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Aphelion

Aphelion Editor

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2 — Shape Tracker masks

Track one feature and carry an editable matte with it: an ellipse, a rectangle, or a hand-drawn polygon. This is the fastest way to isolate a moving subject when a full roto is overkill.

You will build: Video Input → Shape Tracker, with Shape Tracker.mask cutting a replacement into the plate via Merge.

Before you start

  • Video Input → Viewer, file set to samples/source.mp4.
  • Scrub to a frame where the subject is clearly visible.

Steps

  1. Add a Shape Tracker node (Tab → Shape Tracker).
  2. Connect Video Input.frame → Shape Tracker.frame.
  3. Select the node. The control strip shows two rows:
    • Row 1: ◄ Track, Track ►, Clear.
    • Row 2: Draw polygon, Undo vertex, Clear polygon.
  4. Drag the tracking point onto a textured part of the subject (a button, a logo, an eye — anything with contrast).
  5. Set the matte shape in Properties:
    • Shape → Shape = Ellipse (64-point smooth oval), Rectangle (four corners), or Polygon (your points).
    • Shape → Width / Shape → Height = percent of the frame (default 20).
  6. For a soft edge, raise Shape → Feather (0–100 preview pixels).
  7. Press Track ►. Scrub the timeline: the matte follows the track.

Drawing a custom polygon

  1. Turn on Draw polygon.
  2. Click around the subject in the viewport. Each click adds a vertex; the outline closes automatically once you have three or more.
  3. Undo vertex removes the last point; Clear polygon removes them all (without touching the tracked motion).
  4. Turn Draw polygon off to go back to dragging the tracking point.

Polygon vertices are stored as offsets from the tracked point, in frame percent, so they move with the track. Both vertices and shape edits support Undo/Redo.

Using the mask

The Shape Tracker outputs x, y, valid, confidence, predicted, and mask (grayscale, matching the preview size, 1 inside the shape).

To drop a replacement inside the shape:

  1. Add an Image Input with your replacement and a Merge.
  2. Connect the plate (Video Input) to Merge.background.
  3. Connect Image Input.frame → Merge.foreground.
  4. Connect Shape Tracker.mask → Merge.mask.

To keep only the subject and remove the background, invert the matte:

  1. Add an Invert Mask node.
  2. Connect Shape Tracker.mask → Invert Mask.mask.
  3. Feed Invert Mask.mask into Merge.mask (background = new plate, foreground = original footage).

Disabling during a gap

Set Tracking Recovery → During Gaps = Disable Effect During Gap. On a frame with no valid measurement the Shape Tracker emits an empty mask, so a downstream Merge shows the background only. Other policies (Hold, Interpolate, Extrapolate) keep emitting a coordinate so the matte keeps moving.

Troubleshooting

SymptomFix
Matte is locked to one spotThe track failed. Re-check the seed is on texture, then re-track
Matte dissolves to nothing on some framesDuring Gaps is Disable and those frames are gaps — expected. Choose Hold to keep the last position
Mask is the wrong sizeWidth/Height are percents of the frame, not pixels. A value of 20 is 20% wide
Feather looks blockyFeather is in preview pixels; raise it, or raise the Viewer's Proxy Width for a sharper preview
Point placed near the frame edgeSupported — the pattern box is clamped to the frame; the point stays on the feature