Animate and edit paint strokes

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Animate a paint stroke by sketching with Write On
Animate a paint stroke path

You animate a paint stroke by setting keyframes or expressions for its properties. After Effects animates paint stroke properties—even the Path property of a paint stroke—by interpolating values for all frames between keyframes.

By modifying and animating the Start and End properties of a paint stroke, you can control how much of a stroke is shown at any time. For example, by automatically animating the End property from 0% to 100% with the Write On setting, you can make a paint stroke appear to be drawn on over time.

As with all properties, you can link paint stroke properties to other properties using expressions. For example, you can make a paint stroke follow a moving element in your footage by tracking the moving element and then linking the Position property of the paint stroke to the Attach Point property of the tracker.

Scott Squires provides a pair of movies that show how to rotoscope, both painting and masking:

Animate a paint stroke by sketching with Write On

If you choose Write On from the Duration menu in the Paint panel, the End property is automatically animated to match the motion that you used to draw the stroke.

Note: After Effects also includes a Write-on effect.

Select a paint tool in the Tools panel.
In the Paint panel, choose Write On from the Duration menu.
Drag in the Layer panel to apply a paint stroke to the layer.
As you paint, your movements are recorded in real time and determine the rate at which the resulting stroke is drawn to the screen for output. Recording begins when you click within the layer in Layer panel. When you release the mouse button, the current time returns to the time at which you started painting; this behavior is so that you can record more paint strokes for animated playback starting from the same time.

Animate a paint stroke path

Select a paint tool in the Tools panel.
In the Paint panel, choose Single Frame, Constant, or Custom from the Duration menu.
In the Layer panel, drag to create a paint stroke.
Using the Selection tool, select the paint stroke.
To momentarily activate the Selection tool, press and hold V.
Press SS to show the selected paint stroke in the Timeline panel.
Click the triangle next to the paint stroke name to expand its list of properties.
Click the stopwatch for the Path property to create an initial Path keyframe.
Drag the current-time indicator to another time.
While the stroke is still selected, drag in the Layer panel using a paint tool to create a paint stroke. A second Path keyframe appears in the Timeline panel.
By creating a stroke while a stroke is selected, you replace the selected stroke, which is sometimes referred to as stroke targeting.

If you are not satisfied with the way that the path is interpolated, consider creating your path as a mask, using Smart Mask Interpolation to fine-tune the interpolation, and then copying the Mask Path property keyframes to the paint stroke Path property. (See Animate a mask path with Smart Mask Interpolation.)
After Effects interpolates a paint stroke (center) between two different shapes created with the same brush (left and right).