The Perils of Parallax

There are some very simple rules to manage the somewhat complicated idea of where the "screen" is in 3D space. There are also some very simple rules for breaking those rules.

Parallax probably inspires more heated discussions than any topic in stereoscopic 3D (S3D) entertainment production. This article is a cross between a primer and a revolutionary's handbook -- kind of like the dude in high school who wrote beautiful poetry, got A's in calculus, and rode home each night on a souped-up Ducati with a beer stuck in the saddlebag. (You know the guy -- you either wanted to be him or to date him.) 3D is like that: you have to know the rules to understand just how far you can go when you break them.





The classical definition of parallax is the apparent displacement of an object as seen from two different points that are not on a line with the object. Simple? Yes? Well, maybe not, at least when it comes to S3D entertainment: there are as many points of view regarding parallax as there are folks who call themselves stereographers, or at least it seems that way.

Parallax is determined by the amount of separation between the images in a stereo pair. The easiest way to understand parallax as used in stereoscopic 3D, is to employ a concept called the "screen plane." The screen plane denotes the location in space where the images you are watching appear: a vertical plane that is coincident with the screen. Objects that fall "on" the screen plane are referred to as having "no" or "zero" parallax, and generally appear as if they are two-dimensional. Objects that fall behind the screen plane have "positive" parallax, while those that appear in front of the screen have "negative" parallax.

Sometimes you want a spear coming out of the screen. Bwana Devil (1952) is generally considered the first color 3D feature in the US.

(Truly, there is no screen plane. There is simply the path your eyes follow to the current object of interest. "Screen plane" is a manufactured concept to help develop shortcuts in S3D imaging. It's useful, so we keep it, but it's as fictional as Ducati Dude's poetry.)

The "comfort range" for parallax is determined by size of screen and distance of the viewer from the screen. The best way to measure this is relative to screen size, by expressing the offset of the images in the stereo pair as a percentage of screen size. The general recommendation is that positive parallax should not exceed 2% of screen size, and negative parallax should not exceed 1% of screen size. Most television broadcasters who are experimenting with S3D use exactly these specifications and are rather strict about them, as it helps them maintain a comfort level for their viewers, and ensures some uniformity to content standards in the wild west of S3D entertainment.

So, if you're hoping 3net, DirecTV or Sky 3D will license your latest S3D opus, make sure parallax stays within those ranges, or they just might send you packing.


THERE'S ARTISTRY IN BREAKING THE RULES, IF YOU BREAK THEM JUST RIGHT
The current school of thought for S3D production is that the action must take place in positive Z space. ("Z space" is the insiders' phrase for where an object is placed; now you're almost as cool as Ducati Dude.) This makes the audience feel as if they are looking through a window, into the action happening on the other side. As every Peeping Tom knows, windows can be incredibly compelling, but sometimes it helps to have a few things flung through the window to move the action along.

Early 3D movies flung things through the window all the time. In that era, that was the highlight of 3D's appeal. It was important to exaggerate effects because because the imprecision inherent in the medium -- notice that I called it "3D," not "S3D" -- meant that the 3D action needed to be blown out of proportion to have any sort of impact. Beset with the vagaries of aligning strips of film on separate projection devices, and relying on the skills of teenage cinema projectionists, filmmakers could not always be sure that what they envisioned was what the audience actually saw. The solution was to chuck a few spears out of the screen.

But we're past all that now. Digital technology has truly put the "S" in 3D, and while teenage projectionists still abound, it's harder for them to screw up a single digital projector fed by a hard drive. So S3D effects have become more subtle, a device to support storytelling rather than supersede it.

But sometimes you still want the spear, dammit. And that's where going much, much further than 1% with negative parallax can make some sense, if you're aware of these guidelines:

1. 5% is probably the maximum;
2. the object should be centered on the screen and should not break the horizontal edges OR
3. the object can fly in or out of camera, breaking horizontal edges for a fraction of a second. (There is also d) OR it's very dark, but that's rather a soft option for a creative filmmaker, don't you think?)

For a concrete example, go see U23D. There is a moment in the song "Bloody Sunday" where Bono reaches into the camera, singing "Don't cry." For effect, we stretched his hand a bit into negative space. The result was that, every single time the movie is screened, the audience gasps. That's artistry.



Movies like Cave of Forgotten Dreams, just above (director Werner Herzog, left, with archaelogist Wulf Hein), and U23D (top) are reminders that 3D filmmaking need not be gimmicky. Of course, as in the title graphic for this article from Piranha 3D, if you need piranhas jumping out of the screen, nothing else will do.


AND WHAT ABOUT THE GRAPHICS?
Parallax presents a particular problem in live sports production with graphics. Placement in front of the screen is typical, but the temptation is to put it too far front to avoid things like players running through the scores.

Now, a player running through a score could actually be a very compelling effect if managed properly. Several companies are developing tools to do just that. In the meantime, viewers are often stuck with graphics that stick too far out of the screen and that are painful to watch relative to the action on the screen.

The broadcasters mentioned are attempting to handle this by allowing an additional 1% of negative parallax to handle the graphics issue, as well as subtitles. Other practitioners are suggesting that the graphics themselves be moved to parts of the screen that aren't generally filled by the action -- posting scores on the top right, for instance, rather than on the bottom of the screen. One thing is for sure: innovation in this area is just beginning, and we're just getting the first glimpses of 3D graphics treatments.


Werner Herzog and W. Hein in CAVE OF FORGOTTEN DREAMS directed by Werner Herzog. Photo credit: Marc Valesella. A Sundance Selects Release.


THE PARALLAX PARADOX
The idea of a "screen" in S3D also presents an inherent paradox when it comes to parallax: how can something be "in" or "out" of the screen, and yet also "on" the screen? This is most obvious when objects that are "out" of the screen are cut off by the edge of the screen -- more visible on smaller television screens than on large movie screens, yet still a problem for both.


An object that sports any amount of negative parallax and which is cut off by the edge of the screen will be very difficult to look at: our vision systems cannot resolve the paradox. S3D practitioners have developed several techniques to handle this. The first, and simplest, is to live by this rule: Don't use negative parallax on objects that are cut off by the edge of the screen. Of course, this is not always possible, so there's a second technique, called a "floating window."

A floating window creates a sort of proscenium around the screen, so that the screen itself seems to float in space. It's basically a cropping technique. By using it, negative parallax objects that extend beyond the edges of the screen appear to be behind the floating window, and so the paradox resolves itself in our heads. And we can go back to happily munching popcorn as the latest S3D horror film scares the beejeezus out of us.



Sports production by 3ality Digital. New tools from 3ality help manage the placement of broadcast graphics in 3D space.


ACHIEVING BALANCE
Of course, there is one final thing to note when dealing with parallax: a show full of fast cuts between shots with huge disparity in parallax will make for some good eyeball exercises, but it won't be considered great entertainment. The solution is the art of "depth balancing," or slowly adjusting the amount of parallax in scene transitions to avoid eyestrain. Toolsets for managing this process are becoming standard in most S3D post workflows, and calculators are available to help manage the issue during live broadcasts as well. There is some room for artistic choice in the process, but this is clearly an area where following the rules will make for a better production at the end of the day.

So, now you're on your way to being a parallax poet of sorts, ready to create your own works of art in S3D with a minimum of eyestrain and just as little -- or as much -- gimmickry as you feel you need. And, just like the Ducati Dude, you can ride off at the end of the day, surrounded by an admiring crowd and murmurs of "Wow, he/she really knows this stuff."

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Piranha 3D from title image, courtesy of Dimension Films. Cave of Forgotten Dreams, photo by Marc Valesella, a Sundance Selects Release. U23D, courtesy of Revolver Entertainment.