ErodeDilate

ErodeDilate is a channel-morphology image filter. It receives a normal RGBA Image, processes only the selected R/G/B/A channels, and returns a normal image stream. The input does not have to be a matte: it can be footage, graphics, an alpha-bearing key result, or a technical image map.

The sign rule belongs to channel values, not semantic foreground/background. Blackmagic describes negative Amount as erosion: darker areas grow and eat into brighter regions. Positive Amount is dilation: brighter regions grow into darker ones. Therefore the shortcut 'negative shrinks the matte' is only reliable when the useful matte region is actually encoded white.

That distinction matters after keying. With a conventional white subject in alpha, a small negative Amount moves the opaque boundary inward and can remove an outer fringe. With an inverted matte where the subject is dark, the same erosion grows the dark subject region. Inspect the actual channel in the Viewer before deciding which sign represents the intended subject-space correction.

Red, Green, Blue and Alpha are four independent authored controls. They decide which channels participate in morphology. For matte cleanup, Alpha-only is normally the safest starting point because RGB pixels then remain outside ErodeDilate processing. Blackmagic also distinguishes these pre-process channel selectors from the common RGBA controls applied later in the tool pipeline.

Filter selects the operation shape: Box, Linear, Gaussian or Circle. Blackmagic's Tool Reference and Fuse SDK confirm those four choices, but the reviewed user-facing reference does not publish exact numerical native-node kernel weights. Document the observable shape/response and compare filters on the real edge instead of inventing a precise convolution matrix.

Lock X/Y is enabled in the captured default state and exposes one Amount control. Disabling it splits the UI into Amount X and Amount Y so horizontal and vertical morphology can differ. This is useful for directional fringe or anisotropic utility maps, but large axis differences visibly reshape silhouettes and should not be treated as ordinary cleanup.

Blackmagic's Fuse SDK documents an additional Circle constraint: X and Y amounts must use the same sign. That is a useful implementation boundary for the ErodeDilate operation; unusual opposite-sign native-UI combinations should still be treated as a live target-build verification item. Normal matte work should keep both axes on the same side of zero.

Clipping Mode is not a quality setting or kernel type. It controls processing boundaries relative to Fusion's Domain of Definition. Frame evaluates against the full frame, Domain respects the upstream DoD and can clip large filtering at its edge, and None disables the additional source-clipping step. The difference matters most for elements whose active DoD is smaller than the composition frame.

Effect Mask is a separate standard blue mask input that limits spatial contribution. It does not become the source matte being eroded or dilated. If the matte itself must change, its values need to exist inside the Image stream, commonly in Alpha or in an image map prepared by Bitmap/MatteControl-style processing.

Keyframe diamonds beside controls are Fusion's standard animation affordance rather than separate authored ErodeDilate controls. If Amount needs temporal correction, establish a correct static edge first and then judge smooth playback to avoid edge pumping.

Production use is usually conservative and staged. A keyed edge may use Alpha-only erosion followed by a small Blur/MatteControl adjustment and despill. A glow/light-wrap branch may use Alpha-only dilation followed by Blur while a parallel sharp branch preserves the original core. Large morphology rapidly removes hair, thin lines, motion blur and gradients, so judge the result in playback and against contrasting backgrounds.