Rotoscoping

Rotoscoping is another technique for generating a traveling matte. In practice, it’s known as being one of the most time- and labor-intensive methods of doing so. So why is it still in use?

Art

Sometimes rotoscoping is the only way a filmmaker can achieve a particular visual grammar. For years, animators used rotoscoping to capture specific types of real-world movement that couldn’t be effectively captured any other way. More recently, filmmakers have used rotoscoping as a live-action augmentation tool, producing images that take the audience beyond reality. Even the surreal.

Although having its roots in the arts, rotoscoping is used for more commonly for practical applications. Let’s look at some.

Practical Uses

Removing the Unwanted

Let’s say you have a shot with some unwanted elements in the frame. You could draw masks around the regions to remove if:

• The camera’s in a fixed position
• The unwanted items don’t move
• The onscreen talent never intersects with any of these yet-to-be-removed bits,

Chances are, none of these will be true.

To remove the unwanted, you’ll create a garbage matte. Garbage mattes hide the unwanted (i.e. garbage) in the frame. They’re meant to be quickly drawn over regions for removal, then move on. But what if the shape of the unwanted is complex? Or finely-detailed? What if it intersects with something you want to keep in the frame? This is where rotoscoping comes in.

Starting with a frame with the most detail, trace around the unwanted item(s) with a series of points to create a mask hiding the unwanted elements. Frame-by-frame, forwards and backwards in time, you’ll change the position of different points on the mask to match the shape of the unwanted piece. On playback, you’ll refine and adjust these points until the garbage is removed.

Restoring Portions of Shots

With shots intended for keying, you’ll frequently confront color spill on a subject. Also, someone may have worn clothing that matches the color of the background for removal. Once a chroma key’s applied, you’ll see unwanted image loss in the form of splotchy patterns or gaping holes. When key adjustment and refinement isn’t cutting it, how can you bring back these lost portions of your shot?

You’ll create a holdout matte. In most compositing environments, holdout matte creation involves duplicating the keyed shot as a new layer, removing the key from the top layer, creating a mask as a series of points around the item to restore, then changing the positions of the points on the mask as needed.

Like garbage mattes, the precision of a holdout matte’s shape will depend on the area to be restored. Additional color correction may be required if the source shot was chroma keyed.

Combined With Motion Tracking

Yes, rotoscoping is traditionally time- and labor-intensive. But with improvements in motion tracking software, rotoscoping can be less taxing.

Rotoscoping with motion tracking begins with a mask drawn with a series of points at one frame. Unlike manual rotoscoping, motion tracking software can determine changes in a mask’s shape over time. It does this by looking at the source footage in relation to the mask’s shape. Instead of manually changing mask point positions frame-by-frame, you can jump ahead or behind in chunks of time, change those point positions, and the software can determine the motion in between. Results can vary based on the complexity of an object’s shape and/or a shot’s visual fidelity. But, with motion tracking, rotoscoping effort can be significantly reduced.