Trigonometry : Find the length of side Can someone tell me how to calculate the length 'd' from the below figure?

It is from Lecture 06 - Optical flow : https://www.youtube.com/watch?v=5VyLAH8BhF8&index=8&list=PLd3hlSJsX_Imk_BPmB_H3AQjFKZS9XgZm
(At around 16:20 mins.)
Here vector p (to point (u,v)) is resolved into parallel and normal components. So I suppose d is supposed to be perpendicular to the line joining -ft/fy and -ft/fx.
The result for d is also shown in figure. I want to know how to arrive at that. 

The triangle whose vertices are the origin, $-f_t/f_x$ and $-f_t/f_y$ are right, and its altitude is $d$.
Then
$$\frac1{d^2}=\frac{f_x^2}{f_t^2}+\frac{f_y^2}{f_t^2}$$
(I have applied this).
