How to sum up specific pixel intensities in image?

I have a question about image pixel intensity.I want to sum up specific pixel intensities in an image.For example intensitylevel>120. How can I do this?

Answer 1

[Joseph Cheng]

you can use logical ">" to find the values greater than 120

 example = magic(10);
 ind=find(example>50);
 sumintent = sum(example(ind));

[Guillaume]

find is not needed at all here. Use the logical array returned by the comparison directly:

example = magic(10);
isabovethreshold = example > 50;
sumintent = sum(sum(example(isabovethreshold));  %you missed the double sum as well

[Joseph Cheng]

I tried to make it simple based of the level of the question. You don't need the double sum with the find as it'll return a 1xN.

[Guillaume]

True. However, you'll find a lot of beginner use find when it's not needed. In the end, the simplest is probably:

sumintent = sum(example(example(:) > threshold));

Answer 2

[Changoleon]

Do you want to get rid of the intensities less than 120? so that you end up with the pixels intensities over 120? If this is the case you can binarize your image as following:

 a = double(imread('cameraman.tif'));

 figure; imagesc(a); colormap gray

 b = a>120;

 figure; imagesc(b); colormap gray

In case you want the number of pixels who carry the intensity of 1 in your binirized image (image b in example above) you can do the following:

 c = sum(sum(b));     %this is the numbper of pixels that have the value of 1 in the entire binary image