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View Full Version : Render shows cracks between some polygons, VPR does not.



Sebasvideo
03-17-2014, 11:57 PM
I'm having an odd issue I had never seen before. I modeled something in Modo that I brought to render in Layout using FBX, and I had to surface in Layout because FBX does a terrible job exporting surfaces, setting all kinds of odd values. But once I fixed them and surfaced everything the way I wanted everything was fine, until I positioned the camera too close to the device, and although it showed fine in VPR, when I rendered it, it showed some cracks or seams at some of the edges between polygons.

It shows this mostly in places where it has a rather unusual geometry, but nothing too crazy. Before I exported it from Modo, I did a mesh cleanup and it repaired whatever was wrong, and ran it again to make sure nothing wrong was left over. I then exported as an FBX because it seems to be the only export format that brings the model with all the layers in their right positions, even if the surfaces are completely screwed up.

I opened the problematic layer in Modeler and I saw nothing weird with it. Still, I did a mesh repair but I unchecked convert to triangles because I didn't want that. The first render after that made no difference. After that, I used the thicken command and it still happened. Eventually, the only thing that worked was to run a mesh repair again and this time convert to triangles. Only then the render didn't show the cracks.

Still, I'm curious as to why something like this could happen. One thing I checked was that there are no double edges, since in some of the edges that the cracks show were not part of the original geometry, but I had done a bridge, and then subdivided the new polygons because I needed extra geometry to deform things in a specific way. This model has no subpatching at all because that can be a pain when trying to model something very specific, so I opted for using a lot of geometry.

Unfortunately I can't show the model because I had to sign an NDA for this project, and I know that it's hard to tell without seeing it, but can someone think of a reason why this happens? I couldn't find anything on this except an old message from 2008 in this forum, but it was more related to FPrime, which I don't even know what it is. I definitely would like to find a solution that doesn't involve converting to triangles, because even though I did the modeling in Modo, if I have to change some details I can do it in Modeler and avoid having to export to FBX and resurface again.

Thanks

Dodgy
03-18-2014, 12:36 AM
I've had similar problems with CAD geometry. The problem seems to be with 5 point polys which have 3 nearly colinear points, so it looks almost like a quad poly. This causes problem with the f9 render unless you triple or triple/quad the polys. This isn't evident in the VPR renderer as it triples all polys before rendering anyway.

Danner
03-18-2014, 02:21 AM
When you have a non planar quad, there are two ways of triangulating it, [/] or [\] and oddly enough VPR and the renderer sometimes triangulate it in different ways so it might look good in one but not the other. N-gons can also be triangulated differently in vpr and rendertime (modeler openGL and Layout OpenGL also triangulate before display, and it seems like they use different methods). There are a lot of combinations that can look wrong, 3 colinear points as Dodgy said, or any polygon that has a point(vertice) that is closer to the center than it's adjacent points. For example a quad with a delta-wing shape. It can be triangulated by connecting the points closer to each other, or the points further to each other, and the second scenario looks wrong.