Shutter Efficiency – mental ray 3.11

Motion blur interface controls for "shutter shape function"

Motion blur interface controls for “shutter shape function”

Starting with 3.11, mental ray supports a shutter shape function ability for applying arbitrary exposure shapes to the shutter interval.  This allows for motion blur that better matches film.  While programmers can write their own functions, the built in trapezoid shape is good enough for most production needs.

Shutter Efficiency

Motion picture cameras use a rotary disc—a semicircular disk that spin in front of the film gate—as the shutter device.  Light is only captured when the missing part of the disk exposes the film gate.  This is referred to as the shutter interval.  The portion of the complete circle that the disk is missing is referred to as the shutter angle.  A shutter angle of 180˚ is considered normal for a motion picture camera.  A larger shutter angle results in a longer shutter interval and more motion blur effect.

A shutter that is completely open for the entire interval would capture the most light possible and would be said to be 100% efficient.  The physical nature of rotary disks means that the time it takes for shutters to fully open and close is non-zero.  This results in less light being captured at the ends of the shutter interval.  This softens the appearance of motion blur at its edges.

Shutter Shapes and Efficiency

Shutter Shapes and Efficiency

You can conceptualize shutter efficiency with a graph that plots exposure vs. shutter interval.  An idealized shutter would correspond to a box shaped function on this graph.  Real-world cameras have a more rounded shape that can be approximated as a trapazoid.  The minimum efficiency of a trapezoidal function is 50%.  Most real-world cameras are around 80% efficient.

Why do Maya cameras have a default shutter angle of 144˚ if 180˚ is more ‘correct’?

This is because renderers typically—and incorrectly—describe shutter exposure as being 100% efficient.  Using a shutter angle of 144˚ with a 100% efficient shutter matches the total exposure of a real-world camera.  That is:

144˚ × 100% = 180˚ × 80%

144° None (Box), 144° Trapezoidal Shutter @ 80%, 180° Trapezoidal Shutter @ 80%

144° None (Box), 144° Trapezoidal Shutter @ 80%, 180° Trapezoidal Shutter @ 80%

If you decide to apply an 80% efficient trapezoidal shutter shape to your scene, you may also want to adjust your render camera’s shutter angle to 180˚.  This will give you a similar result to the idealized case, except now with more physically accurate (and visually pleasing!) motion blur.

camera shutter angle

camera shutter angle

About bnrayner

I am a VFX developer specializing in 3d rendering. With a background in Physics and Digital Art, I enjoying using math to explain how light interacts with materials and to creating pretty pictures along the way.

Posted on April 17, 2013, in maya, Optimization. Bookmark the permalink. 3 Comments.

  1. Well this is very cool. How much of a strain should be expected at render time? (in comparison to the “legacy” options.)

    • Using the shutter efficiency with Unified Sampling concentrates samples where necessary during motion blur as opposed to just weighting them afterwards. This means fewer eye rays are needed to resolve the motion blur.

      This option previously didn’t exist in any mode. However it can be combined with the other rendering algorithms. Other options are not affected directly and are still used as before.

      So this should make motion blur faster overall but performance depends on the scene and the amount of blur. In typical scenes we see small to modest speed-ups where the quality (smoothness) of the motion blur is concerned. It shouldn’t be detrimental.

    • If you adjust the shutter angle to match, render times should be more or less the same.

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