CompuServe Messages

Andrew, There is a major difference between how programs like AutoCAD do booleans and how 3DS does booleans. This effects the chances of successfully generating the new surface. AutoCAD will only do boolean operations on objects created from solid primatives and simple extrusions. 3DS will do booleans on any objects (or at least attempt to). When doing booleans with solids, AutoCAD has an exact knowledge of the form of the original pieces and can easily predict the boolean result. Solids have know forms, volumes, and definate insides and outsided. They always have fully closed surfaces. AutoCAD cannot do booleans on arbitrary surfaces. When it comes time to put a mesh surface on booleaned solids, AutoCAD will seperatly generate meshes for each surface. The vertices for each surface may not line up and may not be connected. You will have to manually move and weld vertices. 3DS must analyze and attempt to find the surfaces of completely unkown forms and must figure out where the inside and the outside of the surfaces are. Sometimes it doesn't get it right. Several things can complicate the issue. Objects in 3DS don't have to be closed. They can be deliberately open or there can simply be inadvertant gaps in the mesh. This can confuse the routine that determines where the insides and outsides of a form are. Objects with multiple elements can also have this effect, expecially if the elements overlap. Any overlapping faces in the mesh will also confuse the boolean operation. Long thin faces are probably a problem because of rounding errors. Unify/Normals has similar problems with complex or damaged mesh surfaces. It will handle faces with mixed up faces. That is what it is designed for, but if there is a gap in the mesh or overlapping faces or multiple elements, then Unify/Normals will also get confused. 3DS's boolean function, when it fails, may create gaps or overlapping faces. If you do a subsequent boolean, it will compound the problem and Unify/Normals will not help. When you do a complex boolean in 3DS it is often best to do it in stages, one element at a time, checking the mesh carefully between steps for errors. Save a new version of the file between each step. This lets you go back to any given step and start over. You want to do this because you may not catch an error until some later step. I do hope that Yost Group will develop a smarter boolean function for R5 that has less tendancy to trash a mesh when it fails. I doubt that there will be a revolutionary improvement because of the difficulty of doing booleans on arbitrary mesh surfaces, but who knows. This is not meant as a flame at you, but just to let you know that there is not way that AutoCAD and 3DS can just "share code" for booleans. Terry Gilbert Forcade & Associates