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Resolve FX Key · Matte Utility

Alpha Matte Shrink and Grow

A dedicated matte-morphology effect for tightening, expanding and repairing an existing alpha. The panel is compact, but the logic is important: Operation chooses the transformation, Shape defines the kernel, Radius sets the strength, and Process decides whether you are modifying only the alpha or RGB together with it.

Shrink / GrowOpening / ClosingSquare / Circle / DiamondRGB / Alpha process

Mental model

It is alpha morphology, not key generation

Place Alpha Matte Shrink and Grow after the tool that already created a matte. The effect does not decide which colour should become transparent; instead, it reshapes the resulting mask. That is why it is ideal for closing tiny holes after a key, shaving a faint fringe, or slightly expanding a matte before edge blending.

How the matte is transformed

Morph Operation

Alpha Matte Shrink and Grow is not a keyer but a morphological post-process for an existing alpha. It takes the matte boundary and rebuilds it with the selected operation. That is why the effect is placed after the node or FX that already created the key, not instead of it.

  • Shrink contracts the white matte area and removes a light edge halo.
  • Grow expands the matte and brings back lost edges, hair or thin details.
  • Opening applies Shrink first and Grow second, cleaning small debris and isolated white islands.
  • Closing applies Grow first and Shrink second, sealing holes and cracks inside the subject. This is the active mode in the supplied screenshot.
Kernel shape and radius

Shape + Radius

Shape defines the geometry of the kernel Resolve uses to scan the alpha. Circle behaves most naturally on people and soft subjects, Square emphasises horizontal and vertical edges, and Diamond reacts more aggressively on diagonals. Radius sets the strength of the operation: zero changes almost nothing, while a larger radius increases the effect on edges and internal gaps in the matte.

  • Circle is the default choice for keys of people, fabrics, hair and organic forms.
  • Square is useful for UI graphics, monitors, screens, architecture and other rectilinear edges.
  • Diamond is a compromise shape with a stronger response to diagonals and sharp corners.
  • If Radius is too large, Shrink can eat thin details and Grow can create a heavy, bloated edge.

Output logic

Process determines whether you are engineering the matte or the image too

On real jobs, most confusion comes from judging the effect in the wrong process mode. If you want a clean compositing matte, stay in Alpha (RGB Unaffected), enable Preview Alpha and tune the operation there first. Only then evaluate whether RGB also needs to follow the alpha boundary.

What the effect writes to the output

Output · Process

The Process menu decides whether the effect modifies only the alpha or also the RGB channels. This distinction matters because some modes are intended for compositing workflows that pass a matte downstream, while others deliberately affect the image together with alpha.

  • RGB (Blank Alpha) outputs RGB while blanking the alpha. It is mostly a utility/debug mode and less common in everyday work.
  • Alpha (RGB Unaffected) alters only the alpha and leaves RGB untouched. This is the main mode for clean matte engineering.
  • RGB + Alpha applies the morphology to the picture and the alpha together. Use it when the visible edge needs to track the expanded or contracted matte.
Diagnostics and fine control

Preview Alpha + Advanced

Preview Alpha instantly flips the view into a matte preview, so you judge the state of the mask rather than the beauty image. Advanced Options are intentionally sparse: Manual Iteration Control is there for explicit control of the number of passes when the default behaviour is either insufficient or too aggressive.

  • Judge the effect from the black-and-white alpha rather than the RGB image so holes, grey contamination and ragged edges become obvious immediately.
  • If Closing makes the subject too thick, reduce Radius before you start compensating with other FX.
  • Opening is typically useful after noisy keys, Closing after hole-ridden keys, and Shrink/Grow for precise edge fitting.

Workflow

Typical node order: key → cleanup → matte morphology → composite

1. Build the matte

Start with 3D Keyer, Qualifier or another source of alpha. Alpha Matte Shrink and Grow is ineffective if the upstream matte does not exist or is fundamentally wrong.

2. Choose the operation

Use Opening for speckle removal, Closing for patching holes, and direct Shrink/Grow for tight edge fitting. Keep Radius as low as the shot allows.

3. Judge in alpha view

Enable Preview Alpha and work in Alpha (RGB Unaffected). This isolates the matte problem from the beauty image and prevents false decisions caused by colour and lighting.

4. Return to composite

Once the matte is stable, disable Preview Alpha, check the edge against the background and switch Process only if the visible RGB edge must follow the new alpha boundary.

FAQ

Practical questions

What does Alpha Matte Shrink and Grow do?

It performs morphological processing on an existing alpha, letting you shrink, grow, open or close the matte to clean noise, seal holes or fit the edge before compositing.

How is Opening different from Closing?

Opening = Shrink → Grow and is better at removing tiny white specks. Closing = Grow → Shrink and is better at sealing small black holes inside the subject.

When should I choose Alpha (RGB Unaffected)?

Almost always when you need to refine the matte without changing the picture itself. It is the safest mode for a node-based compositing pipeline.

Which Shape should I use by default?

Usually Circle. Square and Diamond are typically chosen only for specific edge geometry or specialised technical tasks.

Next steps

Move between key generation, matte repair and compositing

Previous FX

3D Keyer

Return to the main Resolve FX Key effect that usually creates the matte upstream.

Color page Key

Understand where alpha mixing, key output and key inversion live in the wider node workflow.

Fusion Grow

A related node-based approach to growing and repairing mattes in Fusion.

Resolve FX catalog

Resolve FX Key

The Key section now contains 3D Keyer and Alpha Matte Shrink and Grow.

Resolve FX