Alpha Compositor 21

This effect is used to combine two images using the alpha channels of the images to control the blending. It provides for the standard alpha blending options, and can handle pre-multiplying by any color, although white and black are most common and often run faster.

The alpha compositor effect takes a maximum of two sources and has two parameters.

Parameters

Use the descriptions below to help you understand what the parameters do. To learn how to use parameter atoms, see "Adding Video Effects to a QuickTime Movie"

Name	Code	QTAtom Type	Description
Blend mode	`'bMod'`	`kParameterTypeDataEnum`	Contains the blend mode for the effect.
Pre-multiply color	`'mclr'`	`kParameterTypeDataRGBValue`	If the blend mode is "pre-multiply alpha," this parameter contains the color used in the pre-multiply blend, otherwise it is ignored.

Blend Mode Enum

The blend mode parameter can contain one of the following values:

Straight alpha - perform a standard alpha blend. The alpha channel value of the first source defines the amount of the first source that is included in the composited image, and one minus the alpha channel value of the first source defines the amount of the second source that is included in the composited image.
Pre-multiply alpha - calculates the destination pixel according to the following formulae:



 
    DestinationRed = PreMultiplyRed * (1-alphaC) + temp1 * alphaC

    DestinationGreen = PreMultiplyGreen * (1-alphaC) + temp2 * alphaC

    DestinationBlue = PreMultiplyBlue * (1-alphaC) + temp3 * alphaC

where:



 
    alphaC = alphaB + (1-alphaB) * alphaA

    temp1 = (alphaA * SourceARed + alphaB * sourceBRed)/alphaC

    temp2 = (alphaA * SourceAGreen + alphaB * sourceBGreen)/alphaC

    temp3 = (alphaA * SourceABlue + alphaB * sourceBBlue)/alphaC

Reverse alpha - perform a reverse alpha blend. The one minus the alpha channel value of the first source defines the amount of the first source that is included in the composited image, and the alpha channel value of the first source defines the amount of the second source that is included in the composited image.