int: The number of elements corresponds to the number of possible gray values.
max = property
float: The maximum gray value of the histogram.
mean = property
float: Mean value of all pixels.
median = property
int: The median index (index of the histogram slot, at which roughly 50% of the pixels are darker and 50% of the pixels are brighter than corresponding slot's gray value).
min = property
float: The minimum gray value of the histogram.
mode = property
float: The mode (the most common gray value) of the histogram.