I'm looking for a definition of shadow, mid-tone and high-light regions in digital image. I understand these are related to different parts of the histogram of image, i.e., shadows are the left-part, mid-tones the middle-part and high-lights are the right part of the histogram. However I'm not sure how these are exactly defined. By googling I have found different alternatives: the regions divide the pixels in three parts where each region contains one third of the pixels (from the darkest to lightest), regions are specified by gaussian-like functions (do not understand how single pixel value can belong to multiple regions), etc. ?