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Subdivisions, Mental Ray’s approximation editor and displacement maps

April 5th, 2008 | by Todd Palamar |

Mental Ray Approximation Editor

Elements:

1. Polygon source mesh modeled with all quads, triangles, or a mixture of both
2. Displacement map.

Overview: Convert a rigged, low polygon model of an octopus into a subdivision surface using Mental Ray’s approximation editor. Once established, add a displacement map for further detail. Ultimately this gives you the ability to use the approximation editor to control the tessellation of the polygon object during render time.

Getting Started…

Goal: Let’s face it, render times are no match for production pipeline times. You have to model, texture, rig, light, and everything in-between before you get to rendering. If geometry starts to bog down your computer during the modeling phase your production is headed for certain doom. By using subdivision surfaces and displacement maps through Mental Ray’s Approximation Editor, you can reduce your model to almost a real time state through the entire production pipeline and only deal with high resolution geometry during render time.

This option is only if you want to see the connection immediately in the Hypergraph. If the option is checked when you choose assign the Hypergraph is automatically opened and the input and ouput connections displayed. The option has no bearing on viewing the approximation nodes at a later time.

1. We begin with a model of an octopus type creature. There are two separate pieces to the model, the nautilus shell and the octopus’s body. It has 8,382 total triangles

Four displacement maps cover both pieces.

2. Open the Approximation Editor by going to Window>Rendering Editors>Mental Ray>Approximation Editor. The first option allows you to create single or multiple approximation nodes for each piece of geometry. This is useful to apply different amounts of tessellation to different pieces of geometry or have all geometry use the same node. You can also keep single selected and apply different approximation nodes as you see fit. For this example we keep the default setting of single. The next checkbox allows you to view the nodes in the hypergraph immediately after assigning them. This does not affect viewing the nodes later on. Next, are the four approximation options. Since Maya 8.5 we only need to focus on Subdivisions. It is now only necessary to apply a single Subdivision Approximation node to handle the smoothing of the object and the displacement. 

3. The first node to add is a subdivision approximation node. To add it, select the desired geometry and click the Create option. For the octopus, both the nautilus shell and octopus body are selected. They share the same approximation node.

4. Once created the node can be modified through the attribute editor. Either click the Edit option or select the approximation node in the hypergraph and open the attribute editor. The subdivision node has several different approximation methods to choose from. For this tutorial the Length/Distance/Angle or LDA method is used. Upon selecting it, the length, distance, and Angle options become available for editing. To choose which method to use simple add a value above zero to one of the three sliders. Leaving a parameter at zero prevents Mental Ray from searching for those conditions. Only Distance is used for the Octopus and is set to a value of .1. The next set of parameters to modify is the Min and Max subdivisions. The Octopus was modeled entirely out of quads and contains no real surface detail. Because of this, we need high values to achieve the desired detail. Set the Min Subdivisions to 3 and the Max Subdivisions to 6.

5. Let’s make sure everything looks good. In the render settings switch to Mental Ray and choose Draft for the Quality Presets. Also, turn on Progress Messages under the Translation tab. This forces Mental Ray to give feedback in the Maya Output Window, including the rendered triangle count. (This tutorial assumes the displacement maps are already applied and tweaked to their necessary heights).  

6. Thats it. Check out the final render! To see the maps in action, watch the Quicktime!