DeleD Community Edition

[jones605562000]

I have tried to design a simple house. I used csg subtract to cut holes out for the doorway and the windows. The results are in the image below:



I have holes in my wall around the window in the right square of the editor and several extra lines ( faces?, polygons?) in the top left view and the bottom right view. All the extra lines run diagonally. I am not sure what is happening, if i am not doing something that needs to be done or if there is a problem with the software. To subtract I just make a cube the size I need, then select the model first, cube second and subtract.
this is starting to get really frustrating because this has happened several times. I can fix those problems/areas manually but would prefer to have the csg do clean cuts.

any help will be appreciated.
thanks
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[elementrix]

what if you click on "optimize" when you have the whole object selected.
There seems to be a weird geometry at some place's, is it one object or several?
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[Mr.Fletcher]

If CSG fails, it's mostly because of bad geometry, though the CSG commands don't work perfectly. Try optimizing the object first, weld all vertices that share the same position and be sure there are no holes in either mesh. Also don't worry about the extra edges in the walls, they are just a result of the triangulation by the CSG routines. You can merge the two triangles if they annoy you.
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[jones605562000]

I will try optimizing and welding the vertices and merging the whole object before i cut holes in it. This object was made by subtracting one cube from another so that all the outside walls are part of the same object. the inner walls are seperate objects. on each wall I made sure that some of that wall would be around the area I subtracted out, no subtraction against outside edges of the walls.

The walls are 16 units thick by the way. Don't know if that makes a difference or not, maybe too small an area?
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[Mr.Fletcher]

The thickness should not have any importance. You should also check whether a face of the cube aligns with a face of the other object, the CSG doesn't like that.
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[jones605562000]

Sorry to have to bother everyone again.

By welding vertices and optimizing I was able to create 10 openings
before having another problem. I also made sure that my objects snap
was set to vertices before subtracting holes.

In the following picture is the problem created by the 11th subtract.
The far right window with the white showing through on the right hand
side and highlighted in red on the other views is what created the
problem. The problem is 1 or 2 extra faces highlighted in blue that seems
to have cut through some of the other faces.



The next image is one of those faces highlighted with the face mode and
textured. The face seems to go right through the middle of the walls.



These faces are hard to remove and they make it difficult to do
regular textures on the model because they do divide up the original
faces into random sections.

Is their a limit to how many times you can us the subtract in a row?
I created all the walls first then cut the holes.
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[Jeroen]

I am trying to reproduce this case but sofar, things work as expected. My testmodel does not have innerwalls though, so that's what I am going to try now.

Perhaps you can post your original model (without the holes, only the walls) somewhere? Or maybe you can recreate the model from scratch, saving it between each CSG operation to a different file. If the error happens again, send the previous saved file to my emailaddress. That would greatly help us to solve any problem that might exist.

Btw, do you have auto-optimize on (settable in the CSG tab on the Option screen)?

[jones605562000]

I will rebuild the model and save after each csg operation.
I just checked and the auto optimize setting is on.
I will Post the results when done.

Thanks for taking the time to figure this out for me.
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[Jeroen]

Sure thing! If we can fix bugs like this, we will certainly do so. It's great to see you willing to help us out here. Thanks.

[jones605562000]

I rebuilt the model. Here's The steps that I took To build it.

1. I built 2 cubes so that I could subtract one from the other to create
an open end box.

2. I then scaled the box larger and made sure that it snap was set to vertex.

3. then by using cubes I drew out all the walls.

4. I scaled all the walls simultaneously to the right height.

5. At this point I saved the file. http://www.geocities.com/jones605562000/test_NMO.dxs.txt
Right click and save to download. Rename the file without the .txt
Geocities won't let me upload anything but htm,jpg, or text.

6. I then selected everything and merged all the objects, followed by optimize. http://www.geocities.com/jones605562000/test_MO.dxs.txt
same instructions as above for file download.

7. I made another cube and did a csg subtract. This is the result.
http://www.geocities.com/jones605562000/error.dxs.txt


I didn't even make it past 1 subtract this time.

I hope that all this helps. Thanks
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[Jeroen]

Excellent, I'll get into this a.s.a.p.

[jwatte]

There HAS to be some robust way of doing CSG. Why isn't there an open source library for that already?

For example, I could see how you could do the following:

1) Create a filling tetrahedralization of each of the parameter meshes, building up connectivity information (each face is either connected, or an outside face).
2) Do tetra/tetra polytope clipping between all overlapping tetrahedrons, retaining connectivity information.
3) Union all remaining polyhedra to build a mesh of the outside faces.

This mechanism may leave additional flat, two-sided faces in cases where objects exactly butt up, but that's probably OK. It's the case where your basic geometry connectivity gets messed up that you want to avoid at all costs.

[trucker2000]

I run into this problem all the time while doing csg subtraction of boxes. (same way as was done above) so to work around it, I make my building out of several wall sections. Once I'm done, I weld everything together. That fixes most of the problems I have exporting to torque.
It's something you can do until this gets fixed.
Sorry to not have reported this before Jeroen. I thought it was just the way csg worked.
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[jones605562000]

Thanks for the work around Trucker2000, that helps a lot to making a workable model.

That got me curious as to why single objects had no problem with the csg subtract, but more complex objects made up of several objects did seem to have a problem. I built 2 walls so that I made a corner but didn't merg them together. I just made them touch each other. I cut 4 holes out of each wall and didn't have any problems.

I then started again with that same setup, 2 walls making a corner.
I merged them together and optimized them. I cut 4 holes out of the first wall with out any problems. As soon as I cut out a hole in the second wall I ended up with the hole being cut in the 3d view. There were no faces on the inside edges of that hole. Also, that hole didn't show op in the other views. Same as the window in the top right section of the first picture that I posted. The same thing happened with 2 merged objects that were not optimized.

I thinking that the csg subtract routines don't like objects that have been merged. Maybe this little test I did will help figure out what is happening.
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[Jeroen]

I have examined the model you sent me, jones. CSG isn't working properly on that model because of bad geometry that exists within the model. CSG depends heavily on geometry being setup properly. Take a look at these screenshots showing bad geometry:





The highlighted polygon is totally inside the object. This might cause CSG routines to error, especially when the object is complex.

A method to create good geometry is something like this:

Create a cube and select a polygon


Extrude it a little bit without keeping the original polygon


Select another polygon


Extrude it again


Optimize the result


Using this method, you can create a level like this (unoptimized):


You might wonder how those extra inner-walls were created. I'm sorry for the bad quality of the pictures, but I think the idea is clear.

Select two opposing edges


Split them 3 times and go to vertex mode


Connect the newly created vertices


Select the new polygon


And extrude it. Again, without keeping the original polygon


Using CSG on a model created like this will give better results. You just have to make sure that there are no polygons inside the model anywhere. Also avoid overlapping polygons. And, like trucker already said, welding vertices whereever possible and using optimize is generally a good idea too. Following those rules should help out a great deal.