Update Image RGB value for many pixels with logical index

Hello all, I'm doing my first steps with MatLab, and faced a problem for several hours now that I'm unable to solve.

If have converted an RGB image to a double matrix, which works. Now I want to modify all pixels of the RGB matrix that have a value >=0.7 in the green channel. I determine the indexes of those pixels using a logical Matrix:

logical_mark_green = my_image_double(:,:,2) >= 0.7;  %store logical Matrix containing a '1' at all indexes where value of index 2 (=green) >= 0.7

Afterwards, and that's where I am stuck, I want to overwrite the determined pixels to the colour black:

my_image_double(logical_mark_green, :) = 0;   %overwrite image on indexes stored in logical Matrix with 3D-Array representing black colour

But I only get a corrupted image with a small black line. Can anybody help me out? I appreciate any help. And kindly provide beginner-friendly answers :)

Answer 1

[Ameer Hamza]

One way is to repeat the mask for three layers before applying

img = double(imread('peacock.jpg'))/255;
mask = img(:,:,2) > 0.5;
mask = repmat(mask, 1,1,3);
img(mask) = 0;
imshow(img);

Some other methods are also mentioned here: https://www.mathworks.com/matlabcentral/answers/77660-apply-logical-mask-to-rgb-image

[Benjamin A.]

This works, thanks a ton! Can you explain why repmat solves it? The logical Matrix is n*n, after using repmat it is (n*1)*(n*1)*3, right?

[Ameer Hamza]

Yes, after repmat, the dimensions of mask become n*n*3, which also matches the dimensions of the img variable. The dimensions of the logical mask must match the variable it is trying to change.