Motion Blur 21
In traditional motion picture photography the shutter is a rotating disc with an opening the size of a given angle. Movements during the shutter open time will blur the exposed image. This feature is essential to making computer generated images appear more realistic.
Motion Blur, enable/disable
Motion Samples is the amount of additional motion snapshots taken during shutter open time. The blurring effect will be a continuous straight movement between the snapshots. Values higher than one allow smoother and very fast rounded motion in the blurred effect. A value of two will create a total of three motion snapshots with two linear segments between them during the shutter's open time.
Sample Motion Blur: If this is enabled, then the output image will be blurred when objects are moving. If it is unchecked, the output image will not be blurred, but the motion vectors for any moving objects will still be calculated and available for output as a vector AOV. This allows for post processing motion blur in a 3rd party application (such as Nuke).
Shutter Timing: This determines the range of the time span the motion blur is sampled from. Try visualizing frames as existing on a continuous line:
If you set a keyframe (for say, an object transformation) at frame -1 and another at frame +1, then the object will be transitioning between those two keyframes. When Frame is rendered it takes a snapshot of a moment along that line of movement. To use motion blur we need to tell the renderer where, in regards to that frame marker, we should take the extra time samples that create the blur.
For ease of this example we will say that the Shutter angle is set to 180 degrees, or half of a frame.
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End on Frame: The shutter is completely closed on the frame marker. This means that all the motion samples are being taken between the halfway point of the previous frame and the current frame.
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Center on Frame: The shutter is halfway through its timespan when the current frame renders. This means the motion samples are taken equally before and after the frame point.
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Start on Frame: The shutter begins to open on the frame marker. This means all motion samples are taken during the timespan between the current frame and the next one.
So why is this important?
If you are attempting to match Renderman outputs with images from other CG software or live action photography, you have to ensure that all are using the same shutter timing, otherwise blur tails may not match up and other visual anomalies may occur.
Shutter Angle: This determines how long the shutter is open in degrees. 360 degrees is one full frame, so the default of 180 degrees indicates that the shutter is open for half of the frame time.
Shutter Open Speed: The softness of the shutter opening, where a value above zero fades the beginning of motion blur. You can think of this as the location in the frame where the shutter stops opening.
Shutter Close Speed: The hardness of the closing shutter, where a value below one fades the end of motion blur effect. You can think of this as the location in the frame where the shutter begins to close.
Helpful Tip: Real cameras do not have shutters that open and close instantly, so for a realistic motion blur the open speed and close speed parameters should be used. The shutter angle default of 180 degrees is a very common setting for emulating motion picture cameras.