computer graphics animation, Lecture notes of Computer Graphics

Download computer graphics animation and more Lecture notes Computer Graphics in PDF only on Docsity! UNIT – VIII Computer animation: Design of animation sequence, general computer animation functions, raster animation, computer animation languages, key frame systems, motion specifications. (p.nos 604-616 of text book -1, chapter 21 of text book-2). Computer animation • Some typical applications of computer-generated animation are entertainment(motion pictures and cartoons), advertising, scientific and engineering studies, and training and education. • The term computer animation generally refers to any time sequence of visual changes in a scene. • In addition to changing object position with translations or rotations, a computer-generated animation could display time variations in object size, color, transparency, or surface texture. • Advertising animations often transition one object shape into another: For example, transforming a can of motor oil into an automobile engine. • Computer animations can also be generated by changing camera parameters, such as position, orientation, and focal length. • Computer animations are produced by changing lighting effects or other parameters and procedures associated with illumination and rendering. • Many applications of computer animation require realistic displays. • An accurate representation of the shape of a thunderstorm or other natural phenomena described with a numerical model is important for evaluating the reliability of the model • Also, simulators for training aircraft pilots and heavy-equipment operators must produce reasonably accurate representations of the environment. • Entertainment and advertising applications are sometimes more interested in visual effects. • Thus, scenes may be displayed with exaggerated shapes and unrealistic motions and transformations. • There are many entertainment and advertising applications that do require accurate representations for computer-generated scenes. • In some scientific and engineering studies, realism is not a goal. For example, physical quantities are often displayed with pseudo-colors or abstract shapes that change over time to help the researcher understand the nature of the physical process. Design of animation sequence • In general, an animation sequence is designed with the following steps: •..1 Storyboard layout •..2 Object definitions •..3 Key-frame specifications •..4 Generation of in-between frames • This standard approach for animated cartoons is applied to other animation applications, although there are many special applications that do not follow this sequence. • Real-time computer animations produced by flight simulators, for instance, display motion sequences in response to settings on the aircraft controls. • Visualization applications are generated by the solutions of the numerical models. • For frame-by-frame animation, each frame of the scene is separately generated and stored. • Later, the frames can be recorded on film or they can be consecutively displayed in "real-time playback" mode. Storyboard layout • The storyboard is an outline of the action. • It defines the motion sequence as a set of basic events that are to take place. Pg 1 • Depending on the type of animation to be produced, the storyboard could consist of a set of rough sketches or it could be a list of the basic ideas for the motion. Object definitions • An object definition is given for each participant in the action. • Objects can be defined in terms of basic shapes, such as polygons or splines. • The associated movements for each object are specified along with the shape. Key-frame Specifications • A keyframe is a detailed drawing of the scene at a certain time in the animation sequence. • Within each key frame, each object is positioned according to the time for that frame. • Some key frames are chosen at extreme positions in the action; others are spaced so that the time interval between key frames is not too great. • More key frames are specified for intricate motions than for simple, slowly varying motions. Generation of in-between frames • In-betweens are the intermediate frames between the key frames. • The number of in-betweens needed is determined by the media to be used to display the animation. • Film requires 24 frames per second, and graphics terminals are refreshed at the rate of 30 to 60 frames per second. • Typically, time intervals for the motion are set up so that there are from three to five in- betweens for each pair of key frames. • Depending on the speed specified for the motion, some key frames can be duplicated. • For a 1-minute film sequence with no duplication, 1440 frames are required. • With five in-betweens for each pair of key frames, 288 key frames are required. • If the motion is not too complicated, the key frames can be spaced a little farther apart. • There are several other tasks that may be required, depending on the application. • They include •..1 Motion Verification •..2 Editing and Production •..3 Synchronization of a soundtrack. • Many of the functions needed to produce general animations are now computer-generated. Pg 2 Key frame systems • Each set of in-betweens from the specification of two (or more) key frames are generated. • Motion paths can be given with a kinematic description as a set of spline curves, or the motions can be physically based by specifying the forces acting on the objects to be animated. • For complex scenes, the frames are separated into individual components or objects called cels (celluloid transparencies), an acronym from cartoon animation. • Given the animation paths, the positions of individual objects can be interpolated between any two times. • With complex object transformations, the shapes of objects may change over time. Examples are clothes, facial features, magnified detail, evolving shapes, exploding or disintegrating objects, and transforming one object into another object. • If all surfaces are described with polygon meshes, then the number of edges per polygon can change from one frame to the next. • Thus, the total number of line segments can be different in different frames. Morphing • Transformation of object shapes from one form to another is called morphing, which is a shortened form of metamorphosis. • Morphing methods can he applied to any motion or transition involving a change in shape. • Given two key frames for an object transformation, first the object specification is adjusted in one of the frames so that the number of polygon edges (or the number of vertices) is the same for the two frames. • This preprocessing step is shown in Fig. 8-4. • A straight-line segment in key frame k is transformed into two line segments in key frame k+1. • Since key frame k + 1 has an extra vertex, a vertex is added between vertices 1 and 2 in key frame k to balance the number of vertices (and edges) in the two key frames. • Using linear interpolation to generate the in- betweens, the added vertex in key frame k is transitioned into vertex 3' along the straight-line path shown in Fig. 8-5. • An example of a triangle linearly expanding into a quadrilateral is given in Fig. 8-6. • Pg 5 General preprocessing rules for equalizing key frames in terms of either the number of edges Pg 6 or the number of vertices to be added to a key frame. • Suppose the edge count is equalized, and parameters Lk and Lk+1 denote the number of line segments in two consecutive frames. Then Lmax = max(Lk and Lk+1), Lmin = min(Lk and Lk+1) -(1) • and Ne = Lmax mod Lmin Ns = int(Lmax/Lmin) -(2) • Then the preprocessing is accomplished by •..1 dividing Ne edges of keyframemin into Ns + 1 sections •..2 dividing the remaining lines of keyframemin into Ns sections • For example, if Lk = 15 and Lk+1 = 11, 4 lines of keyframek+1 is divided into 2 sections each. The remaining lines of keyframek+1 are left intact. • If the vertex count is equalized, parameters V k and Vk+1 are used to denote the number of vertices in the two consecutive frames. • In this case, it is defined as Vmax = max(Vk,Vk+1), Vmin = min(Vk,Vk+1) -(3) • and -(4) • Preprocessing using vertex count is performed by •..1 adding Np points to Nls line sections of keyframemin •..2 adding Np - 1 points to the remaining edges of keyframemin • For the triangle-to-quadrilateral example, Vk = 3 and Vk+1 = 4. • Both Nls and Np are 1, so one point is added to one edge of keyframek • No points would be added to the remaining lines of keyframek+1. Simulating Accelerations • Curve-fitting techniques are often used to specify the animation paths between key frames. • Given the vertex positions at the key frames, the positions can be fitted with linear or nonlinear paths. • Figure 8-7 illustrates a nonlinear fit of key-frame positions. This determines the trajectories for the in-betweens. • To simulate accelerations, the time spacing can be adjusted for the in- betweens. • For constant speed (zero acceleration), equal-interval time spacing is used for the in-betweens. • Pg 7