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I’m not the kind to watch a movie in the cinema just because everybody is talking about it. I actually avoid watching the movie for fear of being biased, with my expectations extremely high because of all the recommendations. Imagine my disappointment when I watched Transformers –am not much of an action/CGI fan either (yes, we know you can do it, Hollywood). And no, I still haven’t watched Inception and the plan is to wait around one more year when people have forgotten about it.









However, I did watch Avatar in IMAX. Hypocrite? You can consider that. Fact is, I don’t have the money to buy my own 3D projector, nor the time to make my own 3D glasses. Plus it was my first time in IMAX. Yes, I found it worth the 3D package. No, the story was nothing new, Pocahontas.



So, about 3D motion perception: a study suggests that the recent models of binocular 3D motion perception may not actually reflect the strategies we, as observers, use (Harris and Drga, 2005).

The left and right eye see two different things. This is called binocular disparity. Perceiving depth because of binocular disparity is called stereopsis. Ordinary cameras cannot do this. A special device, however, was introduced by Charles Wheatstone . This device, called a stereoscope, is capable of capturing what the left and the right eyes can see in two different images. The same thing goes on with 3D movies –the “blurred” images are actually two images, and the glasses filter out the other image using colors (Goldstein, 2010).

Direction estimation based on binocular disparity and motion signals in binocular 3D motion has been studied well enough. We can see the results of these studies on the big screen and now almost everywhere around us. However, Harris & Drga (2005) have shown that observers may use neither binocular disparity nor motion signals to obtain the angle of 3D motion for small trajectory angles close to the nose.

In their research, they tested if observers make an estimate of the visual direction of an object at the start and end of its motion and if they use that estimate to determine how the object has moved. The experiment required observers to view a small point-like object moving along a 3D trajectory. They were then asked to move a wooden pointer to reproduce the perceived 3D motion direction of the target.

It was found that they use an estimate of visual direction to set a perceived angle instead of estimates of distances moved in depth, showing the importance of visual direction for perception and control of action. Visual direction is the “angle between where an object of interest is and where the observer is facing” (Harris & Drga, 2005). Of course, in the natural setting, visual direction is not the only way we determine motion.

Imagine this. You are looking at a floating ball that is right in front of you. The floor is just plain white, with no texture gradient whatsoever that can help you determine distance (see this entry for texture gradients). This floating ball moves towards you but not in a straight line. Harris & Drga (2005) suggest that you will estimate the distance of movement the ball made based on the angle between the ball and where you are facing, and not how near the ball is to you.

References
Goldstein, E. B. (2010). Sensation and Perception (8th ed.). Belmont, CA, USA: Wadsworth Publishing.
Harris, J. M., & Drga, V. F. (2005). Using visual direction in three-dimensional motion perception. Nature Neuroscience , 8 (2).