Green screen, masks and cut-outs

Compositing puts pieces of different pictures together: a presenter in front of a new background, a title behind a person, a logo stuck to a moving wall.

How layers stack

Lanes higher on the timeline sit in front of lanes lower down. To put a subject over a new background, put the background on a lower lane (for example Main) and the subject on a lane above it (for example Overlay), at the same time. Anything you remove from the subject's clip lets the background show through.

To move a picture clip in front of or behind others, right-click it and choose Bring to front, Bring forward, Send backward or Send to back.

Choose a method

Key out a green screen

1. Put the green-screen clip on a lane above the new background.
2. Select it. In the Library, open Effects, then the Keying group, and click Chroma key.
3. Open the Inspector's Adjust tab and tune the key in the Effects section:
   • Tolerance: how much of the green is removed.
   • Softness: how gradual the edge between kept and removed is.
   • Spill suppression: takes the green glow off hair and edges.
   • Edge feather: softens the outline.
   • Choke: shrinks (or grows) the edge of what is kept.
   • Amount: how strongly the key is applied.

Luma key removes Darks or Brights by a Threshold. Difference key removes whatever matches a clean shot of the background.

Check the edges at 100 % on a frame with fast movement: hair and hands are where a key fails first. Evenly lit screens key far better than patchy ones.

For fire, smoke or flares shot on black, you don't need a key: select the clip, and in the Inspector's Transform section set Blend to Screen. The black disappears and the bright parts add to the picture below. The other blend modes (Multiply, Overlay, Add, Soft light and more) are in the same menu.

Cut out a person or an object

Cut-outs work on ordinary footage with no screen. They need models on your computer, which you install once in Settings > Models.

Remove the background behind people

1. Select the video clip.
2. In the Inspector's Video tab, find the Cut out section and click Remove background.

Zornov keeps the people and drops everything behind them, through the whole clip. It runs in the background; you can keep working. This needs the MODNet model. The same tool is in the Library under AI** > **Remove background.

Cut out any object

1. Select the video clip and move the playhead to a frame where the object is clear.
2. In Cut out, turn on Pick object.
3. Click the object in the player. Alt-click (⌥-click on Mac) anything that is getting caught but shouldn't be, or drag a box round the object. A blue tint shows what will be taken.
4. Click Cut out to carry it through the whole clip, or Roto mask to make an editable mask that follows it. Clear starts the pick again.

Picking objects needs the SAM 2.1 model.

Once a clip has a cut-out, the Cut out section lets you turn it off and on, Invert it (Keep the background instead, which clears a person out of the shot), or Remove cut-out.

Put a title behind a person

1. Keep the original footage on the Main lane.
2. Put the title on the lane above it.
3. Put a second copy of the same footage on the lane above the title, lined up exactly with the original. With snapping on, it snaps to the original's start.
4. Cut out the person on the top copy.

The person now stands in front of the title, and the background behind the title is the original shot.

Masks

A mask shows only part of a clip.

1. Select the clip and open the Inspector's Video tab.
2. In the Mask section, pick Rectangle, Ellipse or Brush (a free path).
3. Set Feather to soften its edge, and turn on Invert to hide the inside instead of the outside.

Choose None to remove the mask. Once a clip has a mask, the Pins section gets a Mask tool that shows the mask's points on the picture and lists them in the Inspector.

Track mattes

A track matte uses the clip on the lane above as a stencil. In the Transform section, set Track matte to:

• Alpha matte: show this clip only where the clip above has picture.
• Alpha inverted: show it only where the clip above is empty.
• Luma matte and Luma inverted: the same, by the brightness of the clip above.

Put the matte (a shape, a title or a picture) on the lane directly above first. Video inside text is an alpha matte with a title above the footage.

Track a moving thing

Tracking follows a point or a surface through a shot, so a title, a graphic or a mask can stick to it.

1. Select the video clip. In the Video tab, find Tracking.
2. Pick a tool:
   • Point: click a point with detail (an eye, a logo, a corner) in the player. Size sets how big the area it follows is.
   • Plane: drag a box over a flat surface (a screen, a sign, a wall). Its four corners are followed in perspective.
3. Zornov tracks through the whole clip in the background.
4. If the track slips, move the playhead there, choose Correct, drag the point (or corner) to where it should be, and it re-tracks from that frame. Re-track from here does the same without moving it.
5. To make something follow the track, pick it in Layer, choose what follows in Follow (Position, Transform, Corners for a plane, or Mask), and click Attach.

A clip that follows a track says so in its own Tracking section, with a Stop following button.

To replace a screen or a sign, track it with Plane and attach the new picture with Follow set to Corners. To pin a picture's corners by hand instead, use the Corners tool in the Pins section: drag a corner, or Shift-drag to skew.

Stabilise a shaky shot

The Stabilize controls are in the same Tracking section. Smooth keeps an intended camera move and takes out the shake; Lock removes all motion. Smoothness sets how much is smoothed (with Smooth), and Max zoom limits how far Zornov zooms in to hide the moving edges (keep it low, about 110 %). Click Stabilize; Remove takes it off again. Stabilise before you track, and leave alone a shot whose shake is the point.

The compositor

For shots that need several steps (a key, then a clean-up, then a new sky, then a grade), use the compositor, which builds the shot as a graph of nodes.

1. Select a clip, or an adjustment clip to work on the whole picture at that time.
2. Click Compositor in the timeline toolbar or press Shift C. The graph takes the timeline's place.
3. Click Composite this clip with nodes.
4. Use Add node to add steps. The nodes are grouped as Inputs, Merge, Transform, Colour, Filter, Keying, Mattes & roto, Channels, Lens & camera and Cleanup & paint.

Each node has its inputs on top and its result at the bottom. Drag from a node's bottom port to another node's input to connect them, and drag an input away to disconnect it. Select a node to see its settings on the right; click View (or double-click the node) to see its picture in the player. Press Backspace or Delete to delete the selected node. Scroll to zoom the graph, and drag the background to move around it.

Remove comp takes the graph off the clip. Press Shift C again to go back to the timeline.

Let the AI editor help

Some compositing jobs are quicker to ask for than to build by hand: finding where an object is and where there is free space for a title, removing an object or a wire, replacing a sky, matching the grain and light of an inserted subject, or adding a contact shadow. The AI editor can do these and show you the result. See The AI editor.