Some important tips regarding motion blur using the rasterizer with the ray tracer, ...

1) When using the rasterizer for motion blur, if you can get by with it, disable ray tracing. See the end of the Rasterizer Tips page under Usage and Performance, for things to check that may enable ray tracing.

When using both raytracing and the rasterizer together, it is like using two renderers at the same time as both acceleration methods will use up memory for their acceleration techniques. So if you don't need secondary raytracing effects, the render may go much quicker without it enabled.

2) If using ray tracing in addition to the rasterizer, prefer samples motion = 1 if you can. For details on what it provides for the rasterizer shading samples, see the Samples Motion section of Rasterizer Tips. When samples motion is greater than one, not only are there multiple shading samples at different frame times per polygon, but a dynamic BSP tree is used for secondary raytracing. Motion blur for secondary rays are more accurate, but the dynamics BSP tree uses up more memory as well. For small motion, the amount of memory is less, but for complex scenes with a lot of motion, the dynamic BSP tree can take up quite a bit of memory.

In class, we often show how to enable blurred shadows on static objects when using the rasterizer with ray traced shadows. We do this by increasing samples motion to a value greater than 1. The key is to only use this technique when necessary.

It is important to note how to use the samples motion option in the three main packages.

In XSI, it is named Motion Samples.

But in both Maya 8.5 and 3ds max 9, it is tied to the Time Samples parameter, which has a different meaning when the rasterizer is not turned on. When the rasterizer is not on, it represents the number of time samples per spatial sample with 3D motion blur using ray tracing. When the rasterizer is turned on, it is the samples motion parameter which represents the number of shading samples used per polygon. For the rasterizer, the samples are always at the same times, for example 0 and 0.5 for Time Samples (samples motion) of 2. When not using the rasterizer, the times at each spatial sample point with have a QMC time shifted difference. So at Time Samples of 2, each spatial sample will have times spaced 0.5 apart, but at different times, eg, 0.2 and 0.7, or 0.45 and 0.95.

BOTTOM LINE: When using the rasterizer, don't forget to turn samples motion (Motion Samples/Time Samples) down to 1, if it is not needed. It could possibly speed up some motion blurred scene renderings quite significantly. In fact, it might make some frames previously unrenderable, now renderable.