Comp fundamentals

Merges, bounding boxes, channels, and premultiplication — the four things every comp problem turns out to be.

Merge

Merge takes B (the background) and A (the foreground) and combines them with one of twenty-three operations. B decides the format: an A of a different size composites pixel for pixel, because a Merge does not resample — inventing a resize there would be a Resize you did not ask for.

A2…A8 stack more foregrounds into one node, in order. mix dissolves the whole operation back toward plain B. bbox decides whether the result claims the union of both inputs, their intersection, or one of them.

Bounding boxes

Every image carries a bbox — the region it has pixels for. Outside it there are no pixels, uniformly. Blurs grow it, crops shrink it, transforms move it. When an edge disappears at the frame boundary, the bbox is usually the answer: widen it with AdjustBBox before the operation that needed the room.

Channels

Layers are named (rgba, depth, motion, matte, disint.dust…), and data passes are marked as such so no colour transform ever touches them. Route them with ChannelRoute, keep or drop them with FKeep and FRemove, and split a multilayer EXR into a working web with SplitAOVs. The conventions are listed in ref-layers.

Premultiply discipline

Colour operations belong on unpremultiplied pixels; compositing belongs on premultiplied ones. Grade an element with a soft alpha while it is still premultiplied and its edges go dark or bright — the fringe is the giveaway.

1. Unpremultiply before a Grade that has to respect the edge.
2. Grade.
3. Premultiply again before the Merge.

Nodes that must see straight alpha say so on their pages.