Motion Blur and Rotoscoping

I’ve recently been working on a lot of screen replacement shots with Mocha Pro and the one area I’m trying to get a better understanding of is rotoscoping and motion blur. For example, I was tasked with replacing an animated screen over real footage of someone swiping on their phone (i.e there was no way to key this out) so I rotoscoped the thumb to create a matte. The issue I’m having is there is a good amount of motion blur at certain points of the thumb swiping over a real screen which reveals the original image through the motion blur. Is correcting this as simple as rotoing on the hard edge of the thumb and feathering the pixels to match the blur? How do I go about isolating the thumb so the motion blur is not revealing the original screen below while matching the original motion blur? Hopefully, this makes some sense.

All the best,

-Ben

This is a common compositing problem but a pain in the neck.

The way to solve this is to basically make a composite of your fingers and a similarly colored background to the fingers, or if possible even enlarged fingers behind your actual barely feathered edged roto of your fingers.

Then you use your motion blur roto on that layer. That makes it to where instead of having transparent pixels of your screen, you have transparent pixels of a similar color to your fingers.

So you’re making a composite and then applying motion blur on that layer to get something you can composite over your screen without problems.

Does that make sense? Mocha Pro can’t do this for you automatically, but some clever compositing with mocha shapes can.

Thanks Mary!

So if I’m understanding correctly, it would be a cut and replace type scenario in that I would cut past the transparent motion blur thats revealing the screen below in my roto and then in the comp stage (back in AE) enlarge the fingers or use a solid or some form of paint replacement to add motion blur too comping the cut and blur versions together. Is is that correct?

Thanks again for all you help. It definitely makes sense this is something done manually, wasn’t sure if I was heading the right direction.

All the best,

-Ben

I am having a hard time visualizing what you mean so let me show you one option with a picture.

You want something that looks like that.

Or you want something that looks like this monstrosity. Though it would be harder to do.

Then you apply your Mocha Pro plugin with the motion blur on to this composite OVER your screen. Your motion blur will no longer have an old screen ghosted in it.

See what I mean?

My apologies I hope I’m not way off here. So it looks like in the first picture you’re using the feathered edge over a skin color background and applying the mocha pro plug-in with motion blur as kind of a comped patch. The same applies to the second picture but it is a duplicated version of itself. I guess my only confusion is layer order/where you’d place the instance of the mocha pro plugin in AE? Wouldn’t the skin colored layer in the first picture also need to have its instance of mocha so it followed the same shape data of the finger roto then you’d enable mocha pros motion blur on the finger layer? So it would go finger roto on top, skin colored solid below, apply mocha pro with motion blur to the finger roto?

Thanks again,

-Ben

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I am telling you what the images should look like when you are done with the hand comp, I wasn’t really outlining layer order, you should be able to figure that out by adjusting your comp until it looks like the image here, which I just photoshopped to illustrate. :slight_smile:

You need to precomp the solid and the hand roto layer together, then you can apply Mocha Pro on the precomp. Precomping everything and moving all the attributes into the new comp is how you get around any trickyness in AE.

No need to overcomplicate this, it’s just cutting pixels out with transparency, and whatever pixels are in the transparency will be the color of the original plate if you don’t replace them with something else. This is just replacing those original pixels with a medium skin tone color of the hand. It’s less a Mocha fix and more a compositing fix. This works for objects with depth of field transparency, and any feathered or blurred edge. I can’t really walk you through the exact steps because I can’t see your shot, but conceptually, you’re looking to replace BG plate pixels with new information and adjust until you’re happy with the results.

In Nuke this is wrapped into an Edge Extend node and of course you still need non motion blurred roto on top of that node as well.

In Flame it is called Pixel Spread and you still have to composite over it.

In AE, some artists do this with Boris FX Sapphire tools S_Distort and composite over that.

Or you can precomp over a color. There’s a million ways to do this, but the basic concept is you need similarly colored pixels for the motion blur roto.

Some people tile the texture and use the wrap edges option. Some people blur the cutout hand and composite it over itself until it is not feathered anymore, but opaque, then reapply the motion blurred matte on top of that. There’s so many ways to composite and there’s no “right way” there’s just the way that works for your shot.

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Gotcha, that makes sense. Well, hey I truly appreciate your help on this topic; there’s so much great information here you’ve really clarified exactly what I was looking for with several fantastic methods for motion blur replacement.

Thanks again!

-Ben

Happy to help. :slight_smile: The basic issue here is RGB vs Alpha, right? Your RGB pixels have the original plate pixels in them, so when the Alpha (matte) is applied, you see those pixels. To solve this, you have to replace those pixels first before compositing that over the screen. I hope you solve this tricky shot! It’s definitely a slightly more advanced compositing task.

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