bools problem

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shadow
Posts: 35
Joined: Tue Dec 31, 2002 1:33 pm

bools problem

Post by shadow »

hi

I have a strange problem with boolean operations. I make two meshes, e.g. two cubes with different size. When I want to make a union sometimes the result is 'intersection', and I have to use 'intersect' to make a 'union'. But sometimes it works fine. Where is a problem? Both meshes are closed. Can anybody help me?
The box said: "Requires Windows 95 or better"
So I installed Linux...

thorax
Posts: 320
Joined: Sun Oct 27, 2002 6:45 am
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Re: bools problem

Post by thorax »

shadow wrote:hi

I have a strange problem with boolean operations. I make two meshes, e.g. two cubes with different size. When I want to make a union sometimes the result is 'intersection', and I have to use 'intersect' to make a 'union'. But sometimes it works fine. Where is a problem? Both meshes are closed. Can anybody help me?

Try to stop thinking of using booleans as a method of modelling, that's the problem.. Actually to do booleans blender must turn a object into a volumetrically enclosed shape, then break the objects up into sub-objects
and reconstruct the objects back, so what you put in isn't always like what you get out.. I'm not sure blender even does this.. Its booleans aren't fantastic, but booleans in general aren't... Its a feature that carried on over from CSG (constructive solid geometry, primitives represented by math which only defines what is not inside the primitive) that should have..

Read up on CSG and what polygon-based-booleans is, and you will
see what the problem is and why a lot of modellers try to avoid using booleans period.. For a program like POV its okay because POV has volumetrically defined primitives.. The only one blender has is a Metaball.. Everything else is polygons and surface patches..

tordat
Posts: 0
Joined: Sun Apr 13, 2003 7:11 pm

Post by tordat »

Blender is very predictable in bools. You only have to consider that the three option work like they should if both objects have their normals outside. E. g. if one object has the normals inside, intersect will result in difference. That's very predictable, too.
So, if your not sure which orientation the normals of your object have (maybe some in; some out) recalc normals before doing bools.

Meshes don't have to be closed. You can cut e. g. a tube by a plane.

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