Tuesday, January 17, 2012

VFX Back to Basics Series: 1. What is rotoscoping?

This is part of a series on the basic elements of Visual Effects. Each post will talk about a certain element which is one of the basic bricks used for building VFX shots.

In this first post of the series I will talk about rotoscoping.

I am sure that most of the artists out there have dealt with some form of roto work in their career. I usually have to do it when the shooting crew didn't put in the time to set up a proper green screen or when the client wants some modifications on the live action plates which weren't planned for. Removing and replacing commercials signs while people walk in front of them are a classic. The whole point of rotoscoping is to create a cut out image so the image can be split up in several layers.

Although rotoscoping is used differently today it is one of the oldest tricks of the trade. The technique was originally used by classic animators in which they traced over projected live action film to capture the motion of the actors or animals to produce life like animations. It was invented by Max Fleischer in 1915 and has been used in famous features like Disney's Snow White and the Seven Dwarfs and the Beatles animated film Yellow Submarine.

Later on the technique was used  to create mattes to cut out bits out of live action movies. The mattes  were used to create layers during the compositing stage, which was still done with an optical printer, so they could place elements behind the rotoscoped elements or they could add effects which only affected the rotoscoped elements themselves. The famous lightsabers of the Star Wars series is a perfect example of this latter technique. These mattes were still hand painted frame by frame. It did cost the artist quite some time to finish a shot and it was prone to chatter. Chatter can be perceived when de movement of the matte is slightly erratic and wobbles over the edges.

Nowadays we have computers to help us out. We usually start by placing roto curves at specific keyframes and the software will create the curve for the in between frames. The in between frames can then manually be adjusted to create accurate mattes. Quite a bit faster than doing it frame by frame and it reduces possible chatter. Another advantage is that you can feather the mattes to create softer edges and that will make it easier to create mattes for motion blurred or out of focus elements. Roto work can also be combined with tracking. This technique works extremely well when the rotoscoped elements are non deformable and it speeds up the process considerably.

Here is a small example to illustrate one way to use rotoscoping.

original image

A roto shape around the target

Matted out area

Rotoscoping is usually seen as a junior position before people move onto compositing jobs. Although it looks simple enough and not a huge amount of training is needed it can be tricky to create perfect mattes. The big companies have specific roto departments while the smaller ones have their compositors taking care of the work. Most compositing packages like Nuke, Fusion and After Effects have built in roto tools but there is also dedicated software like Silhouette FX and Mocha to take care of the job.

This concludes the first part of the VFX Back to Basics series.
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