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Optimizing Scenes

To get started, let's open the Maya scene,
(Where are the tutorial files?) 


Armed with a few general rules, it's easy to optimize your scenes for maximum performance. You can realize large performance gains by taking the renderer into consideration early in your 3D pipeline, during modeling and shading. All models are not created equal, and you'll find geometry has a substantial effect on render times. It is common to find that inefficient modeling and/or shading creates unnecessary bottlenecks when it comes to rendering, and when shots are ready to render it's usually too late to fix poor, inefficient models and shaders. By planning in advance, you can avoid squandering valuable render cycles. In this tutorial we'll go through some of the steps that you can use to optimize your scenes for RenderMan. 

Make sure you've opened the tutorial file,


RenderMan prefers NURBS and subdivision surfaces over polygons. In general, NURBS and subdivision surfaces will look better and render faster and more efficiently than an equivalent object modeled from polygons. This holds particularly true for complex curved topologies, where polygons generally require a much denser mesh to approximate curved surfaces. 

For simpler models, like a box, the surface type is less important, and polygons may be faster. Generally complex organic objects with lots of curves, like animated characters, are best modeled as subdivision surfaces.

Now Render the Maya scene: 

Render-> Render Current Frame

You should get an image like the one below:


On the left is a subdiv dragon. On the right is a polygon dragon

RenderMan renders true-curved surfaces (tesselating geometry into micropologons on a per-pixel basis) so there are never any tesselation issues with spheres or other curved surfaces. These surfaces will always be smooth, never faceted. If we take a closer look at the alpha image in the scene above, the undesirable faceting of polygons is clear. We can see this in the image below:


The alpha emphasizes the faceting in the poly model


Because RenderMan prefers subdivisions over polygons, go ahead and delete the polygon dragon.  




Whenever possible, use single-sided geometry. This allows the renderer to throw away half of the surface with no further processing, doubling the rendering speed for that object. Obviously, an object should not be single-sided if the interior or back faces can be seen. Next we'll turn the dragon into a single sided object. 


1) Select the dragon in Maya. 
2) Open the Attribute Editor.
3) Toggle off Render Stats-> Double Sided. 

The dragon will now be more efficient to render. Setting double-sided surfaces to single-sided will make a big difference when rendering larger scenes. 



Adding too much detail to a model can slow rendering down. During the modeling phase consider the final placement of an object in a shot, and if a detail is ultimately smaller than a pixel, don't model it. During the modeling stage it can be tempting to build as much detail into a model as possible. Avoid that temptation. Add only the detail that's required and render times will dramatically improve.  

Also, avoid unnecessarily dense geometry. Polygon meshes should be just dense enough so that tessellation artifacts are not a problem. Subdivision surfaces should always have as few vertices as possible. NURBS should have only as many isoparms as required. Extra geometry means extra work for the renderer. Take the case of a NURBS sphere — it is equally "round" with six or 600 isoparms. In either case it's a sphere, but you will notice that the NURBS sphere with 600 isoparms takes much longer to render.  

Ultimately, it's not always feasible to model for each shot, but it is good practice to be aware that the complexity of models has a direct impact on render times. 

The subdiv dragon in our scene has the right amount of geometry. In the image below we can compare this dragon (as seen on the right) with another dragon (on the left) which has too much geometry. Both models look the same when rendered, but the heavier model on the left carries a greater, and unnecessary, overhead.


The model on the left is inefficient — too much geometry. 
The model on the right uses less geometry to define the same form.





Sometimes it's better to apply displacement (or bump) shaders to simple geometry than to actually build complex geometry. Take the example of this dragon. Perhaps we want to add scales. These scales could be modeled outright, which would create a much heavier model. Alternatively, a displacement shader can be used to add scales to the dragon much more effciently. By letting RenderMan handle small details during rendering, you'll be able to render lighter models faster. As a bonus, you will also accelerate your interactive Maya sessions because you are using lighter models. 


To attach scales to the dragon, we'll attach a pre-made displacement shader. 

1) Select the shader "scales_lambert" in the Hypershade
2) From the Hyper shader pick:
        Edit-> Select Objects with Materials
3) Now attach the displacement shader. From the Rendering Menu select:
	Lighting/Shading-> Assign Existing Material-> scales_displaced_lambert

Now render the scene again: 

Render-> Render Current Frame

You should get an image like this, with nice displaced scales:


A dragon with scales added by a displacement shader



Following a few simple guidelines during modeling and shading will have a positive impact on render times:

1) Use efficient geometry (NURBS and subdivision surfaces) for complex organic models.
2) Use single-sided surfaces and process half as much geometry. 
3) Don't over-model. Model “for the shot” as much as possible. 
4) Use shaders to add detail. Let RenderMan do the modeling, and accelerate your renderings and interactive workflow in Maya. 



Pixar Animation Studios
Copyright © 2003 Pixar. All rights reserved.

Pixar Animation Studios
Copyright© Pixar. All rights reserved.
Pixar® and RenderMan® are registered trademarks of Pixar.
All other trademarks are the properties of their respective holders.

RenderMan® are registered trademarks of Pixar.
All other trademarks are the properties of their respective holders.