Roto Brush and Refine Matte
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Separating a foreground object, such as an actor, from
a background is a crucial step in many visual effects and compositing
workflows. When you’ve created a matte that isolates an object,
you can replace the background, selectively apply effects to the
foreground, and much more.
Conventionally, segmentation of a moving image into foreground
and background elements has been accomplished through rotoscoping—defining mattes
by manually drawing Bezier curves (masks) on most frames, with some interpolation.
(See Rotoscoping introduction and resources.)
The Roto Brush tool provides an alternative, faster workflow
for this segmentation and creation of a matte.
With the Roto Brush tool, you draw strokes on representative
areas of the foreground and background elements, and then After
Effects uses that information to create a segmentation boundary
between the foreground and background elements. The strokes that
you make on one area inform After Effects about what is foreground
and what is background in adjacent areas and on adjacent frames.
Various techniques are used to track regions across time, and this
information is used to propagate segmentation forward and backward
in time so that each stroke that you make is used to improve the
results on nearby frames. Even if an object moves or changes shape
from one frame to the next, the segmentation boundary adapts to
match the object.
After you have created a segmentation boundary, you use the Refine
Matte properties to improve the matte. The Refine Matte effect is
also available separately for the improvement of mattes created
using features other than the Roto Brush tool.
Online resources for the Roto Brush tool and Refine Matte effectFor a collection of video tutorials and resources about
the Roto Brush tool, see this article on the Adobe website.
For a video that demonstrates the use of the Roto Brush tool
to quickly create a matte for selective color correction, see the Adobe
website.
For a video that demonstrates the use of the Refine Matte properties
in the Roto Brush effect to improve a matte, see the Adobe
website.
John Dickinson provides a video tutorial on the Motionworks website that demonstrates
the Roto Brush tool.
Chris & Trish Meyer share tips on the Roto Brush tool on
the ProVideo Coalition website.
Roto Brush and Refine Matte overview and workflowActivate the Roto Brush tool by pressing Alt+W
(Windows) or Option+W (Mac OS).
Open the layer that you want to work on in the Layer panel. Note: Double-clicking
a layer in the Timeline panel when the Roto Brush tool is active
opens the layer in the Layer panel.
Preview the movie in the Layer panel to find a frame in which
the greatest amount of the foreground object is in the frame and
in which the separation between the foreground and background is
as clear as possible.
The frame on which you draw your first
stroke is the base frame. (See Roto Brush strokes, spans, and base frames.)
Drag in the Layer panel to draw a foreground stroke on the
object that you want to define as a foreground object. When you
are drawing a foreground stroke, the Roto Brush tool’s pointer is
a green circle with a plus sign in the middle.
The magenta
outline that appears around the foreground object in Alpha Boundary
view mode is the segmentation boundary, the rough line
that separates the foreground from the background. You can also
view the segmentation using other view modes. (See Layer panel view options.)
Alt+drag (Windows) or Option+drag (Mac OS) to draw a background
stroke on the area that you want to define as the background. When
you are drawing a background stroke, the Roto Brush tool’s pointer
is a red circle with a minus sign in the middle.
Repeat the steps of drawing foreground and background strokes
on the base frame until the segmentation is as precise and complete
as possible. You should make the segmentation on the base frame
as good as possible; the segmentation of other frames is based on
the segmentation defined on the base frame.
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.)
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.
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.
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.
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.)
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 toolWhen 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 framesBase frames, Roto Brush spans, and corrective strokesWhen 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 strokesThe 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.
Roto Brush
strokes are in a Strokes property group within the Roto Brush property
group in the Timeline panel.
Many keyboard shortcuts that
work with paint strokes and paint tools also work with Roto Brush
strokes and the Roto Brush tool. (See Paint tools (keyboard shortcuts).)
Press 1 or 2 on
the main keyboard to move earlier and later (backward or forward)
one frame.
Press PP to show paint strokes, Roto Brush strokes, and Puppet
pins.
Press Ctrl+A (Windows) or Command+A (Mac OS) to select all
strokes on current frame.
Roto Brush effect and Refine Matte effect referenceThe Roto Brush effect properties in the Propagation property
group affect segmentation between foreground and background and
how that segmentation information is used for contiguous frames
in a span. Other properties of the Roto Brush effect affect the
matte that is generated based on the initial segmentation. The Refine
Matte effect includes all of the properties of the Roto Brush effect outside
of the Propagation property group (except for Invert Foreground/Background).
- Propagation
- Properties in the Propagation property group (except for
View Search Region) affect all Roto Brush calculations. Any change
to these properties requires a recalculation and propagation of
the segmentation information from a base frame. Also, the results
on a base frame itself are not affected by changes to these properties;
therefore, it’s best to change these properties when the current-time
indicator is a frame or two away from a base frame, so that you
can see the result of the changes.
- Search Radius
- The radius of the area within which After Effects searches
when looking for pixels that match from one frame to the next. You
can change how the search radius adapts to regions with more or
less motion using the Motion Threshold and Motion Damping properties.
If the search radius is too small, some motion may be missed; if
the search radius is too large, extraneous motion may be detected.
- Motion Threshold and Motion Damping
- These two properties control how the search region is constrained
based on motion. Change Motion Threshold to set the motion level
below which is considered no motion, where the search region will
shrink to nothing. Motion Damping affects the remaining areas that
are considered to be in motion. As you increase Motion Damping,
the search region is tightened, with slow-moving areas tightening
more than fast-moving areas. Constraining the search region in areas
with little motion can reduce edge chatter in these regions. Constraining
the search region too much will cause the automatic boundary detection
to fall off the edge of the object.
- View Search Region
- Renders the search region as yellow, and the foreground and
background as a grayscale image (with the background dimmer than
the foreground). The value of this property affects the rendered
output of the Roto Brush effect, not just an intermediate stage
of the effect's operation. Its main use is to help you pick values
for Search Radius, Motion Threshold, and Motion Damping.
Note: For
frames other than the base frame, viewing the search region shows
the area that will be searched on the current frame, based on segmentation
results of the previous frame. So, for frames other than the base
frame, adding a stroke on a frame does not change the search region
shown for that frame.
- Edge Detection
- Choose whether to favor the segmentation boundary calculated
for the current frame in isolation or the segmentation calculated
based on the previous frame when determining the edge between foreground
and background. The Balanced option considers the current frame
and surrounding frames equally. Foreground objects with colors that
match the background will usually benefit from Favor Predicted Edges.
- Use Alternate Color Estimation
- Subtly changes the process by which the Roto Brush effect
determines what is foreground and what is background. Sometimes checking
it helps with segmentation; sometimes it doesn't.
- Invert Foreground/Background
- Inverts which strokes are considered foreground strokes and
which strokes are considered background strokes in the segmentation
phase of the Roto Brush effect.
- Smooth
- Increasing this value reduces the sharpness of the curves
in the segmentation boundary by smoothing along the edge.
Leave this number low when isolating a object with sharp features,
such as hair.
- Feather
- Softness of the segmentation boundary. This property does
nothing if Smooth is 0. In contrast to Smooth, Feather applies across the
edge.
- Choke
- The amount of choking (contraction) of the matte relative
to the value of the Smooth property. The result is very similar
to that of the Choke property in the Matte Choker effect, but the
value is given from -100% to 100% (instead of -127 to 127).
- Reduce Chatter
- Increase this property to reduce erratic changes to edges
from one frame to the next. This property determines how much influence
the current frame should have when performing a weighted average
across adjacent frames to make the matte edges not move erratically
from one frame to the next. If the Reduce Chatter value is high,
the chatter reduction strong, and the current frame is considered
less. If the Reduce Chatter value is low, the chatter reduction
is weak, and the current frame is considered more. If the Reduce
Chatter value is 0, only the current frame is considered for matte
refinement.
 If the foreground object isn’t moving,
but the matte edges are moving and changing, increase the value
of the Reduce Chatter property. - Use Motion Blur
- Check this option to render the matte with motion blur. The high
quality option is slower, but generates a cleaner edge. You can
also control the number of samples and the shutter angle, which
have the same meaning as they do in the context of motion blur in
the composition settings. (See Motion blur.)
- Decontaminate Edge Colors
- Check this option to decontaminate (clean) the color of
edge pixels. The background color is removed from foreground pixels, which
helps to fix halos and the contamination of motion-blurred foreground objects
with background color. The strength of this cleaning is determined
by Decontamination Amount.
- Extend Where Smoothed
- Only functional when Reduce Chatter is greater than 0 and
Decontaminate Edge Colors is selected. Edges that are moved in order
to reduce chatter are cleaned.
- Increase Decontamination Radius
- Amount (in pixels) by which to increase the radius value
for the cleaning of edge colors, in addition to any cleaning that covers
feather, motion blur, and extended decontamination.
- View Decontamination Map
- Shows which pixels will be cleaned by decontamination of
edge colors (white pixels in the map).
Freezing (caching, locking, and saving) Roto Brush segmentationWhen the View menu in the Layer panel is set to Roto Brush,
a Freeze button appears in the lower-right corner of the Layer panel.
Click this button to cache and lock segmentation for all Roto Brush
spans for the layer within the composition work area. This preserves
the matte and saves it with the project, preventing the Roto Brush
effect from recalculating the segmentation when you open the project
again or make changes.
If After Effects has already calculated Roto Brush segmentation
information for a frame when you click the Freeze button, then this
information is cached. If the segmentation has not been calculated
for a frame within a Roto Brush span, then After Effects must calculate
the segmentation before freezing.
Frames with frozen (cached and locked) segmentation information
are represented by blue bars in the Roto Brush span view in the
Layer panel.
Note: If you click Stop in the Freezing Roto Brush dialog box, After
Effects stops adding frames to the cache, but Roto Brush segmentation
is still locked with the segmentation information cached up until
the point that you clicked Stop.
To unfreeze Roto Brush segmentation, click the Freeze button
again.
When Roto Brush segmentation is frozen, you can place the pointer
over the Freeze button to see a tooltip that tells you when the
cached information was created.
When Roto Brush segmentation is frozen, the pointer for the Roto
Brush tool has a slash through it.
The information that is cached and locked is the result of Roto
Brush strokes and the properties in the Propagation property group
of the Roto Brush effect. Making changes to any of these items (for
example, by drawing new Roto Brush strokes or modifying properties
in the Propagation property group) has no influence on the result
of the Roto Brush effect until you unfreeze segmentation. The properties
in the Matte property group are not frozen.
Frozen Roto Brush segmentation information is cached and locked
while the application is running, and the cached information is
saved with the project.
Layer panel view optionsYou can choose these view modes from the Show Channel menu
in the Layer panel, by clicking the buttons in the Layer panel,
or by using keyboard shortcuts. You can use the controls at the
bottom of the Layer panel to change the color and opacity of the
overlays used in Alpha Boundary and Alpha Overlay mode.
- Alpha
- Shows alpha channel of layer (Alt+4 or Option+4).
- Alpha Boundary
- Shows source layer with foreground and background unchanged,
with segmentation boundary overlaid as colored outline (Alt+5 or Option+5).
Note: Alpha
Boundary view mode is turned off when the View menu in the Layer panel
is changed to anything other than Roto Brush.
- Alpha Overlay
- Shows source layer with foreground unchanged and background
overlaid with a solid color (Alt+6 or Option+6).
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