After Effects Tutorials

The Difference Matte effect creates a transparency a mask or a matte by comparing the pixels in a source layer with the pixel in a defined difference layer, and then keying out (eliminating) pixels in the source layer that match both the position and color in the difference layer. It is usually used to key out a static background behind a moving object, which is then placed on a different background. So this effect works the difference layer is simply a frame of background footage (a still plate before any action or person is in the scene). For this reason, the Difference Matte Key is best used for scenes that have been shot with a stationary camera and lighting conditions that do not change. Because of the nature of this effect, and the inherent motion blur of moving objects, the end result can often look quite trashy.

Original (left), Matte Only view of keyed-out subject (center), and composite of subject with new background (right)

Using the difference matte

- Create a new Composition and place you movie footage onto a layer.
- Select that movie footage and find a frame that consists only of the background. Save/ export that background as a still image file and re-import it back into the composition. Alternatively use After Effect 7’s new freeze frame feature Time/Freeze Frame as a separate layer. Make sure the duration is at least as long as you original source movie file where you want the effects to take place.

(If there is no full-background frame in the shot, you may be able to assemble the full background by combining parts of several frames in After Effects or Photoshop.)

- We no longer need to actually see the difference layer because we are just using it as a reference. So you can make it invisible by switching off the video in the timeline.

- Ensuring that the original movie source layer is now selected, choose Effect/ Keying/ Difference Matte.

- In the Effect Controls panel, choose Final Output or Matte Only from the View menu. (Use the Matte Only view to check for holes in the transparency.)

- Select the background file from the Difference Layer menu.
If the difference layer is not the same size as the source layer, choose one of the following controls from the If Layer Sizes Differ menu:
Center Places the difference layer in the center of the source layer. If the difference layer is smaller than the source layer, the rest of the layer is filled with black.

- Adjust the Matching Tolerance slider to specify the amount of transparency based on how closely colors must match between the layers. Lower values produce less transparency; higher values produce more transparency.

- Adjust the Matching Softness slider to soften the edges between transparent and opaque areas. Higher values make matched pixels more transparent but do not increase the number of matching pixels.

- If there are still extraneous pixels in the matte, adjust the Blur Before Difference slider. This option suppresses noise by slightly blurring both layers before making the comparison. Note that the blurring occurs only for comparison and does not blur final output.

- Before closing the Effect Controls panel, make sure that you have selected Final Output from the View menu to ensure that After Effects renders the transparency.