Multi-segment Motion Blur 


Using Multi-segment Motion Blur

MTOR now has the capability to output multi-segment motion blur directives to PRMan. In certain cases the standard, two sample, linear blur doesn't sufficiently capture the motion of an object. You can attach multi-segment motion blur attributes to individual primitives (or Auto Archive nodes). To add multi-segment motion blur to a primitive select the primitive and invoke,

RenderMan-> Attributes-> Motion Samples-> Attach 

This creates an integer attribute named mtorMotionSamples. 

Motion Samples Attribute
From the Motion Samples Attribute window you can select the number of motion samples for the specified primitive. This setting determines how many sub-samples are taken between frames. The default range should be 1 to 6 (PRMan has an arbitrary motion sample limit of 6). When present, the mtorMotionSamples attribute can also be used to tag individual primitives for subframe motion even when your global setting implies frame motion.


The motion samples of an object can be adjusted after the samples are attached from the channels box. It is best to not use more samples for an object than necessary to keep the cost of motion samples to a minimum.


Auto Archiving Cycles
Archivers can be used to make motion samples more efficient. Be sure to familiarize yourself with the Shutter Config parameter of the Render Globals. This parameter must be set to "stationary" so that archives of motion blurred objects can be inserted into the RIB stream independent of the current frame.


Propeller Example

A Propeller
Multi-segment motion blur is useful for creating blur for objects like propellers. Here we have a propeller. It's spinning 180 degrees a frame.


The propeller spins at 180 degree a frame.

Multi-segment Motion Blur
We've attached Motion Samples to the propellers at 6 motion samples per frame. The resulting blur is quite nice. Notice, however, that the segments are still visible; you can see the six "steps" of the motion blur calculations if you look closely at the edge of the blur. These artifacts can be reduced by cleverly offsetting another propeller against the first.


Multi-segment motion blur, 6 samples.

Sub-Frame Motion Blur
Here the multi-segment motion blur is a big improvement over the next best alternative, Sub-Frame motion blur. Here only two samples are made for blur calculations. The result in this case is not too convincing.


Sub-frame motion blur.


Summary

Multi-segment motion blur is a savior for some of the trickier types of motion, like propellers. This technique is more expensive, however, because in order to create six motion samples six times the info has to be put into the RIB file. To minimize the overhead added when using multi-frame motion blur, motion samples can be attached to only those objects that need it, and additionally, these objects can be automatically archived to create a repeating cycle, reducing overhead even further. 

Of course, the most important part of creating motion blur is calculating for the final image, not for physical realism. If a propeller that would be spinning at fifty thousand revolutions per second in the "real" world looks better spinning at two revolutions per second, with a couple of "secondary" propellers thrown in, then do what looks better.


Limitations


 

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