How do i change specific values in a matrix/image

I have a black image (a matrix full of zeros). I also have 3 1x181 arrays with data in, the first two arrays, x and y correspond to the coordinates at which within the image. I want to replace the value '0' at these specific coordinates with the third data set z, in other words change the intensity to z values are the coordinates x y. Can somebody please help me with the for loop, or else I will have to do it manually - for example K(x,y)=z for every data point where K is the black image or matrix with 0s.

Answer 1

[Stephen23]

This is exactly what sub2ind is for. You will need something like this:

im = ... 2D image.
idx = sub2ind(size(im),row,col);
im(idx) = z

If your image is a 3D array (i.e. an RGB image), then you will also need to provide the page dimension as appropriate, or repmat the index and z.

[Panagiota Chatzi]

i have 3 arrays x=[64 180 297 416], y=[1 1 2 2] and z=[807 796 794 793]. X is the x coordinate of my measurements, Y is the slice number because i have a stack of images and z is the length. I created a black image and filled it with zeros and i want to assign intensity values to this specific data points according to z.

so far i wrote this:

 x=[64 180 297 416];
 y=[1 1 2 2];
 z=[807 796 794 793];
   for j=1:length(x)
     for i = 1:length(y)
         ????

 end

   end

I dont know what to put in the loop to give me the right results.Should i use function? should i just use 2-3 for loops?

[Stephen23]

@Panagiota Chatzi: I did not mention loops anywhere in my answer, so it is not clear to me what your comment relates to. As I wrote in my answer, you can easily use sub2ind:

>> row = [ 64,180,297,416];
>> col = [  1,  1,  2,  2];
>> z   = [807,796,794,793];
>> im = zeros(500,2); % 2D image of zeros.
>> idx = sub2ind(size(im),row,col);
>> im(idx) = z;

And we can easily find which elements of the image im are non-zero:

>> [r,c] = find(im) % locations of non-zero elements.
r =
    64
   180
   297
   416
c =
   1
   1
   2
   2
>> im(64,1) % check if the first value is correct.
ans =  807

So sub2ind easily put the correct value into the expected location. I don't see why you need any loops.

[Alex Fratila]

You don't need any loops. Stephen's answer is all that you would need, because the sub2ind function combines your x and y vectors so you don't need to iterate through them manually. Then, the last line assigns the values at the given indices to the values from z. So, with your data:

x=[64 180 297 416];
y=[1 1 2 2];
z=[807 796 794 793];
idx = sub2ind(size(image),x,y);
image(idx) = z;

However, I think part of the issue here is that it seems like you want to save this modified image. Right now, this doesn't modify the original image. If you want to do this, use the imwrite function:

imwrite(image, 'filename');

Answer 2

[Alex Fratila]

Does this work:

for i = 1:181
      image(x(i), y(i)) = z(i);
end

[Panagiota Chatzi]

No unfortunately it does not work. Just tried it.

[Alex Fratila]

In what way? Does it error, or does it not do what you intended?

Also, if you set a breakpoint after changing one pixel, then in the command window you print out the value at that pixel, is it still 0?