find the minimum RGB values of a pixel from an image

Hi! I'm having trouble to get the minimum of Intensity in an RGB image. I want to extract the darkest pixel (so the minimum RGB values) and mark it on the image with x, y coordinates. Could someone help me? Thank you

[Adam]

You might as well just convert to greyscale if it is only the intensity you are interested in, then you only have to deal with a 2d image rather than 3d.

Answer 1

[Rik]

You mean like this?

RGB=uint8(255*rand(100,100,3));%random RGB image

temp_IM=sum(double(RGB),3);%sum the three color channels
[val,ind]=min(temp_IM(:));%find lowest value (first one if there are multiple)
[r,c]=ind2sub(size(temp_IM),ind);%coordinates of lowest intensity pixel

[Ilaria Pintus]

Thank you so much!

In the end I solved it in a way similar to what you suggested to me!

[Guillaume]

There are several way you can define the intensity of a colour but a plain sum of the 3 colour components is not standard. Using matlab's definition of luminosity (see Algorithms in the doc of rgb2gray) , the intensity would be 0.2989 * R + 0.5870 * G + 0.1140 * B, a weighted mean of the three colour components. There are other definitions that are more attuned to human vision.

Note that instead of

RGB=uint8(255*rand(100,100,3));

you could use

RGB = randi([0 255], 100, 100, 3, 'uint8');

And I would use im2double instead of plain double for the conversion.