rendering

Motion Blur

Automatic motion blur facilities are provided by AL. Only the RenderMan render packages currently support motion blur.

For the most part, specifying motion blur is just a matter of turning it on with the "motion-blur" parameter to the render command. The "motion-blur" parameter specifies the duration of time, in frames, that the virtual camera shutter is open when a single frame is rendered. Because motion blur is specified in frame units, the specific frame or frames to be rendered must be specified using the "frame" or "framelist" rendering options.

Examples:

render frame 5 with the shutter open for half a frame:

(render 'frame 5 'motion-blur 0.5)

render frames 5-10 with the shutter open for a quarter of a frame:

(render 'frame '(5 10) 'motion-blur 0.25)

invalid!

(render 'motion-blur 0.25)

Motion Blur Tips

To compute motion blur, the world model is evaluated twice. Once at the specified frame (the "shutter opentime") and a second time at the specified frame plus the duration that the shutter is open (the "shutter closetime"). Usually, a value between 0.1 and 0.5 is used for the shutter duration.

Automatic motion blur will fail if the world model changes structurally (i.e. new gops/gprims added or deleted) between shutter opentime and shutter closetime. This should not be a problem unless the shutter duration is greater than 1 or structural changes occur (like a new object coming into the scene) at fractional frame values.

Motion blur images generally require a higher sampling rate. ri-pixel-samples should be used to increase the sampling to 4x4 samples or higher. Also, the shading rate can be lowered for heavily blurred objects since detail is lost due to blur. See ri-shading-rate and the special "motionfactor" parameter to ri-geometric-approximation (PRMan only).

Selectively Turning Motion Blur On and Off

There may be times when you want to selectively turn motion blur on or off for individual objects.

(motion-blur onoff)

This gop turns automatic motion blur on if onoff is true (#t) and off if onoff is false (#f). The on/off status of automatic motion blur is an attribute of the graphics state and thus can be saved and restored using a separator.

Manual Motion Blur

(motion-blur (time₁ ... time_n) form₁ ... form_n)

time_i represents the time at which form_i is defined. Each form_i must be the same gop or gprim (with different parameters). Render packages which do not support motion blur will only execute form₁.

Automatic and manual motion blur can be mixed. For example, automatic motion blur can be turned on for the entire scene, a specific object can have automatic motion blur turned off, and manual motion blur can be applied to that object. When used this way, the automatic motion blur will set the shutter open and closetimes to 0 and 1 for the entire scene -- therefore, 0 and 1 should be used for time1 and time2 in the manual form of the motion blur gop.

If no automatic motion blur is used in the scene, the shutter will have to be set manually by specifying ri-shutter in the 'option parameter to the render command.

(render ... 'option '(ri-shutter 0 1) ...)