Roto Tools and Techniques: Splines, Shapes, Keyframing and Tracking
Domain focus
This domain covers roto tools and techniques: splines, shapes, keyframing, and tracking. The exam tests Bezier versus B-spline behaviour, shape types, keyframe interpolation, and using tracking to reduce manual keys.
Core rules and concepts
- Bezier splines have control points with handles that set tangent direction and length. B-splines are smoother but less local; moving one point affects a wider region.
- Use the fewest points that hold the shape. Add points at corners or changing curvature, not uniformly around the whole silhouette.
- Keyframe the shape's position, shape, or individual points. Set interpolation deliberately: linear for constant motion, Bezier for eased motion, hold for static sections.
- Track a point or planar surface to stabilise the roto shape, then parent or constrain the shape to the track. This reduces keys and drift.
- Check the matte on a contrasting background and at 100% zoom; use onion skin or ghost frames for comparison.
Worked example
A walking actor needs a matte for a leg. Track a point on the hip; parent the leg shape to the track; keyframe the silhouette at contact and passing poses. Do not key every frame. Reasoning: the hip track carries the overall body movement, so only the leg's changing shape needs keys. If the track slips, the matte will drift even if the silhouette is well drawn.
Traps that cost marks
- Over-keying every frame, creating jitter and slow review.
- Using Bezier handles that overshoot, causing popping at corners.
- Tracking a point that leaves frame or crosses an occluder.
- Forgetting to set the correct interpolation after copying keys.
- Adding many points for accuracy and making the shape harder to edit.