View Full Version : Subds in Mental Ray

05 May 2003, 12:11 PM
Hi There,

I'm having trouble rendering Subdivision surfaces in Mental Ray, using Maya 5.

THey work fine with Maya's renderer but Iget a blank screen and a message about non-quadrilaterals in the base mesh.

The polygon model, before conversion renders fine.

Anyone had this?

Any tips?

05 May 2003, 12:20 PM
If you want to render a sub-d model wit mr, base mesh must be all quad. If it's not, just collapse 1 level up, that makes it all quad. See documents for details.

05 May 2003, 12:32 PM
Thanks augustus

How could I ensure this was the case?

I applied quadrangulate to the poly before conversion and also tried clean up on the subd but without success.

05 May 2003, 01:03 PM
Well, you can use selection costrains to select non-quad faces. Select your model, switch face component mode, then edit polygons->selection->selection constrains. Check all but quads in 'order' section. Now you can select only non-quad faces.

05 May 2003, 01:36 PM
I'll try that.


09 September 2004, 04:25 PM
subd must be all quad faces to render in mental ray. just because the poly model was quads does not mean it will become an all quad subd. if you go to the subdivisions menu and select collapse hierarchy and set it to 1 it will mke your subd model all quads and it will render in MR. However collapsing the hiearchy will make the mesh very dense and more difficult to work with.
The mesh as you see it in its original polygon form converts into the level 0 subd mesh. however when you look around the converted subd mesh network you will see certain areas of localized detail, new edges that end abruptly leaving a few five sided faces. Now it wont render in MR. That added, localized geometry in the subd model is created by maya in the conversion to make the subds smooth. This geometry is on the level 1 surface detail of the subd whereas the rest of the converted poly model is at level 0. When you collapse the hiearchy to 1 you are upping the res of the model to level 1 throughout. Those edges that ended abruptly in the areas of localised detail will now be continued to whatever degree necessary to make the model all quads. This could almost triple the density of your mesh, but will ensure that it will render in MR. I try to model my polys in a way that will not add new geometry at conversion to subds. New geometry is almost always added wherever a 5 sided intersection and a 3 sided intersection are next to each other which is common (also five sided next to five sided or three sided next to three sided intersections). Try to redirect your edge loops to create four sided intersections between your five and three sided intersections. This can be tedious and force you to add geometry but when your model converts to an all quad subd model and does not need to have its hiearchy collapsed you actually save alot of geometry.It will then render in MR. Just check the poly edge network around where added geometry is created in the subd conversion and try some work arounds to break up those five and three sided intersections with four sided ones.

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