Natural & artificial depth perception
I have decided to use this opportunity (my second blog post) to talk about an instrument without which my experiment would have been impossible – a stereoscope. To be honest, until 2 weeks ago its mechanism was a bit of a mystery for me but that quickly changed when I had to disassemble and assemble it again. Luckily, Abi Lee, the PhD student, who made it was around and was able to give me a hand.
Before we talk about a stereoscope, we need to understand the term ‘stereoscopic vision’. Taken literally, it means ‘solid sight’ and refers to the visual perception of the 3D structure of the world. This type of vision is possible due to monocular and binocular mechanisms, that require the use of one and two eyes, respectively. Some of the monocular cues are perspective, image overlap and shading. There are two binocular cues:
- Binocular disparity – the brain uses the difference in image location of an object seen by the left and right eyes to extract depth information.
- Convergence – the effort by the eye’s muscles to focus on a close-up object gives a clue to the brain about an object’s depth.
So far we have discussed our natural ability to see the world in 3D but it isn’t the only way we can perceive depth. Most of us will be familiar with a stereoscopic instrument that allows to us to recreate this perception. 3D glasses are, in fact, a stereoscopic instrument in which each lens has a filter of an opposite colour (usually red and cyan). These filters match the filters of two superimposed on each other images, resulting in each eye receiving a stereoscopic image. This image is then fused by the visual cortex of the brain into the perception of a 3D scene.
A stereoscope also allows our brain to fuse two 2D images into one 3D image but with one important difference in comparison to 3D glasses. Instead of two superimposed on each other images, we are shown a pair of separate images, depicting left-eye and right-eye views of the same scene. In other words, a stereoscope artificially stimulates binocular disparities that are naturally present when viewing a real 3D scene with two eyes. Our natural ability to perceive depth through binocular disparity works well if the combination of eye convergence and focus required is natural. On the contrary, the images shown through a stereoscope often involve an unnatural combination. In fact, you might be able to see depth without a stereoscope with practice but this process can take a long time and cause some eye strain.
I would like to thank Lord Laidlaw for providing these exciting research and leadership opportunities for two summers (and time in between as I had to think about my second summer’s project). Also a big thank you to the Laidlaw team and my amazing supervisor Prof. Harris!
Howard, I. P., & Rogers, B. J. (1995). Binocular vision and stereopsis. New York: Oxford University Press.
Papathomas, T.V., Morikawa, K., Wade, N. Bela Julesz in Depth. Vision 2019, 3, 18.
Rosas H., Vargas W., Cerón A., Domínguez D., Cárdenas A. (2007) Psychophysical Approach to the Measurement of Depth Perception in Stereo Vision. Shumaker R. (eds) Virtual Reality. ICVR 2007. Lecture Notes in Computer Science, 4563.