How can I change the colors in an RGB image to certain colors in MATLAB

Hello,

What I'm trying to do is count the number of pixels below a certain threshold, that part works. What I'm struggling with is I want to change the color of pixels below the threshold (a) to black and the pixels above the threshold (b) to white. Does anybody have any suggestions?

RGB = imread(['Test2before.png']);
B = RGB(:,:,3);
image(B);

% a is the number of pixel of straw
% b is the number of pixel of soil
a = 0;
b = 0;

% B is a RxC matrix
% max. value for c is C
% max. value for r is R
for c = 1:2250   %729
    for r = 1:2050  %349
        if B(r,c) < 100
            a = a+1;
        else
            b = b+1;
        end
    end
end

% percentage is the residue cover
percentage = a/(a+b)

Answer 1

[Cris LaPierre]

A color image has 3 color values: r, g and b. I'd first convert the image to grayscale, and then impose a threshold doing something like this:

img = imread('pears.png');
imshow(img)

bw = rgb2gray(img);
bw(bw<100) = 0;
bw(bw>=100) = 255;
figure
imshow(bw)

[Dillon Thoms]

Thanks for the reply!

I'm fairly new to MATLAB how would that incorporate into the code I provided?

[Cris LaPierre]

I guess I'm curious why you want to just use the green values. I guess it's somewhat arbitrary what color you pick. Image Analyst shows you how to compute the percentage w/o using for loops.

Here's how I would incorporate it.

RGB = imread('pears.png');
B = RGB(:,:,3);
image(B);
thresh = 100;

imshow(B)
% a is the number of pixel of straw
% b is the number of pixel of soil
a = sum(B<thresh);
b = sum(B>=thresh);
% percentage is the residue cover
percentage = a/(a+b)

B(B<thresh) = 0;
B(B>=thresh) = 255;
figure
imshow(B)

If you are really new to MATLAB, consider going through MATLAB Onramp. Pick and choose the chapters you want to learn.

Answer 2

[Image Analyst]

Try this instead of all your code.

RGB = imread('Test2before.png');
B = RGB(:,:,3);
percentage = nnz(B < 100) / numel(B)