Motion Blur
Among the many computationally expensive computer graphics techniques, motion blur is one of the most important in creating believable scenes. Even with fairly limited knowledge of computer graphics elements, viewers will quickly spot that something is wrong if CG elements are rendered without motion blur. Motion blur is a result of motion relative to the camera, such as a camera moving across a still scene, subjects moving across the camera plane, or a combination of the two. Film and still cameras alike produce motion blur and although it can be minimized with an increase in shutter speed, it is an expected phenomenon for any movement recorded on film.

Motion blur is broken down into two basic categories: transformation and deformation. Transformation motion blur happens when something is moved (or transformed) through space. An example would be a ball passing in front of a camera or a camera panning across a scene. With deformation motion blur, the movement is the result of something changing shape. An example of this type of blur would be the swinging of a tail that is part of a sitting dog. The dog is not being translated through space; the points defining the tail are being deformed to create a new tail shape. Deformation motion blur actually involves copying two sets of geometry into the renderer, whereas transformation blur only requires one set of geometry (for each item moving through the scene) and its transformation information. Each motion blur type is captured automatically by the film camera but takes quite a bit of computation to create in computer graphics.

Shutter Speed
Motion blur is a result of motion during the exposure time of a frame of film. With a computer graphics camera, however, the shutter exposes the image plane instantaneously, thereby eliminating motion blur. In order to create motion blur for a CG camera, a camera shutter is simulated. Most film cameras use a 180-degree shutter (although other shutter angles can be used) with a film speed of 24-frames-per-second. In this setup, the shutter is open for 1/48 of a second recording the image, and closed for 1/48 of a second while the film is advanced. The 1/48 of a second represents one-half of the time a single frame of film occupies the screen. For that reason, many 3D computer graphics rendering software packages use a 0.5 (half frame) shutter speed to evaluate motion blur. What this means for the computer camera is the creation of an extra rendered frame to simulate the motion blur.

With two frames, the renderer can compare the difference between the two, evaluate which pieces of geometry have changed location, and blur them according to how far they have moved. With a motion blur shutter speed setting of 0.5, the CG camera will render two images, each one a half frame (1/48 of a second) apart. You can accomplish this several different, but a common method is to render the actual frame and also an additional frame one-half frame later. For instance, if frame 22 is being rendered, then with a 0.5 shutter setting, frame 22 and frame 22.5 could be rendered for comparison. The software automatically does the comparison, applies a blur according to the amount of movement, and outputs a single render for frame 22. It becomes clear why this process is so computationally expensive, because it means rendering two frames for every one that is output and applying some sort of 3D filter to create the blur. Depending on the software package, a shutter speed of 0.5 may also evaluate a frame using frames 21.75 and 22.25 or frames 21.5 and 22 in order to output a motion blurred frame 22. Each method produces slightly different results, but for the purposes of this text, it is sufficient to understand the concept of two frames being calculated for the output of one.

To learn more about lighting and compositing and other topics of interest to animators, check out Inspired 3D Lighting and Compositing by David Parrish; series edited by Kyle Clark and Michael Ford: Premier Press, 2002. 266 pages with illustrations. ISBN 1-931841-49-7. ($59.99) Read more about all four titles in the Inspired series and check back to VFXWorld frequently to read new excerpts.

David Parrish (left), Kyle Clark (center) and Mike Ford (right).

David Parrish went straight to work for Industrial Light & Magic after earning his master's degree from Texas A&M University. During the five years that followed, he worked on several major films, including Dragonheart, Return of the Jedi: Special Edition, Jurassic Park: The Lost World, Star Wars: Episode I — The Phantom Menace, Deep Blue Sea, Galaxy Quest and The Perfect Storm. After five years with ILM and a short stay with a startup company, he was hired by Sony Pictures Imageworks to work on Harry Potter and The Sorcerer’s Stone.

Series editor Kyle Clark is a lead animator at Microsoft's Digital Anvil Studios and co-founder of Animation Foundation. He majored in Film, Video and Computer Animation at USC and has since worked on a number of feature, commercial and game projects. He has also taught at various schools including San Francisco Academy of Art College, San Francisco State University, UCLA School of Design and Texas A&M University.

Michael Ford, series editor, is a senior technical animator at Sony Pictures Imageworks and co-founder of Animation Foundation. A graduate of UCLA’s School of Design, he has since worked on numerous feature and commercial projects at ILM, Centropolis FX and Digital Magic. He has lectured at the UCLA School of Design, USC, DeAnza College and San Francisco Academy of Art College.