Replaces a pixel channel of the first image with the grayscale intensities of the second image.
Returns an image that has a single pixel channel (see channel parameter), initialized to the grayscale intensities of second image. All other pixel channels are initialized to their respective default values (see Channel).
Void pixels in the second image are ignored (i.e. the original pixel remains unchanged). Void pixels in the first image will also be void in the resulting image.