Hi Skwerm, also just some more info - unless you're doing some kind of special effect like a toon render that is specifically looking for open edges in the mesh there should not be any difference between rendering a welded mesh or an unwelded mesh.

That's because the shading of the mesh is supposed to be controlled by the vertex normals data that is also present in the OBJ file.

If you get a different looking render between an unwelded versus a welded mesh, then that means that your program is ignoring the vertex normals that were supplied in the file and instead trying to calculate brand new ones from scratch by averaging together faces that share points in common.

Blender has a problem with doing precisely that in the latest 2.5x versions - currently their OBJ import script just ignores all the vertex normal data in the OBJ file (those are the lines that start with 'vn') and try to calculate new ones just from the polygonal information. That's not good for CAD data because the normals that have been discarded are ones that have come from the original NURBS surface data and it isn't really possible to get the same exact ones back just by averaging the polygon data since the polygons are only an approximation of that original smooth NURBS surface.

Please see this previous post here for a lengthy description of how vertex normals in the OBJ file are supposed to be used by a rendering program:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4272.3


Basically, as long as Blender just throws away the vertex normals in the OBJ file instead of reading them in and using them, it will always have a problem with CAD data. To my knowledge, Blender is the only rendering program that has this particular problem - all the other rendering programs that I am aware of do read in the vertex normals from an OBJ file and use it to shade the rendered polygon instead of just ignoring them and recalculating them from the polygon face normals.

- Michael