AlphaMultiply

Premultiplied RGBA stores semitransparent color already multiplied by alpha. That is the normal representation expected by conventional Merge compositing because edge RGB is attenuated in proportion to coverage.

AlphaMultiply is the closing operation: RGB is multiplied by the existing alpha again while alpha remains the coverage carrier. Conceptually it is the inverse boundary of AlphaDivide.

Blackmagic documents the explicit AlphaDivide → processing → AlphaMultiply pair for operations that need straight/unpremultiplied RGB in between. After Multiply, the branch is ready for ordinary premultiplied compositing again.

The most common mistake is applying AlphaMultiply to an image that is already premultiplied. Semitransparent RGB is then multiplied by alpha twice and the edge becomes too dark: this is double premultiplication, not matte improvement.

The second common mistake is placing AlphaMultiply too early. If Blur, Glow, ColorCorrector or an OFX still needs straight RGB, Multiply belongs after the last such operation.

AlphaMultiply has no effect-specific Controls. The captured Inspector shows common Settings: Blend, Process RGBA, Effect Mask options, Object/Material filtering, GPU, Layers and Comments. Those controls must not be presented as dedicated artistic parameters of the node.

Blend is Fusion's common dry/wet control, not a mathematical premultiply-strength slider. Intermediate Blend values mix straight and premultiplied representations and therefore create a mixed technical state that is harder to reason about downstream.

The optional Effect Mask also gates contribution after processing rather than replacing the source alpha. A locally masked Multiply can leave one area straight and another premultiplied, which should be treated as an intentional special case.

At alpha=1, RGB changes very little. Real diagnosis lives in 0<alpha<1 regions: antialiasing, hair, motion blur, keyed edges, soft masks and optical tails.

At alpha=0, premultiplication suppresses RGB to zero. If an upstream effect intentionally creates visible RGB where alpha remains near zero, such as a broad Glow, AlphaMultiply can correctly reduce that tail; decide the intended compositing semantics instead of treating Blend as a repair control.

For a single color tool that offers Pre-Divide/Post-Multiply, an explicit pair may be unnecessary. For a multi-node straight-RGB section, AlphaDivide and AlphaMultiply make the boundaries visible and reviewable.

Proper QC means compositing over black, white and a saturated background, checking RGB and Alpha separately, and bypassing/enabling Divide+Multiply as a technical pair rather than judging only one endpoint in isolation.