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Aphelion

Aphelion Editor

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6 — Keying and compositing

Pull a chroma key, refine the matte, kill spill, and composite a new background.

You will build: footage → Chroma Key → Matte Edge → Merge, with Despill Pro cleaning the foreground.

flowchart LR
    FG[Video Input: green screen] --> CK[Chroma Key]
    CK --> ME[Matte Edge]
    FG --> DP[Despill Pro]
    BG[Video Input: background] --> MG[Merge]
    DP --> MG
    ME -- mask --> MG
    MG --> V[Viewer]

Before you start

Have a green-screen clip and a background clip. The examples/keying/ folder contains original.mp4, background.mp4, and result.mp4 for reference.

Steps

  1. Add two Video Input nodes: one for the green-screen plate, one for the background. Rename them (select and press F2) so the graph stays readable.
  2. Add a Chroma Key and connect the green-screen plate into its frame.
  3. Select the key colour:
    • Key → Key Color defaults to (0, 177, 64) (chroma green). Click the swatch and pick a pixel from the brightest, most even part of the screen.
    • Key → Tolerance = 10 — how far from the key colour still counts as background.
    • Key → Softness = 20 — the edge transition width.
  4. Add a Matte Edge and connect Chroma Key.mask → Matte Edge.mask.
    • Edge → Choke — negative shrinks the matte, positive grows it. Start at 0.
    • Edge → Feather — 0–40; a value of 1–3 cleans hard video edges.
    • Levels → Black Point = 0, White Point = 100 — raise Black Point to crush translucent screen noise.
  5. Add a Despill Pro on the plate (key_color matching the key) to remove green in the hair/edges. Despill → Strength = 100, Processing → Mix = 100.
  6. Add a Merge:
    • background = the background clip.
    • foreground = Despill Pro.frame.
    • mask = Matte Edge.mask.
  7. Connect Merge.frame → Viewer.frame.

Refining the key

ProblemNode / control
Screen is uneven (bright here, dark there)Lower Tolerance, then use Combine Masks to union two Chroma Keys with different key colours
Detail lost in semi-transparent areasRaise Softness, then lower Levels → Black Point on Matte Edge
Matte has holesAdd a Roto/Shape Tracker mask and combine with Matte Combine Pro (Combine → Mode)
Green fringeRaise Despill → Strength, or add Spill Suppress after Despill Pro
Foreground looks pasted onAdd Light Wrap (Wrap → Amount, Wrap → Blur) after the merge

Combining mattes

Matte Combine Pro takes two Mask inputs (a, b) and a Combine → Mode (add, subtract, intersect, and so on). This is the tool for "key object + roto garbage matte". Combine Masks is the lighter-weight equivalent for simple unions.

Channel-based keys

For non-green keys (a bright sky, a flat backdrop), Channel Mask is often better than Chroma Key:

  1. Channel Mask on the plate, Key → Channel = the channel with the most separation (often Blue for skies).
  2. Key → Low / Key → High isolate the range, Key → Invert flips it.
  3. Refine with Matte Edge exactly as above.

Troubleshooting

SymptomFix
Whole image is keyed outKey colour is wrong, or Tolerance is too high. Re-pick the key colour
Grey/translucent edgesRaise Matte Edge → Levels → Black Point, lower Softness
Background shows through the subjectMatte has holes: add a roto mask and union it with Matte Combine Pro
Foreground darker than the backgroundThe keyed footage may be premultiplied differently. Try a Premult or adjust Merge → Opacity