Not everyone likes 3D movies, and almost everyone agrees that traditional movies later converted to 3D have been disappointing. Until now, James Cameron, famed director and now also deep sea explorer, agreed with them. He has been a harsh critic of 3D film conversions, except that he has now spent a reported $18 million converting his blockbuster movie, Titantic, to 3D. The re-release is timed to coincide with the movie’s 15th anniversary and the 100th anniversary of the Titanic’s sinking. This herculean effort inspired us to take a closer look at how a movie gets converted to 3D after it has already been shot, and what makes Cameron’s effort so daunting.

Before a director — we’ll use the term director to refer to whichever filmmaker has taken on the task of spearheading the conversion — gets into the technical process of starting the conversion, some decisions need to be made about the look and feel of the converted film. Most importantly, the director needs to decide how far from the audience the action and sets are to be. Monsters or bullets appearing to fly out of the screen towards the audience add to the “wow” factor of a movie, but can cause nausea in some moviegoers and may just not be appealing to others. Alternatively, the depth provided by the 3D rendering can be used to show perspective and distance, pushing the background of the scene further away from the audience. Different scenes can combine these approaches, but varying the technique too much leads to motion-weary viewers.

Managing depth with parallax

The key to making a 3D photo or video appear to have depth is parallax. Simply put, parallax describes the way an object’s position or direction changes, depending on viewing angle. We all know that when we cover one eye or the other we see slightly different views from each eye. That makes filming a 3D movie deceptively simple — just place two cameras the same distance apart as our eyes, and let them rip. In reality it is much more complex than that, but at least the director has both views to start with. Converting a 2D movie to 3D requires synthesizing those two views for each of the well over 100,000 frames of a feature-length film.

When creating depth using parallax, the director can use either negative parallax, which makes objects appear closer than the viewing screen, or positive parallax, which makes them appear further away. It’s easier to explain with an illustration, like the one we’ve provided here.

Positive parallax is limited by the “artificial horizon” — the parallax of a point at infinity. The artificial horizon is rendered simply by having objects on the screen appear as far apart as our eyes, about 2.5 inches (6.4cm). For 3D TV that may mean that the left and right images would each be shifted quite a bit to make an object appear at infinity. Unfortunately most people don’t do very well at fusing images that vary by more than about 2.5%, so it isn’t possible to provide unlimited depth cues on a screen smaller than about 100 inches (2.5m) wide — at 100 inches the 2.5 inch desired effect is just about 2.5% of the total screen width. While 100-inch-wide screens are out of reach for most 3DTV viewers, the same effect in a movie theater requires a much smaller relative shift in the images, typically 1% or less.

Negative parallax — objects popping out of the screen — is also limited by our ability to fuse images quickly. Even more than positive parallax, overuse can result in viewer discomfort. A good reason not to sit in the front row of a 3D action flick if your eyes aren’t up to the challenge. Good 3D conversions are designed with the front row in mind, with the 3D “wow” effects fading as you sit further back in the theater.

When a movie is shot in 3D, the scenes can be planned so that the parallax doesn’t change quickly from scene to scene — making it easier for you to follow the action and for their eyes to adjust to each scene. Converting a movie which has already been shot doesn’t allow for that luxury, so in addition to all the other challenges of planning for the 3D conversion, smoothly managing transitions for comfortable viewing is also a necessity.

Next page: Adding depth to a 2D movie