In the video approach a camera monitors the hand positions and attempts to infer from the position of the thumb which phalanx is being thumbed. It also estimates the degree of separation of each adjacent pair of fingers.
This is by far the most technically challenging approach. From the user's point of view however it is also by far the most convenient to use, as the hand remains unencumbered other than by the requirement that it be positioned and oriented to suit the camera. The software should be smart enough not to demand too many concessions by the user, which otherwise can become more of an encumbrance than the switches and contacts of the other approaches.
The ready availability today of miniature black-and-white NTSC or PAL cameras and framegrabbers means that the hardware side of this approach, while certainly sophisticated, is a largely solved problem. In contrast the software side involves determining the position and orientation of the hand as a whole and the phalanxes as its parts, and deciding when a character has been thumbed.
This approach is complicated by the need to take into account the variety of finger positions different users may adopt for comfort. It is further complicated by widely varying lighting and background conditions.
The optimal camera position has its line of sight to the fingers normal to the plane defined by the tip phalanges of the middle-ring-pinky trio when held comfortably in the Open closure. Furthermore the camera should be able to see the tip of the thumb at all times, achievable by positioning the hand and/or camera to view the outer side of the thumb rather than the back.
The determination of when a character has been thumbed is particularly difficult, requiring constant monitoring of a steady and high-speed video stream. This problem is greatly simplified by a hybrid approach involving a switch at the thumb tip which is closed when a character is thumbed. With this approach a video stream is no longer needed and it suffices to simply photograph the hand once at the moment of thumbing the character.