Difference Matte

The Difference Matte creates transparency by comparing a specified image with a specified clip and then eliminating areas in the clip that match those in the image. Though you can use this key to create special effects, you can also use it to key out a static background by selecting the Reverse Key option.

In this way, you can remove a static background behind a moving object (such as a person walking past a stage set), and place the person against a different background. Very often the specified image is simply a frame of background footage (before the moving object has entered the scene). For this reason, the Difference Matte is best used in this way for scenes that have been shot with a stationary camera.

To replace a static background behind a moving object:

1 Find a frame of your clip that consists only of the static background.

2 Save this background frame as an image file. For information about saving a frame, see Exporting a still image.

3 Place the video clip in a superimpose track, select it, and then choose Clip > Video Options > -Transparency.

4 For Key Type, choose Difference Matte.

5 In the Matte section of the dialog box, click the Choose button and select the image you saved in step 2.

6 Select the Reverse Key option to key out the static background.

7 Place the new background in a track below this one to create the composite.

Superimposing and Compositing > Using keys to superimpose and create composites > Difference Matte