How to convert a grayscale image to binary in MATLAB

Question

I have to write a program that converts intensity images into black-and-white ones. I just figured I could take a value from the original matrix, I, and if it's above the mean value, make the corresponding cell in another array equal to 1, otherwise equal to zero:

for x=1:X
    for y=1:Y
        if I(x,y)>mean(I(:))
            bw(x,y)=1;
        elseif I(x,y)<mean(I(:))
            bw(x,y)=0;
        end
    end
end
image(bw)

Unfortunately, the image I get is all black. Why?

I is in uint8, btw. 2-D Lena.tiff image

Answer 1

Use imagesc(bw) (instead of image(bw)). That automatically scales the image range.

Also, note that you can replace all your code by this vectorized, more efficient version:

bw = double(I>mean(I(:)));

Answer 2

Use this :

bw = im2bw(I, graythresh(I));

Here the documentation for im2bw;

using imshow(I,[]);, doesn't evaluate the image between 0 and 255, but between min(I(:)) and max(I(:))

EDIT

You can change graythresh(I) by anyother level. You can still use the mean of your image. (Normalize between 0 and 1).

maxI = max(I(:));
minI = min(I(:));

bw = im2bw(I,(maxI - mean(I(:)))/(maxI - minI));

Answer 3

One problem is that you are always using the mean untill then.

What you will want to do is do this before the loop:

myMean = mean(I(:));

And then replace all following occurences of mean(I(:)) by myMean.

Using the running mean as you currently will definitely slow things, but it will not be the reason why it becomes all black.