You used the Roto Brush tool to create a base frame, which includes a segmentation boundary that divides the foreground from the background. The Roto Brush span appears below the time ruler at the bottom of the Layer panel. When After Effects has propagated the current boundary for a frame, the bar beneath that frame on the time ruler is green.
As you move forward and backward through the footage, the segmentation boundary moves with the foreground object (in this case, the bird and the rock pool). You’ll step through the frames in the span and make adjustments to the segmentation boundary as necessary.
Because the rock pool is stationary, its track matte is consistent throughout the span. Given that, we’ll focus on the bird, which moves around a lot.
- Zoom in on the bird, and then make sure the Roto Brush tool is selected in the Tools panel.
- Press the 2 key on your main keyboard (not the numeric keypad) to move forward one frame.
Tip
To move forward one frame, press the 2 key on your keyboard; to move back one frame, press the 1 key.
Working from the base frame, After Effects tracks the edge of the object and attempts to follow its movement. Depending on how complex your foreground and background elements are, the boundary may or may not conform exactly to the area you hoped it would.
Note
As it propagates the segmentation boundary for a frame, After Effects caches that frame. Cached frames have a green bar in the time ruler. If you jump ahead to a frame further along the span, After Effects may take longer to calculate the boundary.
- Using the Roto Brush tool, paint foreground and background strokes to refine the matte for this frame. If the matte is accurate, you don’t need to paint any strokes.
As you progress through this clip, you’ll notice changes to the segmentation boundary with the bird’s beak. Likewise, you may need to refine the segmentation boundary around the feet and legs.
- Press the 2 key again to move forward to the next frame.
- Use the Roto Brush tool to add to the foreground or subtract from the background as necessary to refine the boundary.
If you make a stroke you don’t like, you can always undo the stroke and try again. As you move through the span, each change you make affects the frames after it. The more you refine your selection, the better the overall results will be. You may find it useful to move forward a few frames to see how changes affect the boundary.
When the bird is moving quickly (such as 0:14 –0:19), you’ll notice that the segmentation boundary becomes more vague, but then “catches up” once the bird slows down again. When a frame displays rapid movement, focus on removing areas that should be excluded (such as the branch behind the bird), rather than adding blurry areas of the bird.
- Repeat steps 4 and 5 until you reach the end of the layer.
- Once you’re satisfied with the bird, zoom out to fit the entire screen, and scroll though the span, making any adjustments as needed.
- When you have completed refining the segmentation boundary for the entire layer, choose File > Save to save your work so far.
Fine-tuning the matte
Roto Brush does a pretty good job, but there may be stray bits of background in the matte, or foreground areas that weren’t included. You’ll clean those up by refining the edge.