The Stereoscopic Experience in Avatar

Few would argue that in recent years things have been looking up for blue people. Gone are the days of hiding from Gargamel -- today's blue people are empowering themselves by playing drums, getting more life-like prostheses, and using their mind powers to burst Viet Cong soldiers like water balloons. Thirty years ago the giant blue people of René Laloux's La Planète Sauvage were infamously keeping human beings as pets, whereas today's giant blue people are winning our sympathies with Ewok-style revolts. Turn, turn, turn.

This is not a movie review site, so I won't be reviewing Avatar here. On the other hand, this site does concern whatever my brain is currently mired in -- this is usually robots and spaceships, but right now it's the fact that I'm directing a high-definition stereoscopic project. As such, today's post will address the stereoscopic aspect of James Cameron's much ballyhooed new talkie.

For the record, I saw the Real D presentation of the film on a silver halide screen approximately 9.5m in width. My glasses were circularly polarized. I was precisely centred on the auditorium's X axis, seated some 12m back from the screen. The girl beside me totally hogged the arm rest.

The most remarkable thing about Avatar's stereoscopy is how gentle it is. In stark contrast to the gimmicky, playful stereoscopy of Robert Zemeckis's A Christmas Carol, James Cameron has kept his stereoscopy understated and strictly realistic.

The vast majority of the action takes place in positive parallax -- that is, the images appear to be recessed behind the movie screen. The intrusions into negative parallax (with features pushing out into the auditorium) were few, non-aggressive, and more often than not peripheral rather than central visual elements.

The point of convergence (the part of the image which is exactly the same in both left and right views) moved throughout each shot, in general tracking with the focus point. This correlation was so strong, especially in the sequences dominated by live-action photography, that I believe manipulation of the point of convergence was likely slaved to the focus operator's control.

In dialogue scenes the speaker tended to be at convergence and in crisp focus, while listeners were softened and shifted either negatively or positively. What this means is that over the course of nearly three hours of cinema what they're asking you to look at is the part of the picture that's easiest for your eye muscles to deal with. Very smart.

The interocular distance appeared to be fixed at human normal (approximately 2.5cm), which keeps the depth at a scale regular people are familiar with. This means all the long shots (a planet in space, features on the horizon) were entirely at convergence -- totally flat. This makes sense, because in the real world our meat-eyes separated by less than three centimeters cannot discern stereo separation of distant objects. To force stereopsis -- as directors using it purely as a gimmick often do -- would mess up the perceived scale of objects and make great vistas appear as if they were miniature. So, hats off to the DOP and DIT for keeping the interocular consistent and credible.

Window violations were rife. (These are instances of negative parallax objects being clipped by the edge of the frame, thereby "breaking" their three-dimensional credibility.) The reason I don't think this was a big problem was because the objects committing the violations were never the targets of our attention. The negative parallax space was almost entirely reserved for things like bugs and bits of dirt or framing elements like pieces of set -- little extras, never the main feature.

(Again, they weren't distracting, but it's worth noting that these window violations would be less likely to be noticed by viewers in an IMAX theatre, or viewers seated much closer to the front of the auditorium.)

As for the look of the picture itself, the light-robbing nature of the two sets of polarizing filters (one set in front of the projectors, one set in front of your eyes) means stills and trailers seen flat seem flamboyantly bright and cartoonishly supersaturated, like George Lucas' Star Wars: Episode I: The Phantom Ate My Menace. The good news is that Avatar looks much more natural once appropriately darkened by polarization.

On the other hand, stereopsis is like an onion...in that you can kind of see through it. The dissonance between the two views creates a subtle but unmistakable scintillation in the eye; in high contrast shots the most strongly depthed elements may show minor ghosting, but even in busy, low contrast shots there is a sort of low-grade sparkling apparent. This is more or less distracting depending on the subject matter on the screen. In my experience, this is not a property of the projection surface itself (it's not random reflections from the silver halide crystals, as some have suggested); I have definitely seen it in rear-projection situations as well. I believe the lion's share of the phenomenon is an optical-psychological artifact of fusing the two views, exacerbated by ghosting or cross-view bleedthrough (caused by poor projector matching, misaligned polarization filters, or overly aggressive stereoscopy in a high contrast situation).

This persistent "glamour" in the depthiest parts of the image is something stereoscopic technology will have to overcome in the future in order to achieve a truly naturalistic picture.

In conclusion, Avatar had the best stereoscopy of any project I've seen to date -- and that's somewhat ironic, because it employs the technique with a comparatively light touch. In the case of stereopsis, less really is more.

Strictly maintaining a human-credible interocular distance, not being afraid to flatten out the big stuff or the far stuff, refusing to use any cheap tricks or gimmicky shots with crap flying out into the audience, tracking the point of convergence with the focus point to reduce eyestrain -- all clever, economical moves designed for the best impact, not the maximum impact.

(We'll return to our regularly scheduled robots and spaceships as soon as possible.)