Roto strategies in FFI.
Based in part on the writings of Scott Steward, below is a guide to strategies for Roto.
Roto is increasingly being to extract foreground elements, yet little has been written about approaches to doing convincing rotos. Let’s start by assuming the roto is being done to extract an element and place it in a very different environment, since roto for just color correction is a far more forgiving problem.
There are a range of things that sell a successful comp based on a roto. We will not discuss matching lighting and camera angles or camera movements here but clearly these have a huge impact. In terms of a successful roto the primary thing you are aiming for is to match the shape of the foreground element, not bubble or chatter the edge and to match the blur or sharpness of the edge.
Strategies for how to do a good roto shape
Scott’s three golden rules of Roto:
1. think like an animator (keyframe)
2. Use as few points as possible, then try not to move them individually
3. Use multiple shapes.
and we add our own
4. keep points linked to the object.
1. Think like an animator.
The first thing you need to do is look for natural keyframes, such a points of deceleration/ acceleration or cycling action- in much the same way an animator does. As you are animating a shape to match the object the more closely your keyframes match the actual motion the better. Inferior strategies such as “animating the shape at every 10 th frame” or “first last and sub divide down” (i.e frame 0 , frame 100 , then frame 50 then frame 25 and 75 etc), all fail in comparision. As the computer will in between directly between any two keyframes – you don’t want the object to have any speed or direction changes between keyframes. The easiest way to do this is to work out those natural keyframes points yourself and match them.
2. Use as few points as possible, and then try not to move them individually.
The more points you have, the more chance you have of leaks – and the longer it will take to animate. If you no longer need points in a shape Delete them with only one click- and that point will just become inactive – but are not lost prior to that point. They can be re-added if needed using Add. A second click will delete the points altogether.
Use Drag multiple select to move groups of points at once.
If your points are created without click-able spline handles – because they are too short or too close to the main axis point itself, you can use hotkey “Q – Drag” to just expand the spline tension control.
3. Use multiple shapes.
Try when defining your initial rotos to not do everything with one shape – use separate shapes, especially for overlapping objects or moving people.
Sometimes parts of the foreground can be handled with a shape which is tracked, while other more dynamically changing parts can be then handled with secondary roto shapes. In FFI tracking points can be annoying as a keyframe is created for each and every frame, in comparison tracking the shape axis and then adjusting the shape is often a fast and very effective solution.
4. Map points to objects.
Do not just place points anywhere, pick points that area easy to follow, and keep them on the approximately same place on the background.
If this is not done, the roto shape may look great at any two adjacent keyframes but in-between the roto effective rotates or goes out of shape momentarily.
Additional tricks and tips.
As FFI does not have motion blur roto shapes it can be hard to roto motion blurred objects. To help it is possible to now use the advanced gradient shapes in the Keyer menu, for variable edge softness.
Tip:
if you get a point on the advanced gradient roto shape that pinches, hold hold the G hotkey and click, this will automatically adjust the shape to fix the problem.
Submitter: Mike Seymour