Blend Alpha Tool

The Blend Alpha tool is used to blend two mattes together under the optional control of a third matte. It has front, back, and matte inputs. It extracts a matte from the front image and composites it over the alpha channel of the back input using a choice of blend modes. The coverage of the front can be controlled by the matte input. The back is the primary input; the output inherits the format of the back input; this tool only affects alpha; if the back is an RGBA image, the color part is simply copied to the output.

Note The alpha output of this tool is always clamped to the [0,1] interval.

The Blend Alpha tool has the following parameters:

Front Channel: Selects which channel to use for the front. Channel selections include Luma, Red, Green, Blue, and Alpha (default is Alpha).
Front Invert: Inverts the front before using it (default is off).
Front Intensity: Specifies the intensity of the front layer (default is 100%; range is [0,10]).
Front Opacity: Controls the opacity of the front in the blending. If a matte image is also used to control the blending, the two are multiplied together. This parameter is never ignored (default is 100%; range is [0,1]).
Back Channel: Selects which channel to use for the back. Channel selections include Luma, Red, Green, Blue, and Alpha (default is Alpha).
Back Invert: Inverts the back before using it (default is off).
Back Intensity: Specifies the intensity of the back layer (default is 100%; range is [0,10]).
Matte Channel: Selects which channel to use for the matte. Channel selections include Luma, Red, Green, Blue, and Alpha (default is Alpha).
Matte Invert: Inverts the matte before using it (default is off).
Matte Ignore: Determines whether or not the matte input is used to modulate the blend. The default is false (meaning that the matte input will be used in the blending equations). Note that if the Matte Input is not chain connected, it will be automatically ignored (no feedback needs to be provided in the UI).
Blend Mode: Determines which blend mode will be used (the default is Normal).

Click the Blend button to view other available modes.

Blend Modes

The following tables (grouped by type) list the available blend modes and describe the resulting blend effect.

Note The Blend Alpha tool has four fewer modes than the Blend & Comp and Glow tools, because the Blend Alpha tool affects only the alpha channel.

Basic blend modes

Mode	Blend Result
Normal	This is the default setting and displays the front input
Average	Adds the front and back, then divides by 2.
Add	Adds the front and back.
Subtract	Subtracts the front from the back.

Blend modes that darken

Mode	Blend Result
Darken	Compares the pixels of the front and back inputs at a given location and selects the darker of the two.
Multiply	Multiplies the color values of the back input pixels and the front input pixels, producing a darker color.
Color Burn	Colorizes darker back input pixels with the front input color.
Linear Burn	Same as Color Burn but with less contrast.

Blend modes that lighten

Mode	Blend Result
Lighten	Compares the pixels of the front and back inputs at a given location and selects the lighter of the two.
Screen	Makes the light areas much lighter, darker areas somewhat lighter.
Color Dodge	Colorizes lighter pixels with the front input color.
Linear Dodge	Same as Color Dodge but with lower contrast.

Modes based on lighting

Mode	Blend Result
Spotlight	Like Multiply but with 2 X the brightness.
Spotlight Blend	Same as Spotlight but also adds ambient illumination to the back input.
Overlay	Darkens or lightens the pixels depending on the back color.
Soft Light	If the front color is lighter than mid-gray, the image is lightened. If the front color is darker than mid-gray, the image is darkened.
Hardlight	If the front colors are lighter than mid-gray, screen mode is applied. If the front colors are darker than mid-gray, multiply mode is applied.
Pin Light	Replaces the back colors depending on the brightness of the front color. If the front color is lighter than mid-gray, back colors darker than the front color are replaced. And vice versa; if the front color is darker than mid-gray, back colors lighter than the front color are replaced.
Hard Mix	Produces either white or black depending on similarities between front and back.

Difference Modes

Mode	Blend Result
Difference	Looks at the front and back inputs, and subtracts the less bright from the brighter one.
Exclusion	Similar to Difference but with less contrast.

Blend Curves

Blend curves are used to adjust the blend of the front and back elements by adjusting the amount of fractional opacity (pixel area) for the front, and one minus front. The latter gives more importance to the back. Blend curves is useful for controlling the blend along the edges of a matte by adjusting fractional values, not 0 and 1 values. Because mattes have fractional values along the edges, the blend curves affect the edge blending.

See Blending Curves with the Allpha Tool for more on blending curves.

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