Select Roto Brush 
in the toolbar (Alt/Option + W ).
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Separate a foreground object from a background using Roto Brush and create mattes using Refine Matte.
Separating a foreground object 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.
Use Roto Brush to draw strokes on representative areas of the foreground and background elements. After Effects uses that information to create a segmentation boundary between the foreground and background elements.
After creating the segmentation boundary, use 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.
Roto Brush 2
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Traditional rotoscoping uses animated masks to isolate an element in a video clip, which can be tedious and time-consuming. Roto Brush 2 uses artificial intelligence to track your subject’s movement so you can cut down on frame by frame refinement.
The default rotoscoping option is Roto Brush 2. If you prefer the older tool (Roto Brush 1), you can select it by using the Version dropdown in the Roto Brush & Refine Matte effect.
What's new in Roto Brush 2
The propogation banner displays in both the Layer and Composition panel which contains the layer to which Roto Brush is applied. In case you have the Layer panel closed, and you are working on something else, you still see the propogation progress.
The Roto Brush and Refine Edge Span shows a light grey chevron pattern, and the base frame is a bright green box at the beginning. When you propogate the frames, green color chevrons display in the middle of the span. For more information, see Strokes, Spans, and Base frames.
The classic controls such as Motion Threshold and Motion Damping are not enabled by default. You can enable them by selecting Enable Classic Controls under the Roto Brush & Refine Edge effect. You can use the features of both the Roto Brush versions together to get the most out of your rotoscoping.
Workflow
If you are familiar with the Roto Brush 1, selecting objects with Roto Brush 2's propagation engine is largely the same:
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Select Roto Brush from the toolbar -
Double click an existing layer in the Composition panel.
This opens up the layer in the Layer panel.
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Select a frame to start from.
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Avoid frames where the object is hidden by other objects in the scene. For example - for a person walking in and out of frame, choose a frame where they are completely in the scene.
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Paint a green foreground stroke on the subject, object, or region you are attempting to roto.
The object to be removed is outlined in magenta. 
Start with one stroke that cuts across the object. See what Roto Brush selects from that and use as few strokes as necessary to add or remove from the selection.
Avoid drawing an outline around the object. Instead, draw through the middle, passing through any regions on the object that have different color or brightness.Don't paint across edges as this can confuse the selection.By default, there is a green cross over the mouse cursor. It is a best practice to set the appropriate brush size before making your first stroke. The brush size is visible in green before you start with your first stroke. To change the size, select Windows > Brushes, and adjust the diameter of the brush. You can also hold Ctrl (Win) or Command (macOS) and drag left and right to change the size of the brush.
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Refine the selection.
Paint extra strokes, or paint background strokes by holding Alt/Option to remove portions of the selection. Draw strokes around the edges that you want to remove.
The magenta line that surrounds the object starts to get refined.
In case you cut a little extra, release Alt/Option and drag over the area to add to the selection. The indication is that the cursor is green during addition. When you are drawing a background stroke, the Roto Brush tool’s pointer is a red circle with a minus sign in the middle.
Foreground stroke, adding to selection 
Background stroke, removing from selection -
Once the base frame is set, start the propagation.
You can start propagation by:
- Pressing the spacebar - This triggers the propagation of the matte from the base frame to the new frame.
- You can also propagate frame-by-frame either forward or backward, if you would like to go slower and evaluate the propagation more closely. Use these keyboard shortcuts to move frame by frame:
- Page Up and Page Down
- Command + Left/Right arrows (macOS) or CTRL + Left/Right arrows (Win)
- Move the CTI (Current-time indicator) to a target frame, or use keyboard shortcuts I/O to move CTI to layer in/out point.
The main difference between pressing the spacebar and moving the CTI is that the space bar propagates, and then displays the results per frame. Moving the CTI waits for frames to propagate, then displays results.
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As Roto Brush propagates the matte from frame to frame, the selection may start to include unwanted regions or "lose" portions of the original selection. To correct this, paint a background stroke (holding Alt/Option) on the first frame where the unwanted change occurs to remove unwanted regions or paint a new foreground stroke to include anything that was missed. As with the base frame, avoid painting across the edge of the selection as much as possible.
Note: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.)
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Freeze the propagation.
After propagating the matte through all the video frames, freeze the propagation using the Freeze button at the bottom of the Layer panel.
Freeze the propagation. After freezing, you can continue to make adjustments to the matte without re-propagating. If you need to add or remove from a selection after freezing, you can unfreeze the propagation with the same button.
Note:You should only freeze once you are satisfied with the Roto Brush matte. What freezing does is that it locks the matte in place so that Roto Brush does not have to re-propagate the edges.
Freezing (caching, locking, and saving) Roto Brush segmentation
If After Effects has already calculated Roto Brush segmentation information for a frame when you select 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 1 span view in the Layer panel (and purple bars in the Roto Brush 2 span view).
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, select 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.
Adjusting and Refining the final matte
- The result of the propagation can be further refined and improved via the controls in the Roto Brush Matte group in the Roto Brush effect, especially with the Reduce Chatter property.
- Additionally, the Refine Edge Tool can be used for highly-detailed edges like hair and further controlled in the Refine Edge Matte property group. There are also options that compensate for motion blur and decontaminating edge colors. It is useful when dealing with mattes with soft edges or fine details like hair.
- Use the Refine Edge tool on other frames until the refinement is as precise and complete as possible. Press Alt/Option to erase Refine Edge strokes.
- If you've used the Refine Edge brush, the Fine-tune Refine Edge Matte option in the Roto Brush & Refine Edge effect properties is selected. Modify properties in the Refine Edge Matte property group as needed. (See Roto Brush effect and Refine Matte effect properties.)
Use the toggles at the bottom of the Layer panel to view the matte results, or open the Composition panel to view the result in-context with other layers.
Layer panel view options
You 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 a 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).
Why should I use Roto Brush 1?
- Older projects that used Roto Brush should still open with Roto Brush 1. If you open a project created in v17.1.4 or earlier, in v17.5, the version is automatically set to the older Roto Brush 1.
- If you are not getting the desired line segmentation results with Roto Brush 2, try enabling classic controls. Doing so creates a hybrid approach, using the best of Roto Brush 2 and Roto Brush 1. This is only available when Version is set to 2.0 and passes the propagation result from the new algorithm through the Classic one for finer control of propagation and more edge details. It turns the old Roto Brush options such as Motion Threshold, Edge Detection, and Motion Damping back on. In the version 1.0, these options are on by default, but in version 2, you need to enable them manually.
Workflow
The controls available to you in Roto Brush 1 by default can be activated in Roto Brush 2 by selecting the Enable Classic Controls option in the Preferences. The procedure to use it is largely the same as Roto Brush 2.
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) defaults to the entire duration of the layer, or to the duration of any empty durations if multiple spans are present. 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 (macOS) a span and choose Remove Span.
To delete all spans, delete the instance of the Roto Brush effect.
Propagation banner
In Roto Brush 2, there is a propagation banner at the bottom of the Layer panel which gives the status of the current propagating frame.
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 (macOS) 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).
The 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).
- Version: To select from the two Roto Brush versions - 1.0 (Classic) and 2.0.
- Quality: Select the quality of detailing around the edges. Standard is faster and doesn’t have quite as much detail around the edges. Best is refined around edges.
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- 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.
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.
When selected, it displays a green box around the subject. It indicates where the Roto Brush algorithm is searching for the subject. If the subject moves out of that frame, the Roto Brush effect is lost. You can increase the percentage to a higher number to cover the entire area. It is slower but more accurate and covers more area.
In Roto Brush 2, Propagation also has this checkbox. This is only available when Version is set to 2.0 and passes the propagation result from the new algorithm through the Classic one for finer control of propagation and more edge details. It turns the old Roto Brush options, such as Motion Threshold, Edge Detection, and Motion Damping back on. In the version 1.0, these options are on by default, but in version 2, you need to enable them manually. If you're not getting the desired line segmentation results with Standard or Best, try enabling classic controls. Doing so creates a hybrid approach, using the best of Roto Brush 2 and Roto Brush 1).
Motion Threshold and Motion Damping
(Available in Roto Brush 1 by default and when you enable classic controls in Roto Brush 2). 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.
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.
- Fine-tune Roto Brush Matte: Use this to refine edges, reduce chatter, shift edges, work on contrast, and feather the selection.
- Refine Edge Matte:
Increasing this value reduces the sharpness of the curves in the segmentation boundary by smoothing along the edge. Leave this number low when isolating an object with sharp features, such as hair.
Softness of the segmentation boundary. This property does nothing if Smooth is 0. In contrast to Smooth, Feather applies across the edge.
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).
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.
Tip: If the foreground object isn’t moving, but the matte edges are moving and changing, increase the value of the Reduce Chatter property. If the foreground object is moving, but the matte edge isn’t moving, decrease 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 the background color. The strength of this cleaning is determined by Amount (In Roto Brush 1, this is named as Decontamination Amount).
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.
(In Roto Brush 1, it is named 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.
(In Roto Brush 1, it is named View Decontamination Map): Shows which pixels will be cleaned by decontamination of edge colors (white pixels in the map).
When 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. Select 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.
Tips to work with Roto Brush
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. After Effects can then extrapolate from those regions to determine where the boundaries are.
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 it; 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 Roto Brush, because switching resolutions requires recalculation of the segmentation information. Fast Previews modes are turned off when you draw a Roto Brush stroke. (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 does not match the frame rate of the layer's source footage item. (See Frame rate.)
Points to note
- Choose your starting frame wisely. The more precise and stable the frame is, the better the propagation is.
- The Roto brush selection can only be made in the Layer panel, so make sure you have your footage open there.
- Older projects that used Roto Brush should still open with Roto Brush 1. If you open a project created in versions before v17.5 that used the older Roto Brush & Refine Edge effect, the version is set to 1. So, old projects render the same even when opened in the new version.
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