Create a monochrome image in which each pixel has the value I = sum of the band levels of the current image
plus a colour image in which the pixel in each band has the value maxLevel * bandLevel / I.
Semi-interactive batch processor which asks the user to point to a detected star in the first frame to be
processed, then the same star is measured in all subsequent frames.
Subtract imf2 from this image, pixel by pixel, the result being about the half-way brightness level
so that both negative and positive results can be seen.