With George Lucas' Star Wars: Episode I—The Phantom Menace 3D currently playing at a number of AMC multiplexes and Titanic 3D poised to repeat the success enjoyed by its director James Cameron with Avatar 3D, the audience for stereoscopic productions is expanding dramatically. With an eye on consumer dollars, a growing number of movie studios are looking through their film vaults and reissuing landmark productions in stereoscopic 3D.
"There are two primary parameters we can use to generate a sense of depth within a stereoscopic image," explains Robert Neuman, stereoscopic supervisor at Walt Disney Animation Studios, who worked on The Lion King 3D and Beauty and the Beast 3D, both of which started life as conventional 2D productions. "Convergence controls the position between near and far within the scene, while interocular distance—sometimes referred to as interpupil distance or, more accurately, interaxial separation—defines the apparent depth of the stereoscopic image."
|For the 2D-to-3D conversion, animators develop a relief map that shows in black where objects are located further within the image and in white for those that will appear closer. This process is easier to visualize without wearing 3D glasses.|
In other words, if an object is far away from us, the light from that object entering our eyes is parallel and both eyes are looking straight ahead. As that object moves closer to us, our eyes converge; the left eye points more and more to the right while the right points increasingly to the left. The amount of stereo effect is dependent upon the separation of the real or virtual camera lenses, which, in turn, defines the relative parallax differences between the left- and right-eye images. If an object in a stereoscopic image is located in the same space for both eyes, it will appear on the surface of the movie screen and will exhibit zero parallax; objects in front of the screen are in negative parallax, and background elements that appear to be behind the screen are said to be in positive parallax, with corresponding negative or positive offsets in the position of the object for left- and right-eye images. The depth of field or focus also provides valuable clues to the relationship between near and far objects within the frame.
During preparation of 3D releases of Disney Studios' iconic animations, Neuman recalls that the first step was to decide where each character and key objects would be assigned in the depth field, "We mark up a rough version of the scene with numbers that indicate the image offset to increase/decrease parallax between the left and right images, from +5 to +10 pixels [on digital images that might be as large as 2,048 pixels wide]. Those depth maps indicate to our animators the parallax settings for each element within the scene. Using our offset data, we can then develop a relief map that shows in black where objects are located further within the image and in white for those that will appear closer—it's a useful way of visualizing the scene without having to don 3D glasses. The grayscale map shows us, in basic terms, what we need to know about the stereoscopic results."