title: Subtract across Numpy array
body: <p>I have a numpy array in the shape (244,244,3). Each holds a RGB image as numpy array. From each of the channels I want to subtract a value. </p>
  
  <p>Of course I can easily do it with a for loop, but I assume that there is a faster way in numpy?</p>
  
  <p>Any ideas?</p>
  
  <p>Many thanks</p>