code: def harris(I):
      m, n = I.shape
      dx = (I[1:, :] - I[: m - 1, :])[:, 1:]
      dy = (I[:, 1:] - I[:, : n - 1])[1:, :]
  
      #
      #   At each point we build a matrix
      #   of derivative products
      #   M =
      #   | A = dx^2     C = dx * dy |
      #   | C = dy * dx  B = dy * dy |
      #
      #   and the score at that point is:
      #      det(M) - k*trace(M)^2
      #
      A = dx * dx
      B = dy * dy
      C = dx * dy
      tr = A + B
      det = A * B - C * C
      k = 0.05
      return det - k * tr * tr
  
label_desc: Use image node_id 1 for calling a global function with example usage: harris(I) and returns: unknown