In this system the sky provides the background. An
overcast sky or sunny weather yield a fairly uniform
background of a whitish color. When the sky is partly
clear and partly cloudy, the background gray level
changes from section to section and time to time,
which creates difficulties in image binarization. To
resolve the problem of gray level image binarization,
when image background gray levels vary from one
area to another, a new adaptive thresholding algorithm,
which we termed recursive BCV thresholding
was developed. This algorithm has been tested in a
quarry in central Sweden.