everything that is visible is within [-1;-1;-1;1] [1;1;1;1] in ndc from your camera. invert thisthe projection matrix to bring the box to world space coordinates and then make them relative to your light source by multiplying with your projection matrix of your light source. Now all you need to do is make a Bounding box of a convex shape with 8 vertices. Shouldn't be that hard.