Right triangles with integer coordinates

The points $P (x_1, y_1)$ and $Q (x_2, y_2)$ are plotted at integer co-ordinates and are joined to the origin, $O(0,0)$, to form ΔOPQ.

There are exactly fourteen triangles containing a right angle that can be formed when each co-ordinate lies between $0$ and $2$ inclusive; that is,
$0 ≤ x_1, y_1, x_2, y_2 ≤ 2$.

Given that $0 ≤ x_1, y_1, x_2, y_2 ≤ 50$, how many right triangles can be formed?