Hasta La Motion!

Another daily update.. This comp manual is still in progress, very few images are documented, so the final chapters which are published may stray slightly from what I’ve shown.

IV. Rotoscope Techniques

A compositor has many responsibilities for the successful completion of a shot.. Almost every greenscreen or live action plate that requires a background changed out needs roto. Often, roto is supplemental and created on the fly by the compositor. You may need to isolate sections of greenscreen to create a better matte. Many people liken rotoscoping to junior level work, when in reality, the techniques necessary to create an efficient matte come with years of experience and time. While some compositors are fortunate enough to have a roto artist help them with the completion of a composite, many don’t. What methods should you use?

I’ll describe articulate character or actor rotoscoping, which is the predominate form of roto that occurs in production. For roto, there are several different approaches and methods that are out there.. Some dictate the keyframing of roto every several frames. Others claim that starting with the beginning and end points of your shots and working inwards is the best way. Even another efficient way is to rotoscope one piece and track that in. Out of all these different methods, what’s the best way? The best way is the most efficient and quick way for the shot that accomplishes the necessary mask without floating or jittering. We refer to floating and jittering as a mask that has an edge that moves around, while the subject being rotoscoped is smoothly moves. This can happen by adding too many points in a rotoscope, or animating the keys too frequently. Only experience will tell you how many points to add, but I will try and clarify some of these methods.

Humans move in a fairly predictable way. They are not jittery, as muscles take time to contract and move limbs. You can take advantage of this by creating a roto keyframes at the extremes of their action. This allows for a smooth roto from extreme to extreme. If there are other changes inbetween them, you can add another keyframe. The big hint here is to not rotoscope an entire actor at one time. You’ll have way too many floating edges, and it will tedious to go and adjust them. Think of each limb as a separate roto piece. The arms, legs and body are separate. Maybe the hat or the cape of another actor is separate as well. By dividing your goal into specific tasks, you’ll be able to complete the roto much quicker. Sometimes your actor is standing fairly still, and there’s no need to rotoscope so many appendages. What do you do then? Depending on the camera movement, if there is any, you can rotoscope a frame of the actor and track that to their movement. You can also apply this same method to numerous roto pieces, track the hat, track the arms, track the shovel, and so forth. Once tracked to a point correctly, you can go back in and adjust the mask , if necessary, again taking into account the extremes of the actors action. Items that don’t move very often are best done with tracking roto, while organic things should be articulate and keyframed.

This brings us to inanimate objects. Boxes, books, wires, airplanes, ropes, and more. While they don’t exhibit the same motion as organic things, the rotoscoping for these are just as challenging. Luckily, there are even more tricks that you can apply when you encounter them! Logic dictates that simple geometric objects are easily rotoscope-able. While not far from the truth, it’s much more difficult that it seems. The problem stems from these objects having a very defined edge, which can be truly show how bad or good a rotoscope job is. A swinging pendulum, for example, is such an interesting rotoscope challenge. Or a boxcrate lying on the ground. These objects are best rotoscoped by using a combination of tracking and keyframing at extremes.

The pendulum example is one where you must rotoscope one extreme, and keyframe it through the motion of its arc. If your compositing package allows for offset rotation of its pivot point, you’re set. Just animation the pivot point of the pendulum and line it up at the extreme ends. The computer will do the rest. Unfortunately, many desktop compositing packages don’t have this offset pivot point capability, and doing so will just keyframe the points in the roto, and not the actual rotoshape. They only way around this is either set another keyframe in between your two extremes, or attach a rotate node after your rotoshape and keyframe that.

Our boxcrate on the ground is another problem entirely. Because it is made up of wooden pieces , you must cut out holes, which can become time consuming when trying to place the holes and figure out which one has been keyframed already. Instead of this, create rotoshapes for each individual brace of the crate and overlap them. While possibly tedious, this will make the edges much nicer, and you won’t suffer from jittery edge syndrome that occur when holes disappear and corners don’t line up.

Ropes and wires are another matter entirely, and may require a combination of all your talent to complete a convincing matte. Often it’s easier to key out wires and ropes than it is to rotoscope them. Unfortunately you may be stuck without a keyable solution, and roto is the only one. Unlike boxcrates and pendulums, rope and wires have a mind of their own. They are erratic and can change direction quickly, which makes keyframing at extremes almost impossible. What’s the best solution? The most unfortunate one, keyframing frame by frame. But like before, divide your goal into separate goals of individual wires or ropes, and life will be much easier. It will still take a long time though. You may be able to keyframe some extreme movements of the rope. Wires are slightly easier, as they’re usually taut. You can get away with creating six points on your rotoshape, maybe eight to ten, even. Two points for each end of the wire, and a set in the middle, to allow for sagging. The other points may be necessary if your wire has kinks. Again, things like ropes and wires are probably best keyed, if possible.

Creating rotoshapes with motion blur is another topic in itself. Some compositing packages allow you to create motion blur on the fly, requiring you to just create a hard edged matte around your actor or object. In this case, most of the work can be done for you. Other times it is necessary to feather your rotoshapes to encompass the actor. When creating these rotoshapes it’s often beneficial to view your composite as you work, so you can see how the motion blur is affecting your comp. Rotoscoping out of focus objects follows the same methodology as with motion blur. Using either soft feathers or blurs while viewing your comp will accomplish the rotoshape.

Another problem which you may encounter involves rotoscoping an extremely dark shot, which contains no discernible edge. In this case, review what’s called for in the completed shot. You will have to guess as to where the placement of the roto should be, and it is better to track a viewable point than it is to haphazardly keyframe points. An offshoot of this is rotoscoping an extreme bright or blown out shot, where your actors edge is bloomed over by sunlight. Again, decide what’s being placed behind the actor or object. It may be necessary to create a clean frame of the foreground without the bloom, which we’ll get into in the next chapter. For such shots, it’s easier to draw a simple curve through the bloomed out portion of the edge, following the curve of the object.