Why Is It Taking So Long to Render?

By now, you've learned that rendering times are hard to predict. After you finish creating an animated scene and set Maya to render a series of frames, it's nearly always a few minutes or hours before the job is done. The scene's complexity is linked to the render time, so scenes with thousands of objects and dozens of lights are slower to render than those with fewer objects and lights. Even simple scenes can sometimes take longer than you might expect, however. What are you supposed to make of it when you come back a day later and only a few frames of a seemingly simple scene have been rendered? Something is obviously putting a heavy burden on the renderer. The following sections cover a few areas to consider as potentially expensive in terms of computation.

Render Globals Settings

Resolution: Large images take longer to render than small images. If your 320x240 render took 10 minutes, you can usually count on a 640x480 render taking 40 minutes because the image has four times the number of pixels. Doubling the dimensions quadruples the pixel space and the rendering time.

Antialiasing Quality: If your renderings exhibit jagged, stair-stepped edges or reflections, you need to increase the antialiasing quality, which lengthens rendering dramatically. Generally, you use the fast, low-quality antialiasing mode for testing and a higher setting for your final production. (Note: If it's the depth map shadows in your scenes that exhibit aliasing, you should raise the Depth Map Resolution for sharper shadows and/or the DMap Filter Size for blurry shadows; this setting has little effect on rendering speed).

Raytraced Shadows: This type of shadow is the sharpest, but it can noticeably slow down rendering. In particular, if the raytraced shadow must project through many small overlapping objects (such as leaves on a tree), you'll probably see a significant decrease in rendering speed.

Raytracing: The Render Globals window contains the option to raytrace, which is used when you render reflections or refractions. It can slow down rendering speed; particularly if you turn up the reflection and refraction settings in a scene full of reflective or refractive objects. For example, if you use 10 reflections, and you're rendering a hall of mirrors, the renderer has to ping-pong the light ray for many of the image pixels through the scene to find its final color. A sample scene called slow.mb included on the DVD illustrates this effect. Similarly, a pyramid of refractive wine goblets would be slow to raytrace if the refraction depth is set to a high number. Usually in this case, you need to use a high number, such as 8 or 10, because the pixels render black if they don't strike a solid object in the number of bounces allotted.

Motion Blur: Use this option when you want to simulate the effect of shutter speed, as in a movie camera. If you're creating an animation in which some scene objects move rapidly through the camera's field of view, you must use motion blur to avoid a strobe effect. There are two types of motion blur: 2D and 3D. The 2D motion blur is a kind of intelligent post-process filter that smears pixels in the rendering and isn't usually very slow to render. The 3D motion blur is more realistic, but definitely requires more time to render.

On the DVD

Chapter_16\slow.mb

Object Attributes

NURBS: All NURBS surfaces must be tessellated to polygons before the renderer can work with them, and each NURBS object can be adjusted for how finely it tessellates. If you set the tessellation level very high, you notice a delay while Maya processes the NURBS object into thousands of polygons. In extreme cases, an immense number of generated polygons can cause the machine to run out of memory. It's a good idea to test tessellation levels if you adjust them from the defaults. You can do this in the Attribute Editor, in the Tessellation section on the object's shape tab. Select the Display Render Tessellation check box to see the tessellation results in the 3D panel as you adjust the settings.

Material Types

Volumetric Material Types: Maya supplies five volumetric materials for non-solid object effects: EnvFog, LightFog, Particle Cloud, Volume Fog, and Volume Shader. These special material types are usually slower to render. If you bury the camera in a Volume Fog, every pixel of the frame looks through a 3D fog effect and takes much longer to render.

Camera Effects

Depth of Field: Maya's normal rendering mode is like that of a pinhole camera: Everything is in perfect focus. You can set the camera to produce depth blurring, where objects closer to or farther away from the focus are progressively blurred. This option usually adds quite a bit to the render time when depth blurring becomes more exaggerated.