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I'm trying to render out some particles that are being emitted from three fast-moving objects and viewed by a following camera. The particles positions are correct, but when motion blur is turned on the particles get blurred backward and forward from the current frame instead of from the current frame and backward, resulting in renders of particles overshooting their emitters once put into their comp.

Here's a sample render of what the render currently looks like. The streaks forward of the upward swoops shouldn't be there.

Cycles motion blur

Blender Internal's sampled motion blur is better about the forward/backward issue, but the particles are moving so fast that the 32 sample max is insufficient for getting a smooth blur; the samples are far enough apart to be dotted.

Blender Internal sampled motion blur

So, is it possible for Cycles to sample backward-only motion blur like BI's sampled motion blur does, or is there something else that can be done to achieve the desired effect?

For those interested, here's a link to the Blend file.

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    $\begingroup$ Note that this is how motion blur actually works. During the time the shutter is exposing (in this case, 1/24 of a second, or one frame at 24 fps) the particles will be some distance behind their exact location on that frame at the beginning of the exposure, and some distance ahead of their location at the end of the exposure. So unless you are going for a more stylized effect, then you may not want to change this. $\endgroup$ – gandalf3 Jul 12 '15 at 7:59
  • $\begingroup$ True, and were the entire shot isolated and coming out of Blender this wouldn’t be a problem, but I’m trying to match the particles with another shot that has been rendered. Comping the two together results in motion blurred particles that are streaked out far ahead of the ship and looks totally wrong. Physically accurate, but visually no good. $\endgroup$ – Gabbagabbahey Jul 12 '15 at 15:08

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