Adobe 12040118 - After Effects Standard Tutorial page 341

Try for a segmentation boundary that is within a couple of pixels of your desired edge. You can modify properties in the Roto Brush effect,
such as Smooth, to refine the initial segmentation further. (See Roto Brush effect and Refine Matte effect reference.)
You can resize the Roto Brush tool's tip to make finer strokes. Broad strokes are best for initial work, but fine strokes are useful for
details. (See Roto Brush strokes, spans, and base frames.)
7. Press Page Down to move forward one frame.
After Effects uses motion tracking, optical flow, and various other techniques to propagate the information from the base frame to the current
frame to determine where to draw the segmentation boundary.
8. If the segmentation boundary that After Effects calculates for the current frame is not where you want it to be, then you can make corrective
strokes to teach After Effects what is foreground and what is background. Draw foreground strokes and background strokes as needed to
correct the segmentation. Corrective strokes propagate in one direction, away from the base frame.
Note: You can also modify properties in the Propagation property group to affect how After Effects propagates the segmentation information
from previous frames to the current frame. (See Roto Brush effect and Refine Matte effect reference.)
9. Repeat the steps of moving one frame at a time and making corrective strokes until you have created a segmentation boundary for the
entire duration that you want to segment.
10. Select the Refine Matte option in the Roto Brush effect properties in the Effect Controls panel and modify properties in the Matte property
group as needed. (See Roto Brush effect and Refine Matte effect reference.)
11. When you are done, click the Freeze button in the lower-tight corner of the Layer panel to cache, lock, and save the Roto Brush
segmentation information. (See Freezing (caching, locking, and saving) Roto Brush segmentation.)
Tips for working with the Roto Brush tool
When drawing strokes to define a foreground object with the Roto Brush tool, begin by drawing strokes along the center of the object's
features. For example, draw a stroke along the skeleton rather than along the outline of an arm. Unlike conventional rotoscoping, which
requires precise manual definition of boundaries, using the Roto Brush tool works by defining representative regions. After Effects can then
extrapolate from those regions to determine where the boundaries are. Before you draw a stroke along a boundary to attempt to get a
precise segmentation, be sure that you've drawn foreground strokes down the center of the object and made at least some rough
background strokes on the other side of the boundary.
If you draw a Roto Brush stroke over the wrong area of the image, undo that stroke. (See Undo changes.) However, if After Effects
misinterprets your stroke and includes or excludes too much of the image, don't undo; further teach Roto Brush by drawing additional strokes
to include or exclude regions.
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 the top
To the top