MultiMedia and animation and animation, Study notes of Multimedia Applications

Download MultiMedia and animation and animation and more Study notes Multimedia Applications in PDF only on Docsity! 13 Computergraphik 1 – Textblatt engl-04 Vs. 10 Werner Purgathofer, TU Wien 3D-Viewing █ Concepts of 3D-Viewing For representing 3D-objects on a 2D-screen in a nice or recognizable way, many techniques are combined. First, the projection has to be defined, which will be described in the next paragraph. After the projection has been set, any of the following qualities can be generated: Wire-Frame-Representation = Only the edges of polygons are drawn. Hidden edges may or may not be drawn. Depth-Cueing = Edges or parts which are nearer to the viewer are displayed with more intensity (brighter, broadened, more saturated), edges which are farther away from the viewer are displayed with less intensity (darker, thinner, grayed out). Correct Visibility = Surface-elements (edges, polygons), which are occluded by other surface- elements, are not drawn so that only visible areas are shown. Shading = Depending on the angle of view or the angle of incident light, surfaces are colored brighter or darker. Illumination Models = Physical simulation of lighting conditions and propagation and their influence on the appearance of surfaces. Shadows = Areas which have no line of sight to the light-source are displayed darker. Reflections, Transparency = Reflecting objects show mirror-images, and through transparent objects the background can be seen. Textures = Patterns or samples are „painted“ on surfaces to give the objects a more complex look (looks much more realistic then). Surface Details = Small geometric structures on surfaces (like orange peel, bark, cobblestones, tire profiles) are simulated using tricks. Stereo Images = A separate image is created and presented (with various techniques) for each eye to generate a 3D-impression. 14 █ 3D-Viewing-Pipeline The viewing-pipeline in 3 dimensions is almost the same as the 2D-viewing-pipeline. Only after the definition of the viewing direction and orientation (i.e., of the camera) an additional projection step is done, which is the reduction of 3D-data onto a projection plane: norm. object- world- viewing- proj.- device- device coord. coord. coord. coord. coord. coord. This projection step can be arbitrarily complex, depending on which 3D-viewing concepts should be used. █ Viewing-Coordinates Similar to photography there are certain degrees of freedom when specifying the camera: 1. Camera position in space 2. Viewing direction from this position 3. Orientation of the camera (view-up vector) 4. Size of the display window (corresponds to the focal length of a photo-camera) With these parameters the camera-coordinate system is defined (viewing coordinates). Usually the xy-plane of this viewing-coordinate system is orthogonal to the main viewing direction and the viewing direction is in the direction of the negative z-axis. Based on the camera position the usual way to define the viewing-coordinate system is: 1. Choose a camera position (also called eye-point, or view-point). 2. Choose a viewing direction = Choose the z– direction of the viewing-coordinates. 3. Choose a direction „upwards“. From this, the x-axis and y-axis can be calculated: the image-plane is orthogonal to the viewing direction. The parallel projection of the „view-up vector“ onto this image plane defines the y-axis of the viewing coordinates. 4. Calculate the x-axis as vector-product of the z- and y-axis. 5. The distance of the image-plane from the eye-point defines the viewing angle, which is the size of the scene to be displayed. In animations the camera-definition is often automatically calculated according to certain conditions, e.g. when the camera moves around an object or in flight-simulations, such that desired effects can be achieved in an uncomplicated way. To convert world-coordinates to viewing-coordinates a series of simple transformations is needed: mainly a translation of the coordinate origins onto each other and afterwards 3 rotations, such that the coordinate-axes also coincide (two rotations for the first axis, one for the second axis, and the third axis is already correct then). Of course, all these transformations can be merged by multiplication into one matrix, which looks about like this: MWC,VC = Rz· Ry· Rx· T Creation of objects and scenes Definition of mapping region + orientation Projection onto image plane Transform. to specific device Mapping on unity image region