Creating the track matte
Now, you will create the track matte to hide the outside scenery behind all areas of the image except the windowpane. To do that, you’ll duplicate the Window Lit layer and use its alpha channel.
- In the sunrise Timeline panel, select the Window Lit layer.
- Choose Edit > Duplicate.
- Drag the duplicate layer, Window Lit 2, up in the layer stack so that it’s above the Window Contents layer.
- Click Toggle Switches/Modes in the Timeline panel to display the TrkMat column, so you can apply the track matte.
- Select the Window Contents layer, and choose Alpha Matte “Window Lit 2” from the TrkMat pop-up menu.
- Press the Home key, or move the current-time indicator to the beginning of the time ruler, and then press the spacebar to preview the animation. Press the spacebar again when you’re done.
- Choose File > Save to save your project.
The alpha channel of the layer above (Window Lit 2) is used to set transparency for the Window Contents layer, so the scenery outside the window shows through the transparent areas of the windowpane.