Thanks Roeland, this helped me find where's the problem.
I did a bit of research and I discovered that ALL UVs are like that by default in Cinema 4D!
Further more, as far as I know, there is NO way to weld UV vertices in C4D!
When I imported the two obj plane files from your example I got the same issue with UVs on both of them.
At first I couldn't tell the difference, but after checking them out in a different 3d app I realized where's the problem..
Also, it's definitely not obj/FBX import issue because new geometry modeled and UV mapped in C4D also has the same issues.
I guess dealing with this in OctaneC4D plugin would slow down geometry updates, so the only way out for C4D users would be for Octane to support unstiched / unwelded / duplicated UVs. Without that, I'm afraid this feature would be useless for C4D users
Let me know if I can help in any way.
Edit: ..Some references:http://www.c4dcafe.com/ipb/forums/topic/85284-weld-uv-points/#comment-583189http://forums.cgsociety.org/showthread.php?p=5698826#post5698826
Supporting it inside Octane would result in the same slowdown.
I'm also not sure if it's a Cinema4D problem, if I import and then export the given OBJ file in cinema4D then the problem goes away. In the exported OBJ file the UV coordinates are merged. I'm not able to tell if this is a specific Cinema4D problem, or something specific that the plugin is doing, or something specific in your scene. I think it is best to check with aoktar
in the Cinema4D forum.
Those topics talk about something different — having exactly one UV coordinate per vertex. That's not required, OpenSubdiv only requires that vertices of adjacent polygons in UV space have shared UV coordinates.