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Defining the base frame

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

Video: Defining the base frame

Now the first thing I want to do is make a foreground selection, teach Roto Brush what I considered to be the foreground object in this layer and namely it's this actor's hands and arms. Although it is just his hands and a little bit of his wrist that go in front of the screen, quite often, it's better in the long run to select large continuous portions of a person's body. If I selected only part of his arm, I might get a matte line, a bit of difference or distinction where the Roto Brush version of his hand in the foreground joins up with the normal copy footage in the background.

Defining the base frame

Now the first thing I want to do is make a foreground selection, teach Roto Brush what I considered to be the foreground object in this layer and namely it's this actor's hands and arms. Although it is just his hands and a little bit of his wrist that go in front of the screen, quite often, it's better in the long run to select large continuous portions of a person's body. If I selected only part of his arm, I might get a matte line, a bit of difference or distinction where the Roto Brush version of his hand in the foreground joins up with the normal copy footage in the background.

So I'm going to select the Roto Brush tool, hover my cursor over the Layer panel, and you'll see this little green cross. This green circling cross indicates I'm about to define the foreground portion of the layer. Now unlike normal painting you don't want to or need to make very precise strokes right along the edges between your foreground and background. Roto Brush is the one who's going to determine where that edge is. What you need to do is make broad strokes initially to capture as much color information for Roto Brush to use and then determine where these transitions may be.

Resizing the Roto Brush is just like resizing an ordinary paintbrush. I'll hold Command on Mac or Ctrl on Windows, click and drag the size of my brush larger or smaller. In this case, since I'm going to be initially defining my foreground along his arm and up his shoulder, I'm going to make my initial foreground brush size pretty darn close to the thickness of his wrist. Once I've done that, I'm going to click inside the foreground. It's very important that you don't accidentally select the part of the background otherwise you'll be teaching Roto Brush bad habits.

Never lie to Roto Brush, you'll get in trouble later. And I carefully place my cursor over the foreground, click and start dragging. As I do so, Roto Brush is going to draw this green trail to show me what I'm selecting as my foreground. Have you accidentally crossover and touch a bit of the background, you'll notice that there's a pink segmentation boundary has selected part of the background and the foreground. That's a bad thing. I'm going to Undo and redraw my stroke making sure I only select the foreground.

Once I release, Roto Brush is going to draw an initial segmentation boundary. Now I'm going to drag my Layer panel a little bit taller so you get to see the entire frame. Now Roto Brush did a pretty good job of finding the hands and arms and a little bit of his hand over here and his shirtsleeve, but it did not do a perfect job. It's missing his fingers in through here. It's grabbled a little bit of the computer screen and computer keyboard through here. So you're next task is optionally teaching Roto Brush where the background is, what it should exclude.

To do that, you hold down Option on Mac, or Alt on Windows, and now you'll see your brush has turned to a red circle with a minus sign in it and indicates you are going to be subtracting from your selection rather than adding to it. Now this brush is too big for me to go down the keyboard or between his fingers, so I'm going to hold Command on Mac, Ctrl on Windows, resize the brush smaller. Now hold down Option or Alt and drag from my background that it got correct to the background that it got incorrect.

I don't dab and dot and point with Roto Brush, I like to draw continuous lines to teach Roto Brush this transition or gradient of colors is what you should be considering all part of the selection or part of the background. I release, it's done a better job of getting rid of the keyboard, but it's still making some mistakes. This is where you need to go into progressively finer and finer detail to keep teaching Roto Brush what's the foreground and what's the background. So I'm going to zoom up to 200%, hold down the Spacebar temporarily to pan around my footage and start looking in greater detail around the shot and how I can correct it.

With my green foreground brush, I'm going to add in the fingers then I'm going to press Command or Ctrl, make a smaller brush, press Option or Alt and start subtracting some of these areas between the fingers that are incorrectly selected. You did a good job up there, I'll get this little bit of keyboard that's missing down here, a little bit of the laptop frame down in this gap, a little bit more right there. Zoom in another level, pan over, make a smaller brush, Option or Alt, and cut into this area between the fingers.

And these are the tricky areas. You'll notice his fingers are also partially blurred, a combination of motion blur of him moving so fast, and also then being out of focus as they get further away from the camera's focal plane. But this pink segmentation boundary is a binary matte. It's just saying here's the centerline of where my eventual transparency transition is going to be. So when I release my mouse, you'll see it has not done a perfect job getting up into this gap. I can keep zooming in and trying to teach Roto Brush more, what's background and what's foreground, but don't expect it to grab every single pixel at this point.

It's just a centerline as long as since within a pixel or two, you can correct it later. Now pick a more of this fingertip that's missing, through here as well, maybe get a little bit more of the finger there, and keep looking at these fine details. It's doing pretty well through here but I'm going to subtract a little bit out right into that gap, a little bit more. If you accidentally make a wrong stroke with Roto Brush, for example, if I accidentally drag into this area, don't try to correct it by making a second stroke.

You're going to be giving Roto Brush conflicting information. Instead, Undo and make sure your brush strokes are only exactly where you mean them to be. Two wrongs do not make it right with Roto Brush. Okay, we've got some tricky areas up here where I need to make some decisions. This area is definitely part of the picture frame behind his hand. However, the flesh tones in the picture are very close to the flesh tones in his hand. I'm probably going to have a hard time creating an automated Roto Brush boundary that differentiates between those heads and his hand.

Meanwhile, this picture frame is a pretty solid boundary. So I think I'm going to make a decision here just making a matte line around here, it's outside of the range of the computer display because that would get me in trouble. As long as I keep a pretty steady line from frame to frame I can get by with just cutting my matte through here. Again, this is not going to get over our replacement screen but I want to eventually have a clean matte line where these joins up with the untouched footage behind. So I'm going to zoom back in. I am going to subtract this part of the picture frame through here, but make sure I get this corner up there.

Come over this direction, the white frame is going to be pretty hard to tell apart from his light blue shirt, but let's make sure we don't get any of this little blue sun that's on top of that picture frame. I'm going to come up here through his shirt collar and again we've got a collar and a background that's very close to his shirt. I'm not going to try to differentiate between his shirt and his background. I'm just going to make sure I have a consistent matte line in this shape. So I'm going to add to the foreground there, maybe a little bit more, and a little bit more.

There we go. I'll keep an eye on that from frame to frame. Okay. It's going to scroll through this. I'm looking at this shirt and I may change my mind at this point. The base frame is what you want to get as right as possible. Make your decisions now rather than later. So even though I talked myself out of trying to get this entire picture frame, I'm going to do a test stroke here to see if Roto Brush can differentiate between a frame and a shirt, and looks like it can. So I'm going to go ahead and try to Roto along the line between his shirt and the picture frame and do that throughout the shot.

The base frame is the time to make these sorts of decisions as to what you want to try to capture or exclude because Roto Brush is going to use this information to determine the rest of the shot and determine your matte from frame to frame. This is looking good, let's go up around his hand, try to get a little bit more into that gap there and here but I can work on these semi-transparent areas later. Looking pretty good through there, pretty good through there.

All right. I've now defined my base frame. The next thing I need to do is to teach Roto Brush how to use this information on subsequent frames earlier and later in time.

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