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Exploring Motion Blur and Decontaminate Edge Color

From: After Effects Apprentice 13: Paint, Roto, and Puppet

Video: Exploring Motion Blur and Decontaminate Edge Color

Finally, let's talk about Motion Blur and Decontamination. Roto Brush has the ability to automatically detect what portion of an edge is moving and give that edge additional blur to compensate just as real footage maybe blurred. Let's go to around Frame 26 where his hand is raising somewhat quickly. If I turn off Use Motion Blur, all I'm going to get is my softened, feathered edge around this area. However, if I enable Use Motion Blur, you'll see that this hand is more blurred than other portions of his body, which are not moving nearly as much.

Exploring Motion Blur and Decontaminate Edge Color

Finally, let's talk about Motion Blur and Decontamination. Roto Brush has the ability to automatically detect what portion of an edge is moving and give that edge additional blur to compensate just as real footage maybe blurred. Let's go to around Frame 26 where his hand is raising somewhat quickly. If I turn off Use Motion Blur, all I'm going to get is my softened, feathered edge around this area. However, if I enable Use Motion Blur, you'll see that this hand is more blurred than other portions of his body, which are not moving nearly as much.

If I want to I can solo this layer and compare without and with. Look at just the Alpha channel without Motion Blur, even edge Feather all the way around, with Motion Blur increase blur where his hand is moving. And I can tweak the motion blur settings to include more Samples Per Frame, basically smooth out any ghosting if there's fast movement, and also set Shutter Angle to match how my shot was originally taken. No Shutter Angle is very little blur.

Increase Shutter Angle is more blur. A lot of footage tends to be shot with the Shutter Angle of 180 degrees but it's good to know the camera settings so that you can match the shutter speed versus the duration of an individual frame to come up with a good Shutter Angle. If the shutter opening time is identical to the duration of your frame, then you want your Shutter Angle to be 360. If the time of the shutter is open is only half of the duration of the frame then 180 degrees, one half of a full circle, is appropriate. And I'll go back to RGB and unsolo my layer because it's good to see things in context just for reality check.

Finally, there is a Higher Quality switch in Motion Blur. Higher Quality always comes at the cost of additional render time. It was not that bad of a hit. I have personally found that toggling it on has given me a little bit cleaner of an edge. For example, around these fingertips as I move. So I'm going to leave Higher Quality on. Now let's talk about Decontamination, and by that we mean how much of the background color is being removed or decontaminated from the edges of your object. Roto Brush does not color-correct the entire image.

It's only color-correcting the edges, and if you want to see where it is correcting go ahead and enable View Decontamination. If you have a particularly soft shot, you might need a wider area of Decontamination because more of the background is seeping through these partially transparent areas around your edges. So you can go ahead and increase the decontamination zone if you so desire but don't do that unnecessarily. You might get some additional color artifact you don't really want in your edges. If I turn it off and increase my decontamination, you can see these additional ghosts that appear.

I'll set that back down to zero. You can toggle the effects of the color-correction off and on. I find it when it's off, and I'm now seeing some of my dark background appear on these edges. That's not desirable so I'll leave it on. If you think it's performing too drastic of a color-correction, if you're starting to see some color-ghosting around your edges, you can reduce the amount of decontamination. Too much, again I'm getting that dark ghosting. 100% is actually giving me the look I desire in this case. Finally, there's Extend Where Smooth and again I'll turn my Decontamination Matte back on.

If Reduce Chatter is greater than zero and if Roto Brush determines it needs to de-chatter an area, Extend Where Smooth says, "Well, let's do some extra color decontamination around those areas." Because I have a very low chatter amount, there's not a lot of additional smoothing of the matte taking place. But if Reduce Chatter was too high value, I'll just artificially put it to 100% right now, you'll see that Roto Brush is doing some more smoothing out of the Alpha boundary and Extend Where Smooth says, "Let's go ahead and do more color decontamination in those areas." In my case well, I decided I need very little Reduce Chatter, Extend Where Smooth does not help me at all.

But it's not one of those parameters where you can toggle it on and off, look very closely at your image and see if it's making a positive change or not. If it's not making a positive change, leave it off and in general that's a rule of After Effects. If you toggle on a switch and it doesn't seem to do anything, turn it back off again because you may not know what it's doing at another frame you're not currently looking at. And by the way, all of these refine matte parameters are not limited to Roto Brush. There is actually a standalone effect called Refine Matte which you can apply after anything that creates an Alpha channel including say a color key operation.

That's another trick to keep in mind. Well, now that we've refined the matte for this particular Roto Brush application, let's go ahead and queue up our RAM Preview. I pressed zero (0) on numeric keypad. Again, if you have a Mac you can press Ctrl+0 on your keyboard. That's to help out people with Mac Books that don't have numeric keypad equivalents. Let's see how we do. I am finding a little bit of artifacting around this particular edge, I'll probably need to go back and clean up the segmentation boundary around here but let's see this full thing in motion because quite often problems on individual frames aren't an issue when you're actually moving quickly.

Now they're playing back at full speed. I see that I do need some de-chattering because I am having some little bumps going on here and there. So if I hit Increase, I Reduce Chatter amount until I see these little artifacts start to disappear. Maybe a little bit more choking and let's RAM preview again and see if that improved the final render at all. I cannot emphasize enough how much Roto Brush is not a "Set it and done effect." It takes a lot of iterative work, a lot of going back and refining what you're doing until you come up with a desirable result.

This is pretty close. I am observing a little bit of trouble right here where a small dark area appears and through the bracelet on his wrist. I can go ahead and keep tweaking parameters, tweak my segmentation boundary, and remember to do that you will need to unfreeze to be able to go back and edit your boundary. And if necessary use other tricks such as masking or even the paint effect to paint back in missing gaps in the Alpha channel or to paint out, if it's the background, that have crept in that I don't want. Still using Roto Brush is a big timesaving as compared to hand masking every single frame or hand painting the Alpha channel for every single frame.

Just follow this procedure of Base Frame, propagation, corrective strokes, refine matte and you'll get a lot better results than other people who have just been applying a Roto Brush, making a couple strokes and saying, "It doesn't work." It does, there's just some assembly required.

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