opacity channel file foreground file background file a picture that has an opacity level will be used as foreground vs a picture that has no opacity channel. the output is a picture that has no opacity channel An image that has an opacity channel. Will be used as foreground. An image that has NO opacity channel. Will be used as background. An image that has NO opacity channel. The opacity channel of the foreground image is used as weighting of how much either picture is seen in the result. default is (1 1 3 #), which is useless. please supply an image.