How can I read an image from a file into two vectors?

I would like to read a black and white image from a file. For this, I use A = imread('img.jpg'); And then I would like to put the coordinates of the black parts of the image into x and y vectors, so that I would be able to use plot(x,y,'.b'). And I do not want to use imshow. Thank you very much.

[Jan]

Is it a grayscale JPEG or a true-color one? What is the dimension of A?

[Image Analyst]

Post your image. If you have a lot of black pixels, then your plot will look like a mess and there is no reason to plot it. Just say what you really want to do, and I don't think plotting should really be part of it.

Answer 1

[Jan]

This is possible by a simple find command:

if ndims(A) == 2  % grayscale image
  [x,y] = find(A == 0);
else
  [x,y] = find(~any(A, 3)); 
end

[Image Analyst]

Be careful - you're using "reverse" definitions of x and y. Your x is rows (vertical) rather than the customary horizontal columns coordinate. I'd recommend one of these

[rows, columns] = find(A == 0);
[y, x] = find(A == 0);

[Alex A]

Thank you! It is exactly what I need.