Computer Vision (17)

Of all the monocular cues that were mentioned in my earlier post, there are just two of them that interest me a lot; motion parallax and accommodation. Motion parallax as explained in the links I had referred to, can easily be observed when you are traveling (say in a train). Objects that are closer to you appear to move faster than the objects that are farther away. If you have understood my post on triangulation, motion parallax is no new concept!
Motion parallax which is a monocular cue is conceptually similar to stereovision which is a binocular cue, in the sense that both of them are perceived due to disparity. In case of motion parallax, to perceive depth along a particular direction you have to move parallel to it. When you are moving in a train, you only capture horizontal disparity between the objects, in the same way as in stereovision we perceive disparity in the direction parallel to the line along which our eyes are placed at that point of time. The first image below is a stereo image pair made into a gif and the second shows motion parallax.

A good animation on motion parallax:http://psych.hanover.edu/KRANTZ/MotionParallax/MotionParallax.html

Motion parallax is in fact mimicking stereovision but at two different instants of time. Imagine that instead of me, I placed a video camera and shot my train journey. If I extracted any two consecutive frames from it I would have got a stereo image pair, one taken after a small delay delta compared to the other. In the case of our eye these two images are captured at the same instant of time, while in the motion parallax case it is equivalent to moving the camera to the second eye’s place to capture the second of the stereo image pair. So, when disparity can solve for depth between a stereoimage pair, why not in case of motion parallax?

You can get more stereo images as shown above here: http://www.well.com/~jimg/stereo/stereo_list.html