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