320 chapter 9 ■ Normal Maps, Displacement Maps, Maya, and Decimation Master
Subdivision Approximations
In mental ray, the subdivision approximation tells the renderer how densely to divide the mesh
at render time. The denser the mesh, the better your fi ne details will appear in the fi nal render.
I’ll describe this as if your displacement map is pressing through the mesh you are rendering—the
fi ner the mesh, the better the details will appear. Consider the mesh with a Parametric subdivi-
sion approximation set to 1 to be like burlap. There is not enough fi neness to this surface to pick
up all the detail of the displacement as it presses through. An Approximation setting of 3 is close
to a cotton, which may show some detail but not the fi nest forms. A subdivision setting of 5 is
like wet silk. This will show all your fi ne detail from the displacement map. These numbers may
vary depending on the original low-res polygon count, but the analogy applies.
Another Approximation option is Spatial. It’s more complex than Parametric but it has a few
interesting options, including the Length control and the View Dependent box.
Select Spatial as the Approx Method and set Min to 3, Max to 6, and Length to 0.01. These
are a good starting point, but all these settings may be adjusted to help achieve better renders.
The Min and Max settings tell mental ray the minimum and maximum number of triangles to
subdivide your model into at render time. The Length value tells mental ray the longest any tri-
angle edge can be and helps ensure that large protrusions and areas of high displacement render
well. The lower this number is, the smaller the triangles in the mesh. I fi nd I get the best results
with this set to 0.01 to 0.001, but it depends on how much RAM you have available.
mental ray will overrun R AM and crash if these settings are too high. The fi ner the mesh is sub-
divided, the more detail you can see from the displacement. High settings are not always necessar y,
and you will need to experiment with these values to fi nd the right combination for each render. In
cases where you have a displacement map that contains more form and fewer fi ne details, these set-
tings can be lower. In most cases, I fi nd Parametric is just fi ne for my rendering needs.Mental Ray Alpha Detection
There have been some changes to the way that mental ray reads alphas and luminance in fi le
textures. To ensure your ZBrush maps are rendered correctly choose Window → Settings And
Preferences → Preferences → Rendering; from the drop-down box select mental ray, and check
the Use Maya-Style Alpha Detection option.Setting Up the Render Settings
Next, we need to change a few settings in the Maya Render Settings window. Select
Window → Rendering Editors → Render Settings to open your Render Settings window.
If it is not already set, choose mental ray from the Renderer drop-down box.
On the mental ray tab is a rollout option called Translation. Open this rollout and set
Export Verbosity to Progress Messages. This will tell mental ray to report useful informa-
tion to the output window when it renders. This includes the fi nal Triangle count, which
when divided by two is the face count of your subdivided model at render time. Compare
this to the fi nal face count in ZBrush to determine how close to the ZBrush subdivision you
are getting with your Approximation settings (Figure 9.37). This can assist in setting your
N Subdivision value accurately. Now open the render view and initiate a render. You can
keep the image in render view for comparison (Figure 9.38). Figure 9.39 shows how raising
the Approximation settings will increase the quality of the fi ne details on the surface.