AlphaDivide

In premultiplied RGBA, color in semi-transparent pixels has already been multiplied by alpha. That is convenient for Merge and normal compositing, but it can be the wrong signal state for aggressive RGB operations such as grading, blur, glow or some third-party OFX processing.

AlphaDivide performs the unpremultiply step: conceptually, it restores RGB to the state before multiplication by alpha. Alpha remains the coverage carrier while the intermediate image becomes straight/non-premultiplied. After the required processing, AlphaMultiply normally returns the branch to a premultiplied state.

Blackmagic documents the standard pair explicitly: premultiplied image → AlphaDivide → color correction → AlphaMultiply. If there is only one color-correction node and it provides Pre-Divide/Post-Multiply, that checkbox can often replace the explicit pair.

AlphaDivide has no effect-specific Controls of its own. The captured Inspector shows only the common Settings page: Blend, Process RGBA, Effect Mask options, Object/Material ID filtering, GPU, Layers and Comments. Those fields are infrastructure controls, not artistic AlphaDivide parameters.

Input is a normal 2D image with premultiplied alpha. The optional Effect Mask is applied as coverage after the tool is processed. This matters because partial masking or partial Blend can leave neighboring regions in different premultiplication states; for a normal technical pair, processing the whole RGBA branch consistently is usually easier to reason about.

AlphaDivide is not a matte erode/grow, keyer or alpha generator. It does not repair the shape of the matte. Use MatteControl, AlphaMatteShrinkandGrow, keyers or mask tools when the coverage itself is wrong.

Pixels with alpha equal to 1 are effectively unchanged by the divide, so the practical difference is concentrated in semi-transparent edges, motion blur, antialiasing, hair, glow tails and other 0<alpha<1 regions.

Judge the operation at the edge, not the opaque center. A dark fringe after RGB correction often indicates incorrect premultiplication handling; a bright or colored fringe later in the graph can indicate that a straight-alpha branch was never returned through AlphaMultiply.

Keep the straight section short. Once the operations that truly require unpremultiplied RGB are complete, place AlphaMultiply before ordinary Merge/output work so the branch has an explicit and predictable state.

GPU, Layers, Object and Material settings belong to Fusion's common tool infrastructure. They can matter in specialized pipelines, but the core AlphaDivide contract remains simple: divide RGB by alpha before processing, then remultiply afterward.