1. A method of selecting a region of interest on a slide, the region being positively contrasted from a surrounding thereof in a marker-only image corresponding to an RGB image of the sample, the positively contrasted region including at least one of a relatively larger nuclei and a relatively higher cell density than the surrounding, comprising: applying a low pass filter to a marker-only image of a sample, the marker-only image being obtained through chromogen separation of the RGB image of the sample; determining a marker-only histogram of pixels in the marker-only image; and binarizing the marker-only image according to a threshold in the marker-only histogram so as to form a mask for discriminating between negative and positive regions of the sample.