How to find out the different intensity values that are used in the image and make out a list of them.Then find the freq. of occurence (probability) of each of intensity values in the image?

How to find out the different intensity values that are used in the image and make out a list of them. Then find the freq. of occurrence (probability) of each of intensity values in the image?

[John D'Errico]

Convert to L*a*b*, then make a histogram of the L* channel?

Answer 1

[Walter Roberson]

grayImage = rgb2gray(YourImage);
[unique_vals, ~, idx] = unique(grayImage(:));
counts = accumarray(idx(:), 1);
probs = counts ./ sum(counts);
bar(unique_vals, probs);

Answer 2

[Image Analyst]

You can use

grayImage = rgb2gray(rgbImage);
[pixelCounts, grayLevels] = imhist(grayImage);
grayLevelFrequency = pixelCounts / numel(grayImage);

It depends on what you mean by intensity or how accurate you want to be. It would be best to calibrate your system using a Color Checker Chart and then convert to LAB and take the histogram of the L channel. This is what I do, and what John suggested above. If you just want to get an idea of graylevels for thresholding or something, then you don't need to go through all that.