Virtual Images | By Jordan Falk
In stereoscopic (3D) cinema, stereographers often discuss the use of a “depth budget” when composing a scene. A depth budget is the total amount of z-axis space that a viewer can comfortably view. Viewing 3D can be immersive and exciting, but can also be confusing and physically uncomfortable if the 3D is poorly done or too extreme. For this reason it’s important to understand how stereoscopic vision works, and how to design with this depth budget in mind.
Typically there are background elements, focal elements, and foreground elements that make up a total depth budget. Background elements appear to recede behind the viewing surface (movie or TV screen, or piece of lenticular lens) of an image. Background depth works the same way for a viewer’s eyes in projected and lenticular 3D, with the left eye seeing the left part of a stereo pair or sequence, and the right eye seeing the right part. This is referred to as “positive parallax” and typically can handle a larger degree of separation between stereo images than foreground 3D, or “negative parallax.” Negative parallax results in elements appearing to float in front of the viewing plane, and is usually used for the “Wow that’s really 3D!” moments in a 3D film or image; for example swords poking out at the viewer, balls shooting out into the audience, or other moments where elements seem to impede beyond the constrains of the screen they are displayed on.
While this effect is certainly exciting and novel, it can be uncomfortable and cause a viewer some eye strain if used in excess. For these reasons we generally suggest a depth budget of approximately 75% background 3D and 25% foreground. It is also a best practice to keep the most important elements on or near the focal plane, where the viewer naturally focuses their view, and will have the most comfortable viewing experience. Use background and foreground elements to build out the scene and add visual interest, and treat the primary elements, like logos, key text and/or main character elements as the central focal point that everything pivots around.
