Two or more images are combined by designating one image as a threshold
image based on information within the images themselves. Pixels in this
threshold image are then segregating into foreground and background
pixels according to some criteria. An alpha value is set to zero for
background pixels indicating that these pixels in the threshold image
will make no contribution in the combined image. The foreground pixels of
the threshold image are set to some value greater than zero. Alpha values
in the non-threshold image(s) are set such that the total value of alphas
for pixels in the images to be combined totals one (indicating 100% total
contribution).