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Combine RGB Masks
The Combine RGB Masks combines 3 input mask images and combines them by assigning each of the output image's RGB channels to the values of the mask images. The alpha channel of the output image will be set to 1.
Here is an example where take the output of the Channel Masks node and combine the masks, swapping the red and green channels of the original image:

For this is the input image:

Inputs and Outputs
Inputs:
Red Mask (Image): A grayscale mask image which will form the output image's red channel.Green Mask (Image): A grayscale mask image which will form the output image's green channel.Blue Mask (Image): A grayscale mask image which will form the output image's blue channel.
Outputs:
Output Image (Image): The output image.