About using filters to manipulate alpha channels
There are filters you can use to manipulate or even replace a layer’s alpha channel. For example, the Keying filters create transparency by generating an alpha channel based on color or luminance values in that layer. Keying filters are usually used on layers that have no alpha channel information; these filters add a new alpha channel to the layers to which they’re applied. For more information on the use of the keying filters, see Keying overview.
The process of improving a keying effect or of customizing an alpha channel created using shape or image masks may require the use of special filters. Although you can apply any filter to a shape or image mask to modify that mask’s effect, the filters described in this section modify a layer’s entire alpha channel, including the sum of all masks and other filters applied to that layer.
For example, if you’ve applied a keying filter, you can use the following filters to modify the resulting alpha channel even though no mask appears for that layer in the Layers list or Timeline.
Channel Blur: This filter, found in the Blur category of the Filters library, lets you selectively blur the alpha channel. You can set the amount of horizontal and vertical blur independently.
Matte Magic: This filter, found in the Keying subcategory of the Filters library, lets you manipulate various qualities of an alpha channel, shrinking, feathering, and eroding the alpha channel to fine-tune it. For more information, see Matte Magic filter controls.