Your thread title mentions “camera” … and that’s only used to establish and find the take-off for precision landing.
The sensors for obstacle avoidance (and the distance to the landing surface when landing in any mode) are ultra-sonic.
In that respect, what @Aij615 said is correct.
Take a look at Did not understand Litchi Reverse - #9 by colinbm to see the sensors (front and down) doing their job most excellently! 