MR Rendering NURBS
Although NURBS curves and surfaces are perfectly curved in all directions, rendering with Mental Ray can result in an image with jagged curves. This happens because mental ray subdivides the NURBS curves and NURBS surfaces into several straight curves or surfaces. This is called tessellation. To control the mental ray tessellation settings we use the 'Surface Approximation' function.
Create new Surface Approximation
To enter the 'Surface Approximation' menu, open the Render Settings. Go to the Options-tab and open the mental ray Overrides section. Locate the Tessellation sub-section. Click the checker box behind the 'Surface Approx' drop-down menu to create a new Surface Approximation node. The attribute editor for 'mentalraySurfaceApprox1' opens.
In the attribute editor for 'mentalraySurfaceApprox1' there are several presets . The 'Angle Tolerance' and 'Pixel Length' presets are usually sufficient. Choose one of these presets and test which preset gives a satisfying result.
Angle tolerance uses an adaptive tessellation method which uses more subdivisions in areas of high curvature. The goal of this method is to add subdivisions where they are needed to capture sharp features, while at the same time using just a few subdivision in large, flat areas where they are not needed.
At first try the 'Angle Tolerance Low Quality' and do a test render. If the curves or surfaces are still jagged, try lowering the 'Angle' value to get a higher quality.
Pixel Length tessellates surfaces based on their size (in pixels) in the final rendered image. Surfaces that are close to the camera will have a lot of subdivisions, while surfaces that are far away from the camera will receive less subdivisions. The goal of this preset is to focus the most attention on surfaces that are more significant to the final image. Using less subdivisions in the surfaces that are further away enables shorter rendering times.
At first try the 'Pixel Length Low Quality' and do a test render. If the curves or surfaces are still jagged, try increasing the 'Max Subdivisions' value or decreasing the 'Length' value until a satisfying result is reached.
Modify existing Surface Approximation
Once you've created your Surface Approximation node in the Render Settings, you can't change it through the Render Settings Menu. Instead, you have to modify it through the Surface Approximation menu.
Go to Window > Rendering Editors > mental ray > Approximation Editor
In the section NURBS Tessellation, on the line Surface locate your previously created Approximation node in the pull-down list.
Click Edit to modify it: