What compositing is
Compositing is the art of combining multiple sources — live footage, green screen, CG renders, matte paintings — into one believable image. In After Effects, every layer is a potential element of a composite.
Keylight 1.2
For green screen, apply Effect → Keying → Keylight. Eyedrop the green backdrop into Screen Colour. Set Screen Gain to ~110 and Screen Balance ~0.5 for green, 0.95 for blue. View → Status to spot edge issues.
Spill suppression and edges
After Keylight, add the Advanced Spill Suppressor to neutralise green tinge on hair and skin. Refine Soft Matte adds 1–2 px feather; Choke pulls the edge in slightly so background doesn't seep through.
Rotoscoping with Roto Brush 2
Window → Effect Controls → Roto Brush 2 (AI-powered since 2020). Paint over the subject in the first frame; After Effects propagates the mask forward. Refine frame-by-frame with the brush set to subtract for hair and fingertips.
Motion tracking
Animation → Track Motion. Pick a high-contrast feature, click Analyze Forward. Apply the data to a null, parent your insert layer to the null — your 2D element now rides locked to the plate.
3D camera tracker
Effect → Perspective → 3D Camera Tracker. After Effects solves the camera move from the footage. Right-click a track point cluster → Create Solid and Camera. Now you can paste 3D text into the scene and it sticks to the floor.
Color match the composite
Add a Lumetri Color layer on top of the inserted element. Sample the plate's blacks and whites with the White Balance tool. Apply grain (Effect → Noise & Grain → Match Grain) so the CG element shares the same texture as the camera footage.
Render passes
Send the comp to Media Encoder as ProRes 422 HQ for editorial, OpenEXR multichannel for VFX delivery. Keep separate render passes (beauty, alpha, depth) so colorists downstream can tweak without re-rendering.