[Note: this blog was originally written as an internal commentary for peers working in the post-production community that work with 3-D, but the issues raised are of interest to a wider community so I have posted this here in its original format]
This is intended as a very brief commentary on “how humans see what we commonly call 3-D.” The purpose of this is to help those of us who work with this kind of service have a better understanding of some of the issues surrounding this technique, and why it may fail for certain viewers. As a service provider, it can only help us to better comprehend the possible feedback from customers, viewers, QC operators, etc as it relates to the possible inability or difficulty to resolve depth perception as used in the theatrical and home viewing of “3-D.”
This is a topic that could fill a large library, and is littered with contradictions, opinions, misconceptions, differing points of view and just plain garbage. I am most likely painting a large bullseye on my back by even starting a blog on this topic… but having a hard head and even thicker skin I will carry on… At the least, maybe this will provoke some commentary, contrary opinion, correction and will further enhance our collective understanding.
This particular article will focus solely on the varying physiological factors than can detract or prevent a person from effectively perceiving “3-D” – I will make no attempt here to discuss how we actually perceive this. That will be for another topic – once I figure out how to reduce that issue to a blog-sized comment.
[Data below is from the National Institute of Health, THX and various vendors of 3-D display devices]
What we commonly call “3-D” is actually nothing more than “2-D plus depth” – a true “3-D” experience would require holography or some similar technology in order to reproduce a continual change of point of view as the viewer moved in space relative to the object (screen). We use the practice of stereoscopy to simulate depth, and furthermore utilize a ‘side-effect’ of our binocular vision that allows us this depth simulation.
The HVS (Human Visual System) did not evolve in order for us to see Avatar in 3-D… our binocular visual alignment served a much more fundamental task: how to find food in a forest (our habitat of choice a few years back), and how to avoid being someone else’s dinner at the same time. There is insufficient time or space to delve into this here, but suffice to say that binocular vision with an interocular distance of 6.5cm (approx. 2.5”) allows one to see around leaves in a forested environment – thereby more clearly seeing things at a distance that might make a good meal – or to avoid the same fate for yourself.
A side effect of this stereoscopic vision is the perception of depth. By adjusting the images presented to each eye to simulate the offsets that track our normal viewing experience, we can trick the brain into believing certain objects are closer or farther away from us. In order for this effect to work properly, the human under question needs a relatively healthy visual system, and there are other issues that factor in as well that can reduce or prevent the “3-D experience” from being appreciated.
It may seem obvious, but blindness in one eye negates the 3-D experience. Approximately 2% of the world population is either bind in one eye, or has sufficiently impaired vision in one eye to prevent stereoscopic vision from working. Advanced Macular Degeneration (a retinal disease) also significantly impairs 3-D vision, and another 1% of the population (<60 yrs old) has this condition. Cataracts negatively interfere with stereo vision as well, approximately 9% of the population (<60 yrs old) has sufficient effect from this disease to affect 3-D vision. Red-Green color blindness also affects depth perception in a stereoscopic environment (this effect holds true even for non-anaglyphic presentations for reasons that are not fully understood at this time) – and that effects another 7-10% of the population.
Even after statistically combining these results to allow for the fact that some viewers will have multiples of the disorders listed above, approximately 18% of the population is physiologically unable to correctly perceive stereoscopic presentations to some degree. In addition to the purely mechanical issues to the visual system listed above, there are further issues that either affect the neurological connection to the brain, the brain’s perception of the visual data presented or other interfering neurological factors. Issues such as epilepsy, vertigo, cognitive processing difficulty, a certain percentage of autistic spectrum disorders, diabetes and other conditions can all reduce or eliminate the possibility of stereoscopic viewing, or make it not enjoyable.
The bottom line is that somewhere between 15-30% of viewers in general will for a multitude of reasons demonstrate a reduced capability for the 3-D experience. That of course leaves many millions of viewers (and their dollars) available for the consumption of this service and the associated products. So this should not be seen as a deterrent to the continuing proliferation of 3-D content and services to distribute the same, but rather as one more knowledge point to have in hand when working in this field.