Over/under exposed detection

overexpose and underexpose detection. It passes test on recognition clearly overexposed and underexposed image. It can recognize also backlight image as underexposed image. Some artist image like sun is label as pending.

I have one question: What is a good way to count number of pixel in a interval of a cv::Mat? For now, i use 2 times of threshold and a countNonZeros, but i think it is not a good one