Motion Blur

mental ray computes motion blur of highlights, textures, shadows, reflections, refractions, transparency, and intersecting objects. There are two methods of defining motion blur:

instance motion blur defines a transformation matrix that controls the orientation of the instanced object, light, or camera after a shutter interval of one time unit, in addition to the regular transformation matrix that defines the orientation at the beginning of the shutter interval. For cameras, this creates true curved blurs. Moving lights cause blurred highlights and shadows.
motion vectors can be defined at object vertices to create internal motion caused by shape changes. Each vertex has a vector associated with it that describes how far that vertex moves in one time unit. For deferred objects (supported by mental ray 3.0 and higher inly), where the vertices are created later during rendering, it is necessary to supply a motion bounding box that describes the lowest and highest expected motion vector components. Cameras and lights do not allow motion vectors because they cannot change shape. mental ray 3.1 supports motion paths with up to 15 motion vectors per vertex.

Both methods can be combined. mental ray supports motion blur both in scanline and raytracing mode. Motion blur does introduce rendering overhead only where blur must be computed; if the camera moves this could be the entire image but if only a small object moves the overhead is small.

A shutter speed may be given for the camera with the -shutter option on the command line or shutter in the options statement, with the default speed of zero turning motion blurring off, regardless of the presence of motion transformations or motion vectors in the scene. The shutter opens instantaneously at time zero and closes after the shutter speed time has elapsed. An object, light, or camera moves by the distance defined by its motion transformation plus motion vector if the shutter interval is set to 1.0. The motion blur trail is shorter for shutter values less than 1.0, and longer for shutter values greater than 1.0.

mental ray 3.1 also allows delaying the shutter open time to shift the shutter open time to times greater than 0. This allows bidirectional motion blurring in output shaders.

There are two alternative motion blur algorithms. The regular motion blur algorithm is based on temporal oversampling, and is enabled if the shutter period is nonzero (i.e. the shutter time is greater than the shutter delay time). The "fast motion blur" mode is based on coordinated spatial and temporal oversampling, which requires far fewer samples (typically by a factor of five) for motion blurred pixels. Fast motion blur mode is enabled if the shutter period is nonzero, and if the time contrast is set to 0. In fast motion blur mode, min samples is forced to be equal to max samples.

Note that the rasterizer ignores these settings.