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[edit] Difference Matting Node

The difference matte node works by determining the distance in color space between two colors, or the difference between two colors. One color is the key and the other color is being check against the key to determine if it should be transparent or not. Difference matting does not rely on a certain background color, but can have less than optimal results if there is a significant amount of background color in the foreground object.

[edit] Inputs

There are two inputs to the difference matte node. The first is an input image that is to be keyed. The second input is a key color. The key color may be selected by using the eyedropper tool in the color select window.

[edit] Outputs

The outputs of the difference matte node are the image with the new alpha channel and a matte that represents the alpha channel.

[edit] Controls

[edit] Color Space

This button group selects which color space the difference algorithm works in. Different color spaces work better for different backgrounds.

[edit] RGB Tolerance Sliders

You can adjust the tolerance of each axis individually so that you can more red variance or blue variance in what you would allow to be transparent. I find that about .15 (or 15%) is plenty of variance if the background is evenly lit. We will cover what to do if it is not evenly lit (such as our spaceship footage in a moment).

[edit] Falloff

You can adjust the amount of transparency that a pixel has by the distance that it is from the actual key location. So that pixels that are closer to the key are more transparent that ones that are further away. This is called falloff. If you want pixels that are far away to be considered very transparent, then use a high falloff value.

[edit] Alpha Threshold

The alpha threshold determines how transparent a pixel should be once it is determined