
                   model = Sketchup.active_model
                   entities = model.active_entities
                   facetemp = entities.add_face pointstemp
                   loop2 = facetemp.outer_loop
                   status2 = loop2.convex?
                   # erase facetemp
                   facetempall = facetemp.all_connected
                   entities.erase_entities facetempall
                   if status2
                     # next statement skips the rest of this if face is convex
                     next
                   end
                 end
               end