Adobe After Effects - Manual (Page 397)

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Work with resolution set to Full when using the Roto Brush tool. Fast Previews modes, such as Adaptive Resolution, don’t work well with the
Roto Brush tool, because switching resolutions requires a full recalculation of the segmentation information. For this reason, Fast Previews
modes are turned off when you draw a Roto Brush stroke. This setting is shared by the Composition and Layer panels. (See Resolution.)
Use the Roto Brush tool in a composition with a frame rate set to match the frame rate of the layer's source footage item. A warning banner
appears at the bottom of the frame in the Composition panel if the frame rate of the composition doesn't match the frame rate of the layer's
source footage item. (See Frame rate.)
When you've gotten everything as good as you can with the Roto Brush effect, you can touch up the matte further using other compositing
features in After Effects—such as by painting on the alpha channel. (See Compositing and transparency overview and resources.)
Roto Brush strokes, spans, and base frames
Base frames, Roto Brush spans, and corrective strokes
When you first draw a Roto Brush stroke, the frame on which you are drawing becomes a base frame. The segmentation information (the
information about what is defined as foreground and what is defined as background) is propagated forward and backward through time—20 frames
forward and 20 frames backward. The range of frames thus influenced by this base frame is its Roto Brush span. Little arrows in the span bar in
the Layer panel show the direction in which the information is being propagated. If you draw a corrective stroke anywhere where the arrows point
to the right, the information from that stroke is propagated forward; if you draw a corrective stroke anywhere where the arrows point to the left,
information from that stroke is propagated backward. If you draw a stroke anywhere outside of a Roto Brush span, then you create a new base
frame and span.
You can work your way forward a frame at a time from a base frame, making corrective strokes, and you don't have to worry about your strokes
changing results on frames that you've already worked on. You can do the same thing going backward from a base frame.
The influence of each corrective stroke propagates forward or backward to affect all frames in that direction within the span, regardless of when
the stroke is made. For example, if the base frame is at frame 10, you make a corrective stroke at frame 20, and then you make a corrective
stroke at frame 15, then frame 20 will be affected by both of these corrective strokes—just as if you had made the corrective strokes in the other
order.
Each time that you make a stroke within a span, the span grows, unless it can't because the span in which you're drawing is adjacent to another
span.
To manually change a span duration, drag either end of a span.
To delete a span, right-click (Windows) or Control-click (Mac OS) a span and choose Remove Span.
To delete all spans, delete the instance of the Roto Brush effect.
When you move to a frame within a span, After Effects must calculate how the stroke information from the other frames in the span affects the
frame to which you've moved. The Info panel shows the message “Roto Brush propagating” as this calculation is being performed. This information
is cached, so this propagation doesn't need to happen every time that you move to a frame. Green bars in a span indicate frames with cached
information. Choosing Edit > Purge Image Caches purges Roto Brush caches, too.
Roto Brush tool, Roto Brush strokes, and similarities to paint tools and strokes
The Roto Brush tool is similar in many ways to the paint tools, though it also has several important differences.
When the Roto Brush tool is selected, controls in the Paint panel are disabled. Roto Brush strokes have a duration of one frame, though their
influence propagates forward or backward within a span.
You can change the brush size (Diameter) for the Roto Brush tool in the same way that you change it for the paint tools. You can use the Diameter
control in the Brushes panel or Ctrl-drag (Windows) or Command-drag (Mac OS) in the Layer panel. Other controls in the Brushes panel do affect
the Roto Brush tool, with the exception of Hardness.
You can copy Roto Brush Path properties, and paste them into masks, shapes, and instances of the Paint effect, just like you can with other kinds
of paths. If you copy an individual stroke, the Roto Brush span information is not copied; however, if you copy the entire Roto Brush effect
instance, it includes the Roto Brush span (and base frame) information.
You can use expressions on the Roto Brush effect's Path property in the same manner as the Paint effect's Path property.
When drawing a stroke with the Roto Brush tool, a new stroke is created even if another stroke is selected. This differs from the stroke
replacement functionality for the paint tools.
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