Stretch a cube according to the following formula for each pixel in the
image:
new_dn = (old_dn - avg) * (variance / stddev)

Here, *avg* and *stddev* stand for the statistical average and
standard deviation of all valid pixels in the current pixel's band,
respectively. The *variance* mentioned is not actually the statistical
variance of the band, but a user-specified parameter, VARIANCE, that affects
how much the histogram of the image is widened.