Question about pij in formula （3）

[FocusK]

I am so sorry to trouble you again. I am not understand the code below.
According to the paper,I know rays is pij = [i j d]，rays[0] is corresponding to i, rays[1] is corresponding to j, … but I can't know the role of K and c_x, c_y. As you say in the paper, i and j are integer in the range [- (r-1)/2, (r-1)/2]. So why do you need to multiply inv_K and rays.
Another, the value of c_x and c_y is always 0?
c_x = 0 c_y = 0 K = np.array([[d,0,c_x],[0,d,c_y],[0,0,1]]) inv_K = np.linalg.inv(K) rays = np.matmul(inv_K, rays.reshape(3,r_h*r_w))