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Motion Blur Roto


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#1 Jed Shepherd

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Posted 11 November 2010 - 08:00 AM

When doing a roto on a person or object that has motion blur, is it best to roto the solid part rather than the edge of the motion blur? I thought this would be the case so that nothing shows through in the motion blur. But if you do this, is it possible to restore a realistic motion blur?
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#2 Phil Connolly

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Posted 11 November 2010 - 09:41 AM

Depends on the shot - but the normal approach is to roto round the solid edges - removing the motion blur where the background shows through.

The motion blur is then put back in during the compositing stage - if done carefully it does look convincing.
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#3 Adam Hunt

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Posted 11 November 2010 - 08:20 PM

Any good compositing package has a roto mask tool that allows you to define both the hard and soft edge of the motion blur. This basically creates a gradient in the matte that preserves the motion blur. A little more work than just rotoing a hard edge, but a lot less work and more realistic than trying to get it back later.

I'm not sure what package you are using but look around for an inner/outer edge type thing. In Shake there are some buttons that allow you to flip between which edge you want to manipulate. It should work similarly in other packages.
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#4 Jed Shepherd

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Posted 12 November 2010 - 02:15 AM

Any good compositing package has a roto mask tool that allows you to define both the hard and soft edge of the motion blur. This basically creates a gradient in the matte that preserves the motion blur. A little more work than just rotoing a hard edge, but a lot less work and more realistic than trying to get it back later.

I'm not sure what package you are using but look around for an inner/outer edge type thing. In Shake there are some buttons that allow you to flip between which edge you want to manipulate. It should work similarly in other packages.


Yeah I'm using shake. Hoping to get some Nuke practice soon though. I can understand if a roto was done on a green screen piece of footage(For whatever reason. Bad green that wont key or something) that roto'ing at the edge of the blur could be done because the keyer would get rid of the green but on a piece of footage with no green mixed into the motion blur would'nt there still be some remaining color from the bg. Also i have tried the 'falloff gradient thing' inside the rotoshape node but did'nt get as good a result as i would have liked. Im not working on anything specifically with this problem at the moment but i just thought i would see what people had to say on the issue.
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#5 Adam Hunt

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Posted 12 November 2010 - 02:22 AM

Yeah I'm using shake. Hoping to get some Nuke practice soon though. I can understand if a roto was done on a green screen piece of footage(For whatever reason. Bad green that wont key or something) that roto'ing at the edge of the blur could be done because the keyer would get rid of the green but on a piece of footage with no green mixed into the motion blur would'nt there still be some remaining color from the bg. Also i have tried the 'falloff gradient thing' inside the rotoshape node but did'nt get as good a result as i would have liked. Im not working on anything specifically with this problem at the moment but i just thought i would see what people had to say on the issue.


Like any VFX, it's case by case and takes a 'feel' for it based on experience. It doesn't work perfectly every time, but I have had quite a bit of success with using it for roto. Some things you can try is placing the 'soft edge' a little further in from the edge, or using gamma or some other kind of node to affect the roll-off of the gradient. I find it's usually easy to work with most backgrounds, but if there are defined bright objects back there it can be problematic.

Also I find it helpful to work with the new background comp'ed in. Often the new background will hide details like that. If you work on a gray or checkered background it may show things that wont actually show up on the final composite.
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#6 Adam Hunt

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Posted 12 November 2010 - 02:25 AM

One other thing you can try is setting up a node chain that blurs the original image along the gradiated matte edge (without blurring the solid part). This sometimes gets rid of patterns that are apparent through the motion blur. Make sure you are blurring the original image but not the matte itself.
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#7 Jed Shepherd

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Posted 12 November 2010 - 02:36 AM

One other thing you can try is setting up a node chain that blurs the original image along the gradiated matte edge (without blurring the solid part). This sometimes gets rid of patterns that are apparent through the motion blur. Make sure you are blurring the original image but not the matte itself.

Thanks for the replies. What your saying makes sense. I was comping some keyed footage the other day and noticed how many problems weren't visible when i had the bg plugged in. So for bluring the matte edge would it be something like a re-order set to aaaa then an edge detect etc combined with blur than added back to the original matte so that the edge is blurry? Or when you say not the matte itself you mean blurring the entire rgb but leaving the alpha alone.
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#8 Jed Shepherd

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Posted 12 November 2010 - 02:38 AM

Ok i re-read what you wrote. Not blurring the matte itself
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#9 Adam Hunt

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Posted 12 November 2010 - 02:40 AM

Thanks for the replies. What your saying makes sense. I was comping some keyed footage the other day and noticed how many problems weren't visible when i had the bg plugged in. So for bluring the matte edge would it be something like a re-order set to aaaa then an edge detect etc combined with blur than added back to the original matte so that the edge is blurry? Or when you say not the matte itself you mean blurring the entire rgb but leaving the alpha alone.


Yeah, blurring the RGB and leaving the alpha (the matte) alone. Yeah, some combination involving an edge-detect on the mask would work. You want basically a separate mask that follows the edge and limits the blur to just the edge.
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#10 Jed Shepherd

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Posted 12 November 2010 - 02:45 AM

Ok thanks. Do you composite professionally?
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#11 Adam Hunt

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Posted 12 November 2010 - 02:52 AM

Yup, I do. I've done work on music videos, shorts, and features as everything from a roto artist to a VFX supervisor.
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#12 Jed Shepherd

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Posted 12 November 2010 - 02:57 AM

Yup, I do. I've done work on music videos, shorts, and features as everything from a roto artist to a VFX supervisor.

Awesome. Is there much point in getting to much better at shake or would i be best to swap to nuke now and get better at that. Im still relatively new to shake but im sure what i've already learnt should transfer to nuke. The guy who taught me what i know in shake composites on feature films and he keeps saying that nuke is the way to go because of how shake was discontinued. Is Shake used at all anymore really?
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#13 Adam Hunt

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Posted 12 November 2010 - 03:17 AM

Whatever you learn in Shake can be transferred to Nuke or any of about a half dozen other node-based compositors. The nodes may be named differently, or there may be some other minor differences, but you can translate any trick you learn or come up with into Nuke.

Personally I have gotten hired on VFX contracts by saying I knew Shake even though they were using something like Digital Fusion and I had never even touched it before. It took all of a 1/2 hour to find my way around in the software. They are all very, very similar.

Shake is not actually dead. Apple killed it as a retail product which was a major F you to small houses and individual VFX artists. But they sold the source code to major operations like ILM and Weta, who maintain their own proprietary versions of Shake which they use as their main compositing software.

Personally I find Shake is still the most streamlined of the node compositors and my opinion the best tool for the job. And until I find a good reason to switch to one of the others I will stick with Shake for any work where I have a choice of software. But I can also be safe in the assumption that it's easy to jump to something else if a job requires it.

If you like Shake than stick with Shake whenever you can. The people who hired you care if the final image looks cleanly composited, not whether you used a piece of software that was technically EOLed.
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#14 Jed Shepherd

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Posted 12 November 2010 - 08:47 AM

Fair enough, I like shake. Just thought i would move to nuke for its 3d workflow. Yeah i have been told that people get onto jobs quite a bit without knowing the software. I guess they just care about your ability to make good images.
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