Rotoscoping in Blender: drawing and masking over a video

Rotoscoping is one of the oldest techniques in animation, and one of the most widely used in visual effects. The principle never changes: start from live-action footage and work over it, frame after frame. This guide explains where the technique comes from and its two uses, then how to put them into practice in Blender, which brings drawing, masking and compositing together in a single application.

Where rotoscoping comes from

The word comes from the rotoscope, a device designed by the animator Max Fleischer, who filed the patent in 1915 and was granted it in 1917. The principle: project a film frame by frame onto a glass panel, so that the animator can redraw the filmed figure on paper. Fleischer used it for his Out of the Inkwell series, in which his clown character moved with a fluidity that was unusual at the time. The technique then spread through the whole of hand-drawn animation, and later through digital filmmaking, where the same word now refers to frame-by-frame masking.

Two uses: animating and cutting out

Rotoscoping for animation means drawing a character or an object by following filmed movement: a dance, a gesture, a walk. It gives realistic proportions and timing. Animators do not copy everything: they simplify the poses and exaggerate what needs to read.

Rotoscoping for compositing isolates a subject in a video: you trace a mask around it that follows its outline on every frame, to separate it from the background, grade it separately, or insert a 3D element behind it. It is routine work in post-production, when no green screen was shot or when it does not cover the whole shot.

Rotoscoping for animation with Grease Pencil

Grease Pencil is Blender's 2D drawing tool: it draws directly in 3D space and can be animated frame by frame. The method:

1. Prepare the video. Set the scene's resolution and frame rate to match the video (Output Properties), and the frame range to its length.
2. Show the video behind the camera. In the camera properties, open Background Images and add the video, either as an Image source (a single image, a sequence or a video file) or as a Movie Clip. Opacity sets its transparency, and Depth places it behind or in front of the objects. Switch to camera view (Numpad 0).
3. Add a Grease Pencil object facing the camera, then switch to Draw Mode. The 2D Animation workspace gathers the drawing tools and the Timeline.
4. Draw frame by frame. With Auto Keying turned on, a stroke drawn on a frame that has no drawing yet creates a new drawing. Move forward one or two frames, redraw, and so on.
5. Turn on Onion Skinning (in the Grease Pencil object properties) to see the drawings of the neighbouring frames through transparency and keep your strokes consistent.
6. Hide the video in the render by keeping it as a background on the camera only, or keep it in the image instead if the drawing should appear over the footage.

Drawing on twos, that is one drawing every two frames, is the classic rate of hand-drawn animation: it halves the work and the movement keeps a hand-drawn feel. The Grease Pencil in Blender guide covers brushes, layers and stroke materials, and the guide to animation in Blender goes over the principles of timing.

Rotoscoping for compositing: masks

To cut out a filmed subject, Blender offers masks. You create them in the Movie Clip Editor or the Image Editor by switching the editor to Mask mode in its header: the mask tools then replace the usual tools.

1. Load the video in the Movie Clip Editor and create a new mask.
2. Trace the outline of the subject with points joined by curves. A mask can contain several layers and several shapes, for example one for the body and one for an arm that passes in front.
3. Animate the mask. Move the points on the frames where the subject changes shape: Blender interpolates between these positions using the mask's shape keys.
4. Make the mask follow automatically. Rather than moving every point by hand, track a detail of the subject with motion tracking, then parent the mask points to the tracking markers: the mask then follows the movement, and all that is left is to correct it.
5. Use the mask in the compositor with the Mask node, which turns it into a black and white image to isolate the subject, grade it separately or place a 3D element behind it.

Masks are independent of the video: the same mask can be used in several places, and also serves as a rough matte to help a green screen key, or as an animated shape in motion design.

Preparing a rotoscoping job

• Choose sharp footage. Fast movement filmed with heavy motion blur makes outlines hard to follow. A high shutter speed during the shoot helps a lot.
• Work on an image sequence rather than a heavily compressed video file: each frame displays faster and without artefacts. The guide on choosing between H.264 and H.265 explains why video compression is not suited to frame-by-frame work.
• Start with the key poses. Draw or place the mask first on the frames where the movement changes direction, then fill in between them.
• Leave a margin on masks and soften their edges, because an outline that is too tight lets a fringe of the background show when the subject moves.

Rotoscoping and 3D animation

The same idea works in 3D: a reference video placed as the camera background helps you match the poses of an animated character, keyframe by keyframe. For a complete character movement, motion capture or an animation library such as Mixamo (in French) often replaces rotoscoping, while frame-by-frame drawing remains the route for 2D styles and cartoon-like renders such as cel shading.