Alpha Tool

Premultiply
Premultiply composites the layer over a solid Background Color, resulting in an opaque layer. This is helpful when creating layers used by other effects, or in certain compositing situations. For example, many composite modes do not behave as expected when composited over a transparent layer. You can use Premultiply on the bottom most layer to force the whole composition to be solid, and fix the problem.

Premultiply modifies both the RGB and Alpha channels.

Unmultiply Black
This option can be used to dynamically create transparency from the luminance of an image while preserving colors visually. The preexisting alpha channel is replaced by the new one generate from luminance. This is ideal for lens flares, fire, smoke and other effects matted over black.

For example, most fire footage is filmed over black. At first you may try using the Add or Screen Composite Modes, however these create unnatural hotspots and do not look realistic. Instead, you can use Unmultiply Black to automatically create an alpha channel from the luminance of the fire. The RGB channels will be dynamically adjusted to preserve the perceptual colors of the fire, but now you can composite it over other layers without using an additive transfer mode.

Unmultiply Black changes both the RGB and Alpha channels.

Remove Color Matting
When an image has been rendered premultiplied with alpha, this mode can be used to remove color matting from the RGB channels. This does not change the input alpha channel, but rather uses the alpha as a guide for removing the Background Color. The result exactly matches the behavior in After Effects when Interpret Footage settings are 'Premultiplied - Matted With Color'. However Alpha Tool has the advantage that it can be applied at anytime on any layer, even on comps and solids.

Unmultiply Alpha only modifies the RGB channels and does not change the Alpha.

Unmultiply Color Key
This mode provides an alternative approach to removing color matting. You simply specify what color you want to be the Transparent Color, then that color is extracted from the RGB channels and placed into the alpha channel.

Unmultiply Color Key has the interesting effect that if the result is re-composited over the Transparent Color, it will visually look identical to the original layer.

Unmultiply Color Key changes both the RGB and Alpha channels.

Ignore [straight RGB]
The Ignore mode removes the alpha channel of a layer by filling the alpha fully opaque, while leaving the RGB channels unchanged. This is useful when effects require the full layer unmatted, such as displacements and color adjustments.

In some situations, After Effects may discard RGB information where the alpha is 0. In this case, you can apply Alpha Tool on the footage earlier in the project before effects and mattes are applied with the Non-Zero Alpha option. This will cause After Effects to save all RGB information.

Ignore only changes the Alpha channel.

Non-Zero Alpha
With the Non-Zero Alpha option, Alpha Tool will prevent After Effects from throwing away color information for pixels with no alpha value. It does this by setting 0 alpha values to 1 bit. This change is imperceptible to the eye, but will cause After Effects to render all pixels in the layer.

To best use this mode, apply Alpha Tool with Non-Zero Alpha on any layer that is masked or whose alpha is modified by effects. In order for Non-Zero Alpha to work, it must be applied before masks and effects. This effect is unable to correct footage that is imported with zero alpha values, unless the RGB information is intact.

Non-Zero Alpha only affects the alpha channel.

Set Matte
The Set Matte mode allows you to set the alpha channel using another layer source. The alpha can be multiplied with the new channel, or entirely replaced. When this mode is chosen several new controls become active. For more advanced options, see the Channel Composite plug-in.