Alpha Divide Node

Alpha Divide does one thing only: it divides R, G and B by the alpha channel. A pure math operation, no parameters.

Its sole purpose: allow a clean color correction on premultiplied content (CGI, PNG/EXR with alpha). Without it, semi-transparent edges get wrong colors after grading (visible halos around the edges of the key).

Always paired with Alpha Multiply, which puts the image back in premultiplied form after grading.

• Effects Library: Tools > Matte > Alpha Divide
• Quick add: right-click in the Node Editor → Add Tool > Matte > Alpha Divide

Premultiplied

The RGB channels already have the alpha built in. A white pixel (1,1,1) with an alpha of 0.5 is stored as RGB = (0.5, 0.5, 0.5), alpha = 0.5. The format of 3D renders and typical PNG/EXR files.

Straight (unpremultiplied)

The RGB channels stay at their full value regardless of the alpha. A white pixel with an alpha of 0.5 = RGB (1,1,1), alpha 0.5. A cleaner format for color grading.

The problem: applying a color correction to premultiplied content creates artifacts on semi-transparent edges (the areas where alpha is between 0 and 1). The fix: Alpha Divide before, Alpha Multiply after.

• Input (orange) - required. The premultiplied 2D image to convert.
• Effect Mask (blue) - optional. Limits the division to an area.

MediaIn (premultiplied CGI)
   ↓
Alpha Divide ← switch to straight
   ↓
Color Corrector / Curves / Brightness Contrast
   (safe grading)
   ↓
Alpha Multiply ← back to premultiplied
   ↓
Merge 2D (final compositing)

Both nodes must sit exactly at these positions. Reversing the order or forgetting one of them breaks the render.

Most Fusion color correction nodes (Brightness Contrast, Color Corrector, Color Curves…) have a Pre-Divide / Post-Multiply option in their Options tab.

Pre-Divide/Post-Multiply (built-in checkbox)

The node does the Alpha Divide before its processing and the Alpha Multiply after, internally. A single checkbox to tick. Enough if you have only one color correction node.

Alpha Divide / Multiply (separate nodes)

Needed when you have several processing nodes between the two. Each node works on straight alpha without having to tick its own Pre-Divide.

Rule: a single color correction node → the Pre-Divide checkbox. Several nodes in a chain → Alpha Divide + Alpha Multiply.

• Grading a premultiplied 3D render - before any chain of Color Corrector / Curves, add an Alpha Divide to avoid halos on the edges.
• Correcting a PNG/EXR with alpha - these formats usually come out premultiplied. Alpha Divide before processing.
• A chain of several corrections - rather than ticking Pre-Divide on every node, one Alpha Divide upstream + one Alpha Multiply downstream = cleaner.
• Debugging halos on a key - if you see white or dark halos around a keyed element after grading, check that the Alpha Divide/Multiply sandwich is in place.
• Compositing fine transparency - smoke, hair, motion blur with a gradual alpha: Alpha Divide preserves the integrity of these delicate edges.

• Always paired with Alpha Multiply - an Alpha Divide without an Alpha Multiply downstream leaves the image in straight alpha, which causes aberrations when compositing.
• No visible effect on its own - Alpha Divide alone changes the underlying RGB values but the visible result is almost identical to the premultiplied one. The difference shows up after grading.
• Pre-Divide keeps it simple for 1 node - if your chain is: MediaIn → 1 Color Corrector → Merge, just tick Pre-Divide/Post-Multiply on the Color Corrector. Fewer nodes in the graph.
• Division by 0 = artifact - pixels with alpha = 0 could in theory cause trouble, but Fusion handles the case by returning 0. No crash, just potentially odd pixels in fully transparent areas.
• Camera footage = not needed - a real filmed plate has no alpha channel, so no premultiplication. There is no point in an Alpha Divide/Multiply sandwich on a plate.