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In order to really make the best use of this article, you may need to visit my actual blog if you're reading this on a mirror such as my livejournal, xanga pages or my LinkedIn, or via an RSS feed, since it contains embedded quicktime movies and such that don't always play nice across those mirrors.

You might also want to pick up a set of glasses like I'm linking over on the right, since you can't make much sense of anaglyph images if you don't have anaglyph glasses! (If you've swiped polarized glasses from a movie theater, those won't work with this.) If you already have a pair of them stashed away in a drawer somewhere, you can play along already! The cheap "throwaway" cardboard ones that sometimes come with comics or advertising materials will work, but the lens quality of real framed sunglass-style glasses will improve the experience.

Note that this article is an attempt to make a pretty technical subject as accessible as possible, so as much as possible I'll be keeping it straightforward and just noting a few things that seem important. There's much more technical information to be written about at another time, but after talking to a friend that's dealing with the same things himself on another project, and realizing that each studio will run into the same issues again and again until somebody writes this stuff down, maybe it was time to get a little of it onto the internet. Since I know the people that find this article are going to range from complete novices to experienced vfx industry professionals, the tone of the article may swing a bit. Rest assured, if it momentarily becomes too mundane or too technical, it'll roll back towards the middle in another paragraph or two!

To make something clear up front, when I refer to stereo films/cg/animation/movies/etc, I'm referring to stereography - the process of creating a 3d image in the mind of the viewer by the presentation of two or more 2d images. I tend to refer to projects as "stereo" projects vs. "3d" projects since I work in visual effects where we think of "2d" as a department and 3d as a variety of cg software package: texture and digital matte painters, roto and paint artists, compositors are 2d artists, and even on a stereo film, the "2d" department does just as much work as they would on a traditional "flat" film.

After working on an in-development feature a few months back, as well as the Hannah Montana 3D concert feature, and the upcoming "My Bloody Valentine: 3D" flick, one thing was really painfully clear: almost nothing about 3d presentation has really been entered into the record. What's been learned by the relative handful of people working on stereo movies has been kept to themselves. Each new team to confront the integration of live action with stereo cg has to invent the whole process from scratch all over again. It's like the beginning of cinema, which makes it exciting and frustrating at the same time.

If you're in the industry, the time to come to terms with stereo production is now: in five years time, we shouldn't expect to see a lot of flat films coming out of the major studios.

So let's jump in and look at a clip now and then talk about what it shows.

Consistent convergence and interocular distance between elements

There are two things that we discuss when talking about stereo cameras: interocular distance and convergence. These are what makes a stereo camera setup different from a traditional camera. All the same old things are there: film speed (though generally this is a CCD or CMOS sensitivity level, not an actual film speed), aperture, shutter angle (or something like it), and field of view. But interocular distance and convergence are new concepts to most people, even though 3d movies have been around for over 80 years.

The video above shows a rotating checkerboard with two geometric objects on it. The interocular distance on the shot is about human-eye normal (that is, the distance between the two "cameras" used in the CG scene is about the natural distance for human eyes). I should also note that this distance is constant in this shot. Some camera rigs, such as the Pace rig that's been quite popular in stereo film production recently, have the ability to vary the interocular distance while the shot is in process (usually in conjunction with a focus pull). When I first heard about this being touted as a promotional point, I was terrified: often, we consider ourselves lucky in visual effects if we were able to get accurate lens info, and now this? Turns out, it's not as hard to deal with as you might think - even if you don't have metadata from the shoot that pairs up interocular distance with each frame. I'll deal with the specifics of that in another post.

The convergence in the above shot is on the cone. Convergence is the distance in front of the eye-pair that each camera's view crosses the other. If you hold your finger about a foot in front of you and look at it, your eyes are converging on your finger. If you then shift your focus to an object beyond your finger, you will notice the image of your finger splitting into two images: this is because your eyes are no longer converging at your finger but rather at another object in the distance.

This is an important thing to note, here, that when composing a scene for stereo presentation, you should always try to set your convergence on the element in frame that you most want to draw the viewer's attention to. Failure to do this will increase complaints from your audience about headaches and eyestrain as they (usually) unconsciously try to compensate! You can feel and see the effect of this by playing the above video and trying to stare at the foreground edge of the checkerboard. The good news is that if this doesn't happen in camera, it's easy to make it happen in post. The dual plates that represent the two eyes of the camera can be panned horizontally until the desired convergence element merges. If the same element exists in both plates at the same position when overlayed, that's where the convergence is! Again, this is something I should cover in more detail in another article - but this info will be sufficient for many.

In the above shot, all of the elements were rendered with the same interocular distance and convergence distance. They appear solid, the geometric objects appear to rest on the checkerboard, and all is right with the world.

Let's see what happens when we try to fool Mother Nature:

Inconsistent convergence and interocular distance between elements

This is the same render of the checkerboard, but in the render of the elements that are sitting on it (which still exist in the same location as before) the camera pair converges just a little bit in front of where it did before. The effect of this is to make them appear farther away from the viewer. It's a subtle effect: our brain still wants to perceive the elements (which obscure the table and thus *must* be in front of it) as being closer to us, but it's fighting with itself: there's a sense that the elements are somewhat indented into the table. The cone is also harder to focus on because it's no longer the point of convergence.

More to come in a later entry...