RESOLVE FX · KEY
Alpha Matte
Shrink and Grow.
Repair geometry.
Do not rebuild the key by accident.
This FX is a compact morphology workstation for an alpha that already exists. The reliable order is defect → operation → shape → minimum Radius → output/QC. Opening and Closing are not generic “cleanup” buttons, and Manual Iteration Control is not a quality switch.
Existing alpha
4 operations · 3 shapes
Preview → Composite
The lab uses independent standard morphology to make the consequences of operator size and repetition visible. It is deliberately not a clone of Resolve’s private implementation.
Advanced Options · Manual Iteration Control
The teaching model treats Operator Radius as a standard morphology-kernel radius and Repeat Operation as literal repeated passes. Resolve’s internal scale and implementation are not claimed or reproduced.
Independent teaching model. It uses standard min/max morphology to make Shrink, Grow, Opening, Closing, kernel Shape and repeated passes observable. It does not reproduce Resolve’s proprietary pixel math, parameter scale, edge handling or RGB processing.
Four actual Inspector captures + an independent matte-morphology lab.
Shrink · Grow · Opening · Closing01MORPHOLOGY, NOT KEY GENERATION
Three decisions keep this tiny panel predictable.
Keep the signal roles separate: upstream tools decide what the subject is; this FX reshapes alpha geometry; the composite decides whether the repaired edge actually works.
It processes an existing matte; it does not generate the key
The effect starts after alpha already exists from 3D Keyer, HSL/Luma Keyer, a Qualifier, Window, Magic Mask, roto or an external matte. If the upstream selection is conceptually wrong, morphology cannot rediscover the subject; it only reshapes the signal it receives. Fix the matte source first, then use Alpha Matte Shrink and Grow for local geometric refinement.
Choose the operation from the defect, not by taste
A fringe around the white region suggests Shrink; a missing boundary suggests Grow; isolated white specks suggest Opening; small black holes and breaks suggest Closing. If one matte simultaneously has a large leak, holes and missing hair, that is a signal to return to the key/roto or split the job across nodes—not to push Radius harder.
The minimum sufficient Radius preserves detail
Morphology benefits from small corrections. The harder the operator pushes through the matte, the faster it destroys hair, motion blur, translucent edges, thin straps and holes that should remain. Manual Iteration Control is useful only after the normal Radius has proven insufficient; repeating operations increases the intervention, not quality by itself.
Stable behavior cross-check: Blackmagic Design · Resolve FX Key ↗
02THE ACTUAL RESOLVE 21 PANEL
Four captures cover the visible panel; advanced controls are explained as conditional controls.
The screenshots are interface evidence, not presets. Closing, Circle and Radius 0.000 simply record one captured state.
01 · DEFECT FIRST
Morph Operation
- Morph Operation
Chooses Shrink, Grow, Opening or Closing. Shrink contracts the white/opaque region; Grow expands it. Opening is a sequence useful for removing small foreground specks or protrusions; Closing is useful for sealing small background holes and breaks inside the matte.
CheckName the defect in Preview Alpha first. If you cannot explain why Opening or Closing is the right operation, the operation was probably chosen too early.
02 · OPERATOR GEOMETRY
Shape + Radius
- Shape
Square, Circle and Diamond define the morphology-operator geometry and therefore its behavior around corners and angles. Circle is the default and gives an even organic expansion; Square tends toward rectangular geometry; Diamond behaves differently on diagonals and sharp corners.
CheckAt a small Radius, compare Shape on a diagonal or corner. If the silhouette becomes visibly rectangular or diamond-like, reduce Radius before trying to compensate with another node.
- Radius
The primary scale for the automatic Morph Operation. Higher values push Shrink/Grow/Open/Close farther through the matte. It is not a blur radius: topology and boundaries are changed morphologically, so thin structures can disappear abruptly.
CheckRaise Radius from zero only until the target defect disappears. Then immediately inspect hair, motion blur, fingers, straps and legitimate holes in the subject.
03 · WHAT SIGNAL LEAVES
Output · Process + Preview Alpha
- Process
Determines which channels the Morph Operation is applied to and what the effect returns downstream: RGB (Blank Alpha), Alpha (RGB Unaffected), or RGB + Alpha. For ordinary matte engineering, Alpha (RGB Unaffected) is usually the safest choice because alpha geometry changes while RGB remains untouched.
CheckIf the effect appears to have “broken the image,” check Process first. RGB (Blank Alpha) intentionally does not output a working alpha; RGB + Alpha changes more than the matte alone.
- Preview Alpha
A diagnostic view of the output alpha. It removes the distraction of the beauty image and makes holes, speckle, edge thickness and thin-detail loss immediately visible. Preview does not replace the final composite check.
CheckTune morphology in alpha view, then turn Preview Alpha off and inspect the edge over real bright and dark backgrounds in motion.
04 · ONLY IF NORMAL RADIUS IS NOT ENOUGH
Advanced Options · Manual Iteration Control
- Manual Iteration Control
Enables direct control over how the morphology operators are applied. Once enabled, the conditional controls Operator Radius and Repeat Operation become relevant. This is an advanced branch, not a “higher quality mode.”
CheckDo not enable manual iteration automatically. First prove in Preview Alpha that the normal Radius cannot solve the narrow problem without unacceptable collateral damage.
- Operator Radius
A conditional slider Blackmagic describes as controlling the size of the operator when Manual Iteration Control is enabled. The teaching model represents it as an independent kernel radius, but does not claim Resolve’s internal scale.
CheckA small Operator Radius repeated several times can differ visually from one larger pass around corners and thin details. Do not assume equivalence without an A/B on the actual matte.
- Repeat Operation
A conditional control that repeats the operator, increasing the effect of Operator Radius. Repeats can change the topology of fine detail very quickly, so add them incrementally and return to QC after each change.
CheckIf a new repeat removes legitimate fine detail, step back: the morphology has already exceeded the safe correction even if the large silhouette looks “cleaner.”
03ONE DEFECT → ONE OPERATION
Shrink, Grow, Opening and Closing change different geometry.
Opening and Closing are especially easy to misuse because the overall silhouette can look similar while small topology changes dramatically.
Shrink
EDGE / WHITE REGION ↓Remove a thin white fringe or tighten an overly wide matte slightly.
Hair, motion blur and thin details disappear first.
Grow
EDGE / WHITE REGION ↑Restore a slightly missing boundary or expand the matte before further edge work.
The edge becomes heavy and background can be captured.
Opening
SHRINK → GROWRemove small white specks and thin protrusions outside the main shape.
An oversized operator removes legitimate small foreground elements.
Closing
GROW → SHRINKSeal small black holes and narrow breaks inside the white matte.
Can close openings that should legitimately remain transparent.
A clean-looking white silhouette is not the goal by itself. A correct matte can contain gray semi-transparency for hair, glass, motion blur and defocus; morphology should preserve those structures when the shot needs them.
04MINIMUM-SUFFICIENT WORKFLOW
Repair the named defect, then stop.
The graph has two views: the normal repair path and the recovery path when morphology starts damaging legitimate detail.
Name the geometric defect
Fringe, missing edge, white speck or black hole lead to different operations. Classify the defect before touching Radius.
Teaching workflow, not a literal Resolve node graph. The central rule is minimum sufficient morphology: classify the defect in alpha, repair only that geometry, then return to the real composite and motion QC.
Useful upstream/downstream references: 3D Keyer ↗ · Qualifier / Windows / Tracker ↗
05REPEATABLE MATTE TASKS
Four exercises with an acceptance check.
Each recipe starts with a defect visible in alpha and ends with a composite/motion check, so the operation does not drift into blind cleanup.
Tighten a light fringe after a key
Move the matte inward just enough to remove background fringe without eating hair and motion blur.
PrepareThe upstream key is already stable. Set Process to Alpha (RGB Unaffected) and enable Preview Alpha.
- 01
Choose Shrink and start with Circle.
- 02
Raise Radius in the smallest useful steps until the target fringe disappears.
- 03
Immediately inspect hair, motion blur and fingers elsewhere in the frame.
- 04
Turn Preview Alpha off and compare the edge over bright and dark replacement backgrounds.
The fringe is gone without visibly thinning the silhouette, and semi-transparent detail survives in motion.
Remove isolated white specks
Clean small foreground noise outside the subject without aggressively shrinking the entire silhouette.
PrepareSmall white dots/islands are visible in the B/W matte while the main boundary is already correct.
- 01
Choose Opening and Circle.
- 02
Keep Radius minimal: the operator only needs to exceed the specks, not the subject structure.
- 03
If earrings, hair or straps disappear with the specks, reduce Radius and repair the upstream key locally.
- 04
Check several frames before and after; temporal chatter may create new specks.
Noise disappears while legitimate thin foreground elements remain stable.
Seal small black holes
Restore foreground density in small holes without filling legitimate transparent openings.
PreparePreview Alpha shows local black dots or narrow cracks inside the subject.
- 01
Choose Closing and Circle.
- 02
Raise Radius only until the defective holes close.
- 03
Inspect gaps between fingers, hair, fabric mesh and other legitimate openings.
- 04
If repairs require different scales, split the tasks across nodes rather than using one large Radius.
Defective holes are sealed without flooding legitimate internal geometry.
When Manual Iteration Control is justified
Use controlled repeated operators when one automatic Radius is insufficient, while measuring collateral damage.
PrepareThe normal Radius has already been tested and operation/shape are correct, but a specific structure needs a stronger result.
- 01
Enable Manual Iteration Control only after keeping an A/B of the base version.
- 02
Start with a small Operator Radius and one Repeat Operation.
- 03
Add repeats one at a time, checking thin detail and silhouette error after each.
- 04
If a strong operator is needed only locally, constrain the upstream matte/window instead of strengthening the effect globally.
The extra repeat solves the stated defect while detail loss remains acceptable and intentional.
06MORPHOLOGY IS DOWNSTREAM OF SELECTION
Use a keyer to decide what belongs; use this FX to reshape what was already selected.
Choosing the simpler correct tool keeps the graph auditable and prevents a morphology node from hiding a broken key.
| Tool | What it solves | Use it when |
|---|---|---|
| Alpha Matte Shrink and Grow | Geometry/topology of an existing alpha | The selection is already correct but needs a small shrink, grow, opening or closing adjustment |
| 3D Keyer ↗ | Generates a color key from strokes and includes its own matte cleanup/despill. | When the problem is separating screen from foreground by color, not merely reshaping an existing alpha. |
| HSL Keyer ↗ | Builds a key from Hue/Saturation/Luma ranges. | When you need an explicit component-based color qualifier with controllable H/S/L ranges. |
| Luma Keyer ↗ | Builds alpha primarily from luminance. | When the subject separates by brightness and color sampling would be unnecessary. |
07WHEN THE MATTE GETS WORSE
Collateral damage is diagnostic information.
If hair disappears, gaps close or playback chatters, the operation has exceeded the safe scale or the real cause lives upstream.
01Shrink removes fringe but hair disappears
Radius is already too large for the finest legitimate detail. Reduce it until hair survives, then fix the residual fringe through the upstream key, a localized matte or a separate edge branch. Do not compensate with a later Grow; it cannot restore the original transparency structure.
02Closing makes the subject look bloated
Closing expands before contracting, so an oversized operator can alter corners and narrow gaps even when the overall area looks similar. Reduce Radius/Operator Radius and inspect the outer silhouette as well as the hole.
03Opening removes wanted small features
The operator has become comparable in size to earrings, straps, hair clusters or fine graphic detail. Reduce Radius, change Shape only when corner geometry is the issue, or clean the specks upstream/with a local mask.
04The effect seems not to change alpha
Check Process: RGB (Blank Alpha) does not output a working alpha. Then enable Preview Alpha, confirm that an input matte actually exists, and verify that Radius/Operator Radius is not zero.
05Edge color looks odd after RGB + Alpha
You selected a mode that affects more than alpha geometry. For normal matte engineering, return to Alpha (RGB Unaffected), stabilize the matte independently, then solve edge color/despill with dedicated tools.
06Several repeats improve a still but the matte chatters in playback
Repeated morphology amplifies frame-to-frame variation in the source matte. Reduce repeats and find the upstream temporal instability—tracking, key sampling, compression/noise or animated roto. Final QC for this FX must include motion.
Check against two contrasting backgrounds
A dark background reveals bright fringe; a bright background reveals dark contamination and over-shrink. One convenient replacement can hide alpha defects.
Inspect at 100% and in playback
Morphology can look perfect fit-to-screen while damaging hair and motion blur at pixel level. After still QC, play the problem section.
Do not treat spill with alpha geometry
Green/blue contamination in foreground RGB is a separate despill/color-correction problem. Shrink can hide spill only by sacrificing real edge detail.
08FAQ
Practical Alpha Matte Shrink and Grow questions.
What does Alpha Matte Shrink and Grow do?
It morphologically reshapes an existing alpha/key matte: contracting, expanding, removing small white specks through Opening, or sealing small black holes through Closing.
Are Opening and Closing just Shrink and Grow?
They are compound morphology operations: conceptually, Opening can be understood as Shrink → Grow and Closing as Grow → Shrink. They target small structures/holes rather than simply shifting the entire outer boundary.
Which Shape should I use by default?
Circle is the documented default and is usually the most neutral on organic edges. Square and Diamond are useful when the matte’s corner/angle geometry needs a different operator.
When should I use Alpha (RGB Unaffected)?
When the task is to change matte geometry while keeping image RGB untouched. It is the safest baseline mode for downstream compositing.
What does Manual Iteration Control do?
It exposes direct control over operator application through Operator Radius and Repeat Operation. Blackmagic describes Repeat Operation as altering the effect of Operator Radius to create more extreme adjustments.
Why not just use a large Radius?
Because morphology does not understand image semantics: a thin hair, a legitimate gap and an accidental hole are only geometric structures to the operator. A large operator destroys fine legitimate detail first.
09SOURCES & BOUNDARIES
Current labels come from Resolve 21 captures; stable morphology behavior is cross-checked against Blackmagic documentation.
The interactive lab intentionally uses standard educational min/max morphology. It demonstrates operation order, kernel shape, radius and repeated passes, but does not claim Resolve’s proprietary scale, edge handling, GPU implementation or RGB-channel morphology.