Figure 4.

There is a noticeable overlap in the histograms of the foreground and the background in the blue channel. Histograms are slightly overlapped in the green channel. In the red channel, histograms are not overlapped and easily separable. Therefore, by setting 0 to the pixels lower than the threshold value, $\theta$ and setting 255 to the pixels higher than the $\theta$, we can easily generate the background mask from the red channeled image. Source of fundus photograph: STARE data set and image file: im0139.ppm.