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For the next several movies I'd like to show you how to use the Roto Brush tool, which was introduced in After Effects CS5. Roto Brush is a very special paintbrush that looks for the edges of objects in a layer and separates these elements from each other by creating an alpha channel that basically separates a foreground object on the background around it. If you have access to the Exercise Files, first open Roto Brush 1 Butterfly starter (RB1-Butterfly*starter). This is a really simple application of Roto Brush and I want to start with just to show you some basic concepts.
In the scene I have some footage of tulips in the background and a little animation of a butterfly flying across the scene in the foreground that we created in the earlier After Effects Apprentice video lesson. I'll render up a quick RAM preview here, and you'll see that our butterfly wafts in front of the flowers. Well, that's kind of nice. But to be honest, I think it will look more realistic if a butterfly wasn't flying basically below the level of the flowers. You'd expect the butterfly to be above them and therefore it should fly behind these foreground tulips.
I could just duplicate the tulip player and mask it out and put that copy on top to obscure the butterfly, but these tulips are shaking a little bit in the wind. So that means I would have to animate this mask and probably update it every single frame. That would be a lot of work. Well, this is where Roto Brush can help us save you some time. It's not an automatic click-and-you're-done sort of thing, but it can reduce the amount of labor required to pull off a composite like this. First thing I'm going to do is duplicate my Tulips layer.
I need to have a copy of it in front of my butterfly to block out or obscure the butterfly as it goes past. Secondly, I'm going to see what frames the butterfly touches the tulips. There's no point in doing more work than necessary. So I'm going to trim down this duplicate layer to only include the frames we need. I'm going to hold Option on Mac, Alt on Windows, press the left square bracket ( [) to set the in-point for this layer. Then move to where the butterfly is a little bit later. I did some in front of the second tulip as well and it clears it about there. Hold Option or Alt, press right square bracket (] ) and set the outpoint for the layer.
Now when I drag that Tulip player on top, it just covers the area where the butterfly crosses the tulips in question. There's a couple of more set up things you need to do to use Roto Brush. One, you must be at full resolution. You really need to calculate every pixel to get an accurate idea of what Roto Brush is up to. It's also a good idea to be at 100% magnification at the same time. So I'm going to lock that in. Next, just like with the Paint tool work with the Rotor Brush needs to be performed in the Layer panel.
So I'm going to double-click this duplicated layer to open it in the Layer panel. And here is my trimmed segment that I want to use Roto Brush on. Now that we've done this prep work, in the next movie we can actually start using the Roto Brush tool.
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