COM337 COMPUTER GRAPHICS Other Topics Animation, Surface Details, Global Illumination Kurtuluş Küllü based on the book by Hearn, Baker, and Carithers
There are some other important issues and topics that we couldn t look at Computer animation Textures Bump mapping Global Illumination Ray-tracing Radiosity Environment mapping Photon mapping
Computer Animation Not only for movies and cartoons also used in advertising, scientific and engineering studies, training and education Not only about motion Any time sequence of visual changes in a picture (translations, rotations, scaling, color, transparency, texture, shape) including changes to the camera
Computer Animation Two methods for a motion sequence are Real-time animation Each stage of the sequence is viewed as it is created Frames need to be produced quickly enough Frame-by-frame animation Each frame is separately generated and stored They can later be played
Double Buffering Employs two refresh buffers One buffer is used to refresh the screen while the other one is modified When back buffer is ready, swap buffers to change roles Removes or reduces flickering and some other problems
Creating an Animation Sequence Common development stages: Creating a storyboard Storyboard shows the outline Defining objects Polygons, splines, etc. Specifying key-frames Key-frame shows the scene at a certain time More intricate motions generally require more key-frames Generating in-betweens (in-between frames) Interpolation At 24 fps, a 1-min. sequence requires 1440 frames. If 5 in-betweens are generated between every key-frame pair, only 288 key-frames are needed.
Morphing Transforming shapes from one form to another Can also be done by key-framing La La Faim (Hunger) (Peter Foldes, 1974) One of the first computer animation films. Awards for Hunger included a Special Jury Prize at the 1974 Cannes Film Festival, a BAFTA Award for Best Animation Film and a Silver Hugo at the Chicago International Film Festival. The film was also nominated for an Academy Award for Best Animated Short Film at the 47th Academy Awards.
Articulated Figure Animation Articulated figures are hierarchical structures with rigid links at rotary joints Used to animate people, animals, insects, Mocap (motion capture) is an effective and common method in producing such animations
Textures Representing a surface with polygons or other primitives is not enough to obtain realistic objects Real world objects have fine detail
Texture Mapping A common method in which a pattern is mapped onto the geometry of the object Texture pattern is often a 2D image
Bump Mapping Texture mapping improves realism but is limited in reproducing rough surface effects Small light-intensity details can be obtained by bump mapping (distorting the surface normal for illumination calculations)
Global Illumination Illumination models in CG are approximations The Basic (Phong) Illumination Model we looked at before was a local model (it only considers the object at hand, the light, and the camera) More realistic lighting effects are produced by global illumination models (also considering contribution of light reflected from other surfaces) Ray-tracing, radiosity, photon mapping
Ray-Tracing Ray casting was described when we looked at CSG and visible surface detection Ray-tracing is mainly the same idea Instead of only detecting the visible surface, continue to bounce the rays to calculate intensities Involves considerable computation
Radiosity Surfaces are divided into smaller surfaces (patches) For each pair of patches, determine how well they can see each other (a coefficient called view/form factor) These are used to solve a linear system of rendering equations (yielding the radiosity, or brightness, of each patch) Takes into account diffuse object-object reflections and soft shadows
Progressive Radiosity Solves the system iteratively corresponding to bounce levels
Environment Mapping Sometimes called reflection mapping An enclosing environment map around an object (or objects) holds the intensity information from lights, sky, and other background objects A cube or another shape can be used instead of a sphere
Photon Mapping Radiosity becomes more difficult to apply as scene complexity increases Photon mapping provides a general global illumination method efficient in complex scenes 1 st pass: Photons are emitted from light source, they strike objects, and they can then be reflected, transmitted, or absorbed Add information about an intersection to the photon map 2 nd pass: Once photon map is built, render the image using a modified ray-tracing approach Like radiosity, photon map is view-independent