how to split a gray image into dark, middle and bright pixels

how to split a gray image into dark, middle and bright pixels

Answer 1

[Image Analyst]

You would have to know the algorithm they're using because there are quite a few gray levels that seem to be in all three classes. I think there is quite a bit of overlap in what they call dark, middle, and bright.

[Elysi Cochin]

can you tell any simple method...

[Image Analyst]

Simple method is thresholding

darkPixels = grayImage(grayImage < 100));
middlePixels = grayImage((grayImage >= 100) & (grayImage <= 200));
brightPixels = grayImage(grayImage > 200));