Show video inside a graphic

A graphic can’t read a video’s pixels, but it can shape one. Make the graphic’s track a matte and the video behind it shows only where the graphic draws, or, inverted, only where it doesn’t.

Menu: Track ▸ Matte
Agent tools: set_track

Show video inside a graphic

Put the video on a track, and the graphic on the track directly above it. See Insert a graphic.
Line the graphic clip up with the stretch of video you want cut. Where the matte track has no clip, the video shows uncut.
Select the graphic clip.
Choose Track ▸ Matte ▸ Alpha.

The graphic itself is no longer drawn. What it shows now decides where the video can be seen. To go back, choose Track ▸ Matte ▸ Off.

Choose a mode

Mode	The video shows where the graphic is
Alpha	Opaque
Inverted Alpha	Transparent
Luma	Bright
Inverted Luma	Dark

Off makes it an ordinary track again.

Write a module for a matte

A matte module is written and called exactly like any other, and the cut follows each frame as it animates. Fill the window in white so it works in every mode.

This one opens a rounded window with a spring:

export const meta = { name: "Window", durationInFrames: 90 };
export const schema = { radius: { type: "number", default: 24, min: 0, max: 200 } };

export function render(frame, props, ctx) {
  const t = spring({ frame, fps: ctx.fps, damping: 18 });
  const w = ctx.width * 0.6 * t;
  const h = ctx.height * 0.6 * t;
  return svg(ctx, `<rect x="${(ctx.width - w) / 2}" y="${(ctx.height - h) / 2}"
    width="${w}" height="${h}" rx="${props.radius}" fill="#ffffff"/>`);
}

Its radius property appears on the Graphic sub-tab as usual, and you can animate it. See Write a graphic module.

What the clip’s settings still do

The graphic clip’s position, scale, rotation, opacity and actions still shape the window, so you can move or fade it with the same controls as any clip.

Its blend mode, drop shadow and reflection do nothing. They’re kept, and they apply again if the track stops being a matte.

The video’s own drop shadow follows the cut-out shape.

Good to know

Only the track directly behind the matte is cut, so tracks further back and the canvas show through the hole. Matte tracks has the whole story, including what happens with two matte tracks in a row and how a matte looks on the timeline.

With an agent

set_track sets a track’s matte to alpha, alpha_inverted, luma or luma_inverted, and none turns it off. See Tools: tracks, markers and canvas.

Show video inside a graphic