Motion blur workflow

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  12 December 2012
Motion blur workflow

I'm looking for motion blur workflow suggestions. I see three options:

1. Generate a velocity pass and apply motion blur in compositor. Downside is shadows and other lighting effects don't appear in velocity pass.
2. Apply intelligent auto blurring in compositor. Downside is, for many effects, such as particles, the intelligent motion blur doesn't work.
3. Have my 3d package produce the motion blur and export as a separate pass. Downside is I lose creative control at compositing and rendering takes a lot longer.

1 seems the best perhaps by generating a shadow pass and faking the motion blur. 3 is most accurate. 2 might work very well for most cases.

Any recommendations. Anybody done this in a major production who can speak to how it was done there?

Just for reference, I'm using 3dsmax/vray + AE.
  12 December 2012
Depends on what you're blurring, but 9 out 10 times I just fake it in the comp. RSMB plugin will work great on some things, not so great on other things. Sometimes you have to render in layers to not have artifacts.

Sometimes I do the velocity pass too.
  12 December 2012
If you are using AE, ReelSmart Motion Blur may help in some cases:

Good luck with your project!
  12 December 2012
You don't mention what type of project this is - cg animated, live-action vfx, arch-viz, other?

With a few exceptions - such as replicating stop-motion animation in cg (and even then stop-motion occasionally uses motion blur), personally I'd always go with rendered motion blur even with the hit in render times. Most major film productions will render motion blur rather than try deal with it as a post process - post-motion-blur often doesn't give good results.

It's accurate and gives high quality results, compositors don't have to spend time making post-motion-blur look good.

How important is creative control of the motion blur to you? How much control do you want to have over the motion blur in compositing? Are you going for cartoony motion blur effects or do you just wish to occasionally reduce or exaggerate it in certain places?

It's physically-based so if photo-realism is your goal then it's a much better choice - I've always found it kind of confusing that people would go to great lengths to use physically-based shading techniques (even with the hit in render times) and then skip on the motion blur.

Much better results when dealing with rotating objects - for example, helicopter blades and car tyres - but also certain types of movements look better if you've got some curved motion blur in there rather than just plain old linear motion blur.
  12 December 2012
If you've got the rendering budget for it and a compositing pipeline that can deal with it, do rendered motion blur. It always looks better. Edit: If you're using V-Ray, is it possible that shade maps or whatever they're called can speed things up? I know they can be used for stereo rendering and DOF; I don't know if it would help for 3D motion blur. Also, always use adaptive DMC or fixed sampling when rendering motion blur and DOF. Adaptive subdivision is really bad at that sort of thing.

If the render times for 3D motion blur aren't within your rendering budget or you absolutely have to be able to tweak the motion blur in post, do motion vectors and blur in post. One problem with this is that it will never produce rotational blur. This can be a problem, but it might not matter much. Even big studio productions with "real" 3D motion blur sometimes use linear blur; it's not always a problem.

If neither works for whatever reason, carefully apply optical flow motion blur (ReelSmart Motion Blur, or whatever).

Also, you can always do a combination of various types of motion blur for your different passes. For example, if you render particle effects separate from the main pass, you can do real 3D motion blur on the particles and then do a motion vector blur on the main pass. You could even do optical flow blur on the reflections, since you wouldn't have any motion vectors for them.

Last edited by CaptainObvious : 12 December 2012 at 04:18 PM.
  12 December 2012
Like what most of the other guys have said it really depends on the production/project your on and whether you have the rendering budget to complete the project.

We generally will render motion vectors, and do the blur in comp, however with the introduction of Arnold, motion blur costs almost nothing to the render time and we will render motion blur sometimes along with DOF.

So to sum it up really, we use all three techniques depending on the production.
  12 December 2012
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