Roto101 – How to separate what you want when you don’t have green screen
In the last post, we talked about when you have something/someone you want to keep but something you don’t want, you put a green/blue screen to cover what you don’t want. You have to keep what you want in the range of green/blue screen so the vfx artists can use some keying tools to get rid the green/blue screen and then replace it with something else. But what if for some reasons you didn’t use a green/blue screen when you shot and now you change your mind and want to replace something in the shot? Or what if the actor moved too big of a range and some part of him was out of the green/blue screen and you didn’t notice? This is where roto is needed.
Rotoscoping, in short roto, is a very time consuming work. Roto means drawing a mask frame by frame around the area you want to keep. If you have some experience with photoshop, this is similar to using the pen tool to draw a mask to keep what you want. But in photoshop, there is only one picture to deal with. A shot usually contains at least 30 frames and it could be up to hundreds or even thousands frames. So you have to adjust your mask every frame to follow the action.
| The arm of the actor is outside of green screen. In order to separate his arm with the background, roto is a necessary step here. This shot is 90 frames long and his arm is outside of green screen 40 frames. This means during those 40 frames, the vfx artist has to keep moving those lines and dots to match the arm movement.
|Just moving the lines and dots around to follow the object you want to keep still can be very difficult sometimes. Imaging you have to separate a woman from background and her hair is flying in all different directions. If the background is a very different color or brightness from the woman, you might be lucky and be able to do keying plus some roto. But if the background is very similar color or brightness with the woman’s hair, the situation will be tricky. An experienced artist will have to combined several different techniques and use several different tools plus a lot of time rotoing to get this done. And this means you might get a large bill by the end. Even so sometimes it is just simply can’t be done. The best way to avoid this situation is plan well before shooting.The other situation that need roto is to fix problems. Such as wire removal or cleaning some undesired stain. The artist has to draw a roto shape(mask) to follow those wire or stain and patch it up flawlessly. This kind of work happens in pretty much every film. We’ll talk about this more in other post. By now, you should have a idea of what roto means. Since roto has to be done frame by frame, it’s not too hard to understand why you should plan well ahead to avoid unnecessary roto. But sometimes, in order to get realistic lighting, the VFX supervisor will decide not to have a green screen and then apply roto in post production to separate a foreground person. In this case it might be more challenging to create a realistic CG background to replace the green screen rather than just doing roto on the foreground. The VFX supervisor will take in many factors like the amount of shots, and the abilities of the vfx team when deciding which way to go. Generally, loose hair, semi transparent objects (like glass, silk or lace) and other things with fine, detailed edges require a green screen to get a good alpha mat. A VFX supervisor or VFX consultant should advise the wardrobe and hair people to reduce these things.