Understanding Rotoscoping - The Process Every VFX Artist Should Know
Rotoscoping is a vital process within the visual effects pipeline. Whether you need to take out elements from live-action footage or bring elements in, you need to have a strong understanding of this very important technique. This article will give you an understanding of rotoscoping and its use, as well as some helpful tips when rotoscoping for your next piece of footage.
In its more traditional use, rotoscoping meant tracing over live action footage frame by frame for use in animated films. That way the animator could get very realistic movements by tracing over the actor in the scene. You can kind of think of it as today's motion capture.
In visual effects, however, the process, although similar, has a different purpose. Rotoscoping for VFX is used to create a matte or mask for an element so it can be extracted out to place on a different background, masked out so colors can be changed or any other set of reasons. The rotoscoping artist (or roto artist for short) will trace an object using a set of tools within the compositing software to create a new alpha channel for a specific part of an image sequence or video.
Unlike computer generated imagery that can easily add an alpha channel to its images, footage taken directly from a camera has no alpha data so the roto artist will need to manually create that alpha by tracing over the elements within the video. A rotoscoping artist will need to create different shapes around an object and animate those shapes to match the movement on each frame.
Depending on the complexity of the shot, the process of rotoscoping can take hours or even days to complete. The use of blue and green screens can make the process of compositing different elements into a scene much easier, but not every shot can take advantage of blue or green screens, so rotoscoping still plays a large role in the production of visual effects. Because you can find rotoscoping used in just about every movie and television show that utilizes visual effects, it's no surprise that roto artists are a vital role within the VFX pipeline.
There are many different compositing applications that have the tools needed to begin rotoscoping. Rotoscoping is an art form in itself so mastering it will not be something done overnight, but the following tutorials will give you a strong foundation in creating good rotoscoping data: Your First Day of Rotoscoping in NUKE, Introduction to Rotoscoping in After Effects, and Introduction to Rotoscoping in Fusion.
Rotoscoping TipsKeep Control Points to a Minimum
When you're first beginning to learn rotoscoping, your first instinct might be to create as many points as possible in order to properly outline the element within the footage. However, it is better to use a minimal amount of points and only what is needed to trace the subject. If a matte's outline changes over several frames, it can give it a jittery look when played back, and it's much harder to keep track of all the points.
A good technique is to find the most complex shape that the subject is in, and outline that first so you know the most points you'll ever need for that rotoscope. The rotoscoping tools in your compositing software have very powerful curve manipulation abilities that are able to create complex curved lines with a minimal amount of points.
Create Separate Shapes
If the object or person is a very complicated shape, do not try and use one shape for the entire subject. Often times it is best to separate different limbs, fingers, etc., into different sub-shapes. For instance, parts that are going to be moved independently of each other should be separated, like the hand, the fingers, the forearm, and the arm. If you were trying to follow the outline of a complex movement, two characters fighting for example, it would be nearly impossible to follow the actions accurately with one single outline.
Use as Few Keyframes as Possible
When you are animating the rotoscope to follow along with the movement of the element in the footage, you typically don't need to create a new keyframe for every frame. The computer will automatically interpolate between keyframes and will generally produce the desired results.
The same way that an animator would first draw in the key poses, you can do the same thing when rotoscoping. Find the frames where the most movement occurs, and adjust the shapes accordingly. If the outline is not following perfectly with the subject you can then go in and add another keyframe to make that adjustment.
Study the Footage
Before you start rotoscoping, you should study the footage you will be working with. Are there large changes in direction? Do parts of the subject get obscured by other elements within the scene? Are there big camera shakes that could be stabilized to make the rotoscoping process easier for you? By knowing the footage, you'll be able to better determine the timeframe for the project, as well as the best way to approach the rotoscoping.
Consider Animation Principles
Rotoscoping is a lot like animation; you are creating keyframes and animating the outline of the subject within the footage. The same way that an animator would think about arcs, ease in and ease out, so should you. After all, the animation principles were based off of real-world studies and you'll see these principles in action while tracing your footage.
By thinking about these things while you are rotoscoping, it will help you place the keyframes in the right spot and figure out early on where the ease ins and ease outs would occur in the footage, and how your keyframes should be placed to follow with the subject.
Rotoscoping may seem tedious and not that exciting, but it is important for any VFX artist to know, even if you are more of an effects person who likes to create the explosions, you should still know this technique and the importance of it within the pipeline. Next time you're tasked with rotoscoping, try implementing some of these tips to help with the process.
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