Gray image thresholding using the Triangle Method

Computes a threshold the Triangle method.

The Triangle method is due to Zack (Zack GW, Rogers WE, Latt SA (1977), "Automatic measurement of sister chromatid exchange frequency", J. Histochem. Cytochem. 25 (7): 741–53.

A line is constructed between the maximum of the histogram at b on the gray level axis and the lowest (or highest depending on context) value a on the gray level axis where the histogram is significantly larger than 0. The distance L normal to the line and between the line and the histogram is computed for all values from a to b. The level where the distance between the histogram and the line is maximal is the threshold value (level). This technique is particularly effective when the object pixels produce a weak peak in the histogram.