Rotoscoping is the process of manually altering film or video footage one frame at a time. The frames can be painted on arbitrarily to create custom animated effects like lightning or lightsabres, or traced to create realistic traditional style animation or to produce hold-out mattes for compositing elements in a scene and, more recently, to produce depth maps for stereo conversion.
The art of rotoscoping changed considerably with the introduction of digital tools such as Flame, Mocha, Silhouette, Digital Fusion, Nuke and Adobe After Effects. With a thorough knowledge of rotoscoping, digital artists can create better live-action or CG composites as well as amazing visual effects. Various rotoscoping techniques are covered below, including matte creation, effects painting, paint touch-up, digital cloning, stereoscopic conversion and motion tracking, as well as a brief history of the craft and summary of the tools.
Effects painting is generally used to quickly add new elements to a scene. Instead of creating elaborate particle effects in 3D simulation software like Maya, many effects can be done faster by a skilled artist using a paintbrush or airbrush in a paint application. Effects like lightning or light-sabres can be painted one frame at a time. More advanced roto tools offer auto-paint capabilities which allow you to record brush strokes and then play them back over a selected range of frames. Some roto applications also allow you to add jitter to the brushes, as well as add the ability to paint the stroke out over time.
Most paint work done in the rotoscoping process is used for touching up film or video footage. This includes removing wires and rigs, removing logos, dust busting, scratch removal, etc. In these circumstances, the roto tool must be able to provide temporal and spatial cloning. Spatial cloning is a type of cloning which takes pixels from one position of the frame, and paints the source onto another position on the frame. Photoshop’s rubber stamp tool is an example of spatial cloning. Temporal cloning allows you to paint pixels from one frame in a sequence to another frame.
Creating hold-out mattes, sometimes referred to as masks or alpha channels, is a major part of the compositing process. A matte is a grayscale clip which is used to stencil portions of the background footage. Anything in the black area will be obscured, and anything in the white area will show through. Any gray area in the matte will be semi-transparent. Roto artists are expected to cut precise mattes with consistent edges which will not chatter. If the matte is sloppy, the shot will look fake. The best compositor will produce unacceptable work if provided with poor mattes. Mattes can be created with three different techniques like extraction, rotosplining and painting. For most situations a combination of these three techniques will have to be used.
Motion tracking is a computer based process which analyzes a pixel or sub-pixel in a clip, and follows that pixel or sub-pixel to find the exact coordinates on each frame. There are two primary uses for motion tracking. The first is for stabilization, and the second is for match moving.
Roto has seen vast growth with the matching explosion in stereo productions. There are actually two types of stereo roto work: Stereo Workflow and Stereo Conversion. With stereo workflow, the material has been shot with two cameras and the task is to match as closely as possible roto in the right eye with roto in the left. Failure to match the rotos for certain operations can lead to viewer fatigue and actual motion stiffness.