Using if else statements to change values of pixel in an image

So I am trying to create a function that takes in an image and for every rgb value less than 128, it will change its value to zero, otherwise, it will change its value to 255. However whenever I try using my code, I just get a white, blank image. I am a self taught learner

 function newimage = extremecontrast(inimg)
[r,c,l]= size(inimg);
for i = 1:r
    for j= 1:c
        for k = 1:l
        if inimg<128
            inimg = 0;
        else
            inimg = 255;
        end
        end
    end
end
newimage = inimg[i,j,k];
end

Answer 1

[Adam]

I very much doubt you need three nested for loops for this, but

if inimg<128

is not going to do what you want if inimg is an RGB image. I assume you want

if inimg(i,j,k) < 128

and the same for everywhere else you overwrite the whole of your input variable.

Something like

newImage = 255 * round( inimg / 255 );

ought to do the job though instead I think.

[PenguinForce]

So you're saying to use the last bit of code you just suggested, so in doing this method I'd be able to get rid of my nested loops entirely shouldn't I?

[PenguinForce]

That worked, very interesting way to significantly decrease three fourths of the code. Thank you

[Adam]

Yes, that is the power of vectorisation within Matlab (well, other languages too, but I am only really familiar with it in Matlab).

Plus using tricks like knowing that scaling the data down to between 0 and 1 will allow the round function to round the top half of the data range to 1 and the bottom half to 0 to effectively binarise it about the centre point.

It takes a while to get used to using all these kinds of things and I am still very much learning, but it is fun to try to work out neat ways to code these things even if it isn't faster than the longer way (in this case I imagine it should be a lot faster though).