The Prob­lem

Most full-blown 3D appli­ca­tions come with “sub­di­vi­son sur­face edit­ing” facil­i­ties of some sort. The tools used for this, how­ever, are basi­cally just polyg­o­nal mod­el­ing ones, applied to the con­trol cage rather than the sub­di­vi­son sur­face itself. As they often got devel­oped before sub­di­vi­sion sur­faces even got added as a fea­ture to such appli­ca­tions, they are more or less ter­ri­ble at guar­an­tee­ing that the sur­face one’s edit­ing is actu­ally stay­ing man­i­fold.

How­ever, non-manifold geom­e­try fre­quently pro­vides to be the opener for a can of worms at ren­der time. Just recently, a col­league at work asked me for help to fix a model done in Maya. While look­ing fine in Maya’s view­port, it would ren­der as a sub­di­vi­son sur­face with com­pletely screwed up tex­ture coor­di­nates in Pho­to­Re­al­is­tic RenderMan.

The rea­son being that it con­tained non-manifold geom­e­try. The ren­derer detected this and threw the offend­ing parts of the mesh away, print­ing a warn­ing mes­sage. Due to a bug though, it didn’t adjust the rela­tion­ship with the tex­ture coor­di­nate data accord­ingly. Which led to the bad image.

Now there’s two solu­tions: report a bug to the ven­dor of the ren­derer or make sure your geom­e­try is ‘good’ (man­i­fold) for starters.

With dead­lines loom­ing, the lat­ter is com­monly the only option. What we tried first in the case at hand was hence to run Maya’s Poly­gon Cleanup tool on the mesh to ensure man­i­fold­ness. Maya’s tool not always detects cer­tain sorts of non-manifold con­nec­tiv­ity though. As was the case with the mesh at hand. So we couldn’t fix the prob­lem in Maya.

I’ve come across this issue numer­ous times in recent years. Hav­ing ‘good’ meshes come out of the mod­el­ing depart­ment can save you so much trou­ble later down the pipeline, its impor­tance can’t be stressed enough.