MatteControl

MatteControl is not a keyer and it does not invent a matte from nothing. Its job begins once alpha or a helper matte already exists: it combines that data with an image, repairs transparency geometry and prepares the resulting RGBA for normal compositing. Diagnose it in both Alpha and RGB, then test the result over a real background.

The node has five physical inputs with distinct roles. Background is the primary image and the destination that receives the resulting alpha. Foreground is an optional donor image whose Red, Green, Blue or Alpha can be selected by Combine. Garbage Matte, Solid Matte and Effect Mask are three different mask semantics, not interchangeable mask sockets.

A fundamental workflow is embedding a prepared alpha into an RGB image. Feed the RGB image to Background, feed the image/matte carrying the desired alpha to Foreground, choose Combine Alpha, then enable Post Multiply Image when the downstream composite expects premultiplied RGBA. This is the workflow demonstrated in Blackmagic Design training.

Combine and Combine Op define the channel math. Combine selects the donor channel or service mode, while Combine Op determines whether that data is copied, added, subtracted, intersected or otherwise combined with the current alpha. Verify the donor first, the operation second and edge shaping third instead of changing all three at once.

Garbage Matte normally removes coverage that should never remain in the foreground: set edges, rigging, microphones or unusable screen areas. Solid Matte does the opposite by restoring or holding opaque coverage where the main key lost useful subject detail. Effect Mask replaces neither; it only localizes the completed MatteControl operation.

Once the alpha logic is correct, shape it with Filter/Blur, Contract/Expand, Gamma, Threshold Low/High, Restore Fringe and Invert Matte. These controls should refine a usable key, not compensate for a fundamentally incorrect extraction. Large changes can easily destroy hair, motion blur and translucent detail.

Post Multiply Image belongs to the RGB/alpha relationship. After alpha is copied or changed, transparent pixels may still contain RGB values that composite incorrectly. Post Multiply multiplies RGB by the finished alpha so conventional Merge math receives premultiplied RGBA; it does not improve the matte shape itself.

The Spill tab corrects edge color rather than matte geometry. Spill Color/Suppression remove green- or blue-screen contamination, Fringe Gamma/Size/Shape define the affected edge region, and the Red/Cyan, Green/Magenta and Blue/Yellow axes restore natural color. Judge despill only after alpha is stable.