Fundamentals of Visual Effects, Compositing and Roto Workflow
Domain focus
This domain introduces visual effects, compositing, and the roto workflow. The exam tests the purpose of a matte, the order of roto operations, alpha channels, and how roto output feeds compositing.
Core rules and concepts
- VFX combines live-action footage with computer-generated elements. Compositing layers images using alpha channels, masks, and blend modes.
- Roto creates a matte, usually a black-and-white image, where white is the area to keep and black is the area to remove. It is used for colour correction, paint, screen replacement, and layer separation.
- A standard roto workflow: ingest and check colour space, identify the frame range, analyse motion, draw simple shapes, keyframe the silhouette, refine edges, feather, and export with the correct naming.
- Premultiplied alpha stores colour already multiplied by alpha; unpremultiplied alpha keeps colour separate. Mixing them causes dark or bright fringes.
- Motion blur and grain should be considered at the roto stage if the matte will be used for a final composite.
Worked example
An actor's arm crosses a window and needs colour correction. Draw a shape around the arm on the first frame, keyframe at the start and end of the movement, then check every frame for drift at the elbow. Feather about 1-2 pixels and export a 16-bit matte. Reasoning: the arm crosses a high-contrast background, so a soft edge prevents a visible cut-out; checking every frame catches changes in silhouette.
Traps that cost marks
- Drawing too many control points, which makes the shape hard to animate.
- Assuming one keyframe per second is enough for fast motion.
- Forgetting premultiplied alpha when importing into the compositor.
- Exporting a matte without checking the frame range.
- Treating roto as a one-time task rather than an iterative review process.