With Kdenlive we can make a third eye appear on an actor's forehead, using the two-dimensional tracking algorithm integrated into the program. And with a little trick we can perfectly control the position and alignment of the eye.
Kdenlive is able to detect the movement of an object in a two-dimensional video: and we can take advantage of this function to avoid manually building all the key frames of a "composite" transition. Great for making gunshots or stains appear on a wall.