4. Video and Animation. Contents. 4.3 Computer-based Animation. 4.1 Basic Concepts. 4.2 Television. Enhanced Definition Systems

Transcription

1 Contents 4.1 Basic Concepts Video Signal Representation Computer Video Format 4.2 Television Conventional Systems Enhanced Definition Systems High Definition Systems Transmission 4.3 Computer-based Animation Basic Concepts Animation Languages Controlling of Animation Display of Animation Transmission of Animation 1 / 48

2 Overview 4.1 Basic Concepts In this part the basic features of video signal representation are covered. These are digitization, transmission (different encoding schemes) and visual representation (frame rate, size, etc.). Furthermore an overview of computer video formats is given. 4.2 Television In this section current international television standards (PAL, NTSC, SECAM) are presented. The evolution from existing television standards to future high definition systems is highlighted. Especially the problem of data rate reduction for transmission is addressed. 4.3 Computer-based Animation The basic principles of motion video are pointed out. Different techniques for describing, controlling, transmitting and displaying of animations are presented. 2 / 48

3 4.1 Basic Concepts: Video Signal Representation Video signal representation includes: visual representation transmission digitization Important measures for the visual representation: 1. Vertical Detail and Viewing Distance Smallest detail that can be reproduced is a pixel. Ideally: One pixel for every detail of a scene. In practics: Some details fall between scanning lines. Kell factor: only 70% of the vertical details are represented, due to the fact that some of the details of the scene fall between the scanning lines. (Determined by experience, measurements). 3 / 48

4 4.1 Basic Concepts: Video Signal Representation The Kell factor of 0.7 is independent of: way of scanning, i.e. whether the scanning is progressive (sequential scanning of lines) or whether the scanning is interlaced (alternate scanning, line 1, line 3,... line n-1, line 2, line 4,...) Scan lines A detail where only 2 out of 3 components can be represented 4 / 48

5 4.1 Basic Concepts: Video Signal Representation Geometry of the television image is based on the aspect ratio, which is the ratio of the picture width W to the height H (W:H). The conventional aspect ratio is 4/3 = 1.33 (W/H = 4/3) :525 = 4:3 (NTSC standard) Modern systems use 16/9 = 1.77 The viewing distance D determines the angle α subtended by the picture height H. This angle is measured by the ratio of the picture height to viewing distance: tan(α) = H/D. α D H 5 / 48

6 4.1 Basic Concepts: Video Signal Representation 2. Horizontal Detail and Picture Width Picture width for conventional television service is 4/3 picture height.(see previous slide) 3. Total Detail Content of the Image total number of picture elements = number of vertical elem. number of horizontal elem. = vertical resolution 2 aspect ratio = /3 (for NTSC) 4. Perception of Depth (3D impression) In natural vision: angular separation of the images received by the two eyes. Television image: perspective appearance of objects, choice of focal length of camera lens (however: usually not real), changes in depth of camera focus. 6 / 48

7 4.1 Basic Concepts: Video Signal Representation 5. Luminance and Chrominance Color perception by the human brain through the additive composition of Red, Green and Blue light (RGB). In contrast to the substracting composition of painting colors, where rather filters (red painting color reflects red light and filters out the rest) than light are mixed. The three R, G, B signals could be transmitted separately. 7 / 48

8 4.1 Basic Concepts: Video Signal Representation Color encoding during transmission uses: one luminance (brightness) signal Y (from 0/dark to 1/white), where Y = 0.30 R G B (R = G = B = 1, i.e. full red, green, blue Y = 1) due to the color sensitivity of the human eye and two chrominance (color) signals. U = c 1 (B-Y); V = c 2 (R-Y); c 1, c 2 = constants reflecting perception aspects of the human eye and the human brain!) Color impression due to: relative intensities of Red, Green, Blue. combinations of these intensities. 8 / 48

9 4.1 Basic Concepts: Video Signal Representation 6. Temporal Aspects of Illumination Motion is represented by a rapid succession of slightly different still pictures (frames). A discrete sequence of pictures is perceived as a continuous sequence of picture (due to Lucky weakness of the human brain). Between frames, the light is cut off briefly. For a realistic presentation, two conditions are required: repetition rate must be high enough to guarantee smooth motion the persistence of vision must extend over the interval between flashes. 7. Continuity of Motion To perceive continuous motion the frame rate must be higher than 15 frames/sec. For smooth motion the frame rate should be frames/sec. 9 / 48

10 4.1 Basic Concepts: Video Signal Representation 8. Flickering Through slow motion a periodic fluctuation of brightness, a flicker effect, arises. How to avoid this disturbing effect? A first trick: display each picture several times. E.g.: 16 pictures per second: very inconvenient. display every picture 3 times 3 16 = 48 Hz. To avoid flicker a refresh rate of at least 50 cycles/sec is needed. Computer display achieves 70 Hz of refresh rate by the use of a refresh buffer. TV picture is divided into two half-pictures by line interleaving Refresh rate of 25 Hz (PAL) for the full TV picture requires a scan rate of 2 25 Hz = 50 Hz. 10 / 48

11 4.1 Basic Concepts: Video Signal Representation 9. Temporal Aspect of Video Bandwidth The eye requires a video frame to be scanned every 1/25 second. Scan rate and resolution determines the video bandwidth needed for transmission. During one cycle of video frequency (i.e. 1 Hz) at most two horizontally adjacent pixels can be scanned. Vertical resolution and frame rate relates to horizontal (line) scan frequency: vertical lines (b) frame rate (c) = horizontal scan frequency Horizontal resolution and scan frequency relate to video bandwidth: horizontal lines (a) scan frequency / 2 = video bandwidth video bandwidth = a b c/2 (since 2 horizontally adjacent pixels can be represented simultaneously) 11 / 48

12 4.1 Basic Concepts: Video Signal Representation A computer system with a resolution of a = 1312 and b = 800 pixels out of which are visible and a frame rate c = 100 Hz needs: a horizontal scan frequency of Hz = 80 khz and a video bandwidth of khz / 2 = MHz /2 12 / 48

13 4.1 Basic Concepts: Video Signal Representation Encoding of the three color signals R(ed), B(lue), G(reen) As the human eye is more sensitive to brightness than to chrominance: Separation of brightness information (luminance Y) from the color information (two chrominance channels U and V). This means: RGB = Separation of Red, Green, Blue YUV = Separation of Brightness Y, and of two chrominance channels U and V. The three parameters correspond to the three basic color values. 13 / 48

14 4.1 Basic Concepts: Video Signal Representation Possible Coding: YUV signal (used by CD-Interactive and Digital Video Interactive) Y = 0.30 R G B U = (B-Y) = R G B V = (R-Y) = R G B The resolution of Y is more important than the resolution of U and V. more bits for Y than for U and V. (Y : U : V = 4 : 2 : 2) Coding Ratio System of 3 equations for: determining Y, U, V from R, G, B or recalculating R, G, B from Y, U, V The weighting factors in the calculation of the Y-Signal compensate the color perception misbalance of the human eye. 14 / 48

15 4.1 Basic Concepts: Video Signal Representation Transmission white green red blue black Different colours and their respective realisation by: R, G, B or by: Y, U, V 15 / 48

16 4.1 Basic Concepts: Video Signal Representation YIQ signal (basis for the National Television Systems Committee-System, NTSC) YIQ: in principle nothing but a slight variation of YUV scheme. NTSC: TV standard in the USA. Y I Q = = = R R R G G G B B B Composite signal: Individual components (RGB, YUV or YIQ) are composed into one signal. Basic information consists of luminance information and chrominance difference signals. Use appropriate modulation methods to eliminate interference between luminance and chrominance signals. 16 / 48

17 R G B 4. Video and Animation 4.1 Basic Concepts: Video Signal Representation A Typical NTSC Encoder Y Matrix Adder 4.2 MHz low pass filter Composite NTSC out I Matrix 1.3 MHz low pass filter Quadrature Modulation Q Matrix 0.6 MHz low pass filter Subcarrier 17 / 48

18 4.1 Basic Concepts: Video Signal Representation Required bandwidth to transmit NTSC signals is 4.2 MHz, 6 MHz including sound. The luminance (Y) or monochrome signal uses 3.2 MHz of the available bandwidth. The I-signal has 1.5 MHz of maximal bandwidth, the Q-signal 0.5 MHz. The I-signal is In-phase with the 3.58 MHz carrier wave, the Q-signal is in Quadrature (90 degrees out of phase) with the 3.58 MHz carrier wave. Picture Carrier Luminance Chrom. Subcarrier Chrominance Sound Carrier MHz 18 / 48

19 4.1 Basic Concepts: Video Signal Representation 19 / 48 Digitization For image processing or transmission the analog picture or video must be converted to a digital representation. Digitization consists of: Sampling, where the color/grey level in the picture is measured at a MxN array of pixels. Quantizing, where the values in a continuous range are divided into k intervals. For a satisfiable reconstruction of a picture from quantized samples 100 or more quantizing levels might be needed. Very often 256 levels are used, which are representable within 8 Bits. A digital picture consists of an array of integer values representing pixels.

20 4.1 Basic Concepts: Computer Video Format Video Digitizers / Raster Displays Video Digitizers Characteristics IRIS VINO Sun Video Frame Resolution 640 x x 240 Quantization 8 Bits/pixel 8 Bits/pixel Frame Rate 4 Frames/sec 30 Frames/sec Most often used displays are raster displays. Colors are displayed by the use of a Color Look Up Table (CLUT or lut), where a limited number of n colors out of a color space of m colors are chosen (n << m). (Only a subset of the global set of colors is used for any actual picture.) 20 / 48

21 4.1 Basic Concepts: Computer Video Format Raster Displays System Architecture Video controller displays the image which is stored in the frame buffer by cycling through the display buffer at a refresh rate of typically > 60 Hz (in order to avoid flickering effects). CPU Peripheral Devices System Bus Memory Frame Buffer Video Controller Monitor 21 / 48

22 4.1 Basic Concepts: Computer Video Format 22 / 48 Video Controller Standards A B C = A B Video Controller Standards Resolution Colour Graphics Adapter (CGA) 320 x 200 Colour presentation 2 Bits/pixel Storage capacity 16 KBytes Enhanced Graphics Adapter (EGA) Video Graphics Array (VGA) 8514/A Display Adapter Mode 640 x x x Bits/pixel (i.e. 16 colours are possible) 8 Bits/pixel (256 colours) 8 bits/pixel 112 KBytes KBytes KBytes Extended Graphics Array (XGA) Super VGA (SVGA) 1024 x x Bits/pixel 16 Bits/pixel Kbytes KBytes 1024 x Bits/pixel KBytes

23 4.2 Television: Conventional Systems Video format standards for conventional television systems: NTSC (National Television Systems Committee) Exact frame rate of Hz to maintain 4.5 MHz between the visual and audio carriers. Delay between frames: 1000 ms / approx. 30 frames per sec. = 33.3 ms. The chrominance signal C for NTSC transmission can be represented as: B Y 2.03 R Y 1.14 ( ω t) + cos ( t) C sin c ωc = (B-Y) (R-Y) For transmission the signal is shifted up to 3.58 MHz. 23 / 48

24 4.2 Television: Conventional Systems PAL (Phase Alternating Line, invented by W. Bruch/Telefunken 1963) Frame rate of 25 Hz, delay between frames: 1000 ms / 25 frames per sec. = 40 ms. Quadrature amplitude modulation similar to NTSC, but color carrier is not suppressed. Phase of the R-Y (V) signal is reversed by 180 degrees from line to line, to reduce color errors that occur from amplitude and phase distortion during transmission. The chrominance signal C for PAL transmission, with U = B - Y, can be represented as: C = = U sin V 1.14 ( ω t ) ± cos ( ω t ) c U = B-Y ( B Y ) sin ( ω t ) ± ( R Y ) cos ( ω t ) V = R-Y c c c SECAM (Sequential Couleur Avec Memoire) Based on frequency modulation, frame rate of 25 Hz. 24 / 48

25 4.2 Television: Enhanced Definition Systems (EDTV) System Total Lines Visible Lines Vertical Resolution Horizontal Resolution Video Bandwidth Aspect Ratio NTSC -i MHz 4/3 NTSC -p MHz 4/3 Pal -i MHz 4/3 PAL -p MHz 4/3 SECAM -i MHz 4/3 SECAM -p MHz 4/3 -i interlaced, -p progressive (non-interlaced) Optimal Viewing Distance 7 m 5 m 6 m 4.3 m 6 m 4.3 m 25 / 48

26 4.2 Television: Enhanced Definition Systems (EDTV) Improved Definition TV (IDTV) EDTV systems are conventional systems which offer improved vertical and/or horizontal resolution by some tricks. Comb filters improve horizontal resolution by more than 30% according to literature. Separate black and white from color information to eliminate rainbow effects while extending resolution. Progressive (non-interlaced) scanning improves vertical resolution. Insertion of blank lines in between active lines, which are filled with information from: above line below line same line in previous picture 26 / 48

27 4.2 Television: Enhanced Definition Systems (EDTV) Other EDTV developments are: IDTV (Improved-Definition Television) Intermediate level between NTSC and HDTV (High-Definition Television) in the U.S. Improve NTSC image by using digital memory to double scanning lines from 525 to One 1050-line image is displayed in 1/60 sec (60 frames/sec). Digital separation of chrominance and luminance signals prevents cross-interference 27 / 48

28 4.2 Television: Enhanced Definition Systems (EDTV) D2-MAC (Duobinary Multiplexed Analogue Components) Intermediate level between current television and HDTV in Europe. Uses time-multiplexing for component transmission. 64 microsec. time slot is divided into: 34.4 msec luminance signal, 17.2 msec chrominance signal and 10.3 msec voice + data 105 Bit Chrominance U/V Data + Sound Luminance (more important than chrominance, i.e. more time, more bits) 10.3 microsec microsec microsec. 64 microsec. 625 lines (574 visible), aspect ratios 4/3 and 16/9 are supported Audio/data are transmitted in duobinary coding with a rate of 105 Bits / 64 msec = 1.64 MBits/s 2 high quality stereo channels or up to 8 channels of lower audio quality 28 / 48

29 4.2 Television: High Definition Systems (HDTV) Composite Coding HDTV is characterized by: Higher Resolution, approx. twice as many horizontal and vertical pixels as conventional systems, (lines transmitted > 1000), total bandwidth 5-8 times larger than today. Aspect Ratio: 16/9 = Preferred Viewing Distance: between 2.4 and 3.3 meters. 29 / 48

30 4.2 Television: High Definition Systems (HDTV) Digital Coding is essential in the design and implementation of HDTV: 1. Composite Coding (sampling of the composite analog video signal i.e. all signal components are converted together into a digital form) is the straightforward and easiest alternative, but: cross-talk between luminance and chrominance in the composite signal, composite coding depends on the television standard, sampling frequency cannot be adopted to the bandwidth requirements of components, sampling frequency is not coupled with color carrier frequency. 30 / 48

31 4.2 Television: High Definition Systems (HDTV) Alternative: 2. Component Coding (separate digitization of various image components): the more important luminance signal is sampled with 13.5 MHz, the chrominance signals (R-Y, B-Y) are sampled with 6.75 MHz. Global bandwidth: 13 Mhz MHz > 19 MHz Luminance and chrominance signals are quantized uniformly with 8 Bits. Due to high data rates (1050 lines 600 pixels/line 30 frames/sec) bandwidth is approx. 19 MHz and therefore substandards (systems which need a lower data rate) for transmission have been defined (see the section on Transmission). 31 / 48

32 4.2 Television: High Definition Systems (HDTV) 32 / 48 Worldwide 3 different HDTV systems are being developed: United States Full-digital solution with 1050 lines (960 visible) and a scan rate of Hz. Compatible with NTSC through IDTV. Europe HD-MAC (High Definition Multiplexed Analog Components), developed by EUREKA lines (1000 visible) and a scan rate of 50 Hz. Halving of lines (625 of 1250) and of full-picture motion allows simple conversion to PAL or D2-MAC. HD-MAC receiver uses digital image storage to show full resolution and motion. Japan MUSE is a modification of the first NHK (Japan Broadcasting Company) HDTV Standard. MUSE is a Direct-Broadcast-from-Satellite (DBS) System, where the 20MHz bandwidth is reduced by compression to the 8.15 MHz available on the satellite channel. Full detail of the 1125 line image is retained for stationary scenes, with motion the definition is reduced by approx. 50%.

33 4.2 Television: High Definition Systems (HDTV) System Total Lines Visible Lines Vertical Resolution Horizontal Resolution Total Bandwidth Scan Rate (Hz) Camera HDTV USA MHz p HD-MAC MHz 50 p MUSE MHz 60 i NTSC i MHz i PAL i MHz 50 i SECAM -i MHz 50 i -i interlaced, -p progressive (non-interlaced) Scan Rate (Hz) Display p 100 p 60 i i 50 i 50 i 33 / 48

34 HDTV data rates for transmission 4. Video and Animation 4.2 Television: Transmission (Substandards) 34 / 48 total picture elements = horizontal resolution vertical resolution USA: 720,000 pixels 24 Bits/pixel 60 frames/sec. = MBits/sec Europe: 870,000 pixel 24 Bits/pixel 50 frames/sec = 1044 MBits/sec Reduction of data rate is unavoidable, since required rates do not fit to standard capacities provided by broadband networks (e.g. 155 or 34 MBits/sec). Different substandards for data reduction are defined: Substandards Sampling Rates (MHz) Luminance Chrominance Data rate (Mbits/sec) Substandard 1 Substandard 2 Substandard = 5/ = 3/ = 5/ = 1/ = 2/ = 1/ fits into a 140 Mbit/sec channel

35 4.2 Television: Transmission Further reduction of data rates is required for picture transmission Sampling gaps are left out (only visible areas are coded): Luminance has 648 sample values per line, but only 540 of them are visible. Chrominance has 216 sample values per line, but only 180 are visible. 575 visible lines: ( ) samples/line 575 lines/frame = 517,500 samples/frame 517,500 samples/frame 8 Bits/sample 25 frames/sec = MBits/sec Reduction of vertical chrominance resolution: Only the chrominance signals of each second line are transmitted. 575 visible lines: ( ) samples/line 575 lines/frame = 414,000 samples/frame 414,000 samples/frame 8 Bits/sample 25 frames/sec = 82.8 MBits/sec Different source coding: Using an intra-frame working ADPCM with 3 instead of 8 Bits/sample 414,000 samples/frame 3 Bits/sample 25 frames/sec = MBits/sec 35 / 48

36 4.3 Computer-based Animation: Basic Concepts to animate = to bring to life Animation covers changes in: time-varying positions (motion dynamics), shape, color, transparency, structure and texture of an object (update dynamics) as well as lightning, camera position, camera orientation and focus. Basic Concepts of animation are: Input Process Key frames, where animated objects are at extreme or characteristic positions must be digitized from drawings. Often a post-processing by a computer is required. 36 / 48

37 Composition Stage 4. Video and Animation 4.3 Computer-based Animation: Basic Concepts Foreground and background figures are combined to generate an individual frame. Placing of several low-resolution frames of an animation in an array leads to a trail film (pencil test), by the use of the pan-zoom feature (This feature is available for some frame buffers). The frame buffer can take a part of an image (pan) and enlarge it to full screen (zoom). Continuity is achieved by repeating the pan-zoom process fast enough. 37 / 48

38 In-between Process 4. Video and Animation 4.3 Computer-based Animation: Basic Concepts Composition of intermediate frames between key frames. Performed by linear interpolation (lerping) between start- and end-positions. To achieve more realistic results, cubic spline-interpolation can be used. Interpolated frames Rather unrealistic motion (in most cases) Key frames 38 / 48

39 4.3 Computer-based Animation: Basic Concepts 39 / 48 Spline 1 Spline 2 more realistic motion achieved by two cubic splines x 0 x1 x 2 A function s: is called cubic interpolating spline to the points a = X 0 < X 1 <... < X n+1 = b, if 1. s is twice continuous differentiable 2. i = 0,..., n which means is a polynomial of degree 3 This line is smooth, because the polynomials have equal primary and secondary derivations at the points X 0, X 1,...,X n+1

40 40 / Computer-based Animation: Basic Concepts Calculation of successive cubic splines: S 0 (x) S 1 (x) S 2 (x) S 3 (x) f(x 0 ) f(x 1 ) f(x 2 ) f(x 3 ) x 0 x 1 x 2 x 3 x 4 s i (x) are polynomials of degree 3. Let s 0 (x) be given. Then s 1 (x) = a 3 x 3 + a 2 x 2 + a 1 x + a 0 is constructed as follows: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ˆ 2 6 ˆ 2 3 ˆ ˆ x s a x a x s x s a x a x a x s x f a x a x a x a x s x f a x a x a x a x s = + = = + + = = = = = 4 equations for a 3, a 2, a 1, a 0

41 4.3 Computer-based Animation: Basic Concepts Changing Colors (two techniques): 1. CLUT animation: changing of the Color Look Up Table (CLUT) of the frame buffer. This changes the colours of the image. 2. New colour information for each frame: Frame buffer: 640 x 512 pixel 8 Bits/pixel 30 frames per sec. = 78.6 MBits/sec data rate for complete update. 1. is much faster than 2. since changing the CLUT requires the transmission of only Kbytes (here 2. is more than 300 times faster than 1.). 41 / 48

42 4.3 Computer-based Animation: Animation Languages Categories for Animation languages: Linear-list Notations Events are described by starting and ending frame number and an action (event). 17, 31, C, ROTATE HOUSE, 1, 45 means: between frames 17 and 31 rotate the object HOUSE around axis 1 by 45 degrees, determining the amount of rotation at each frame from table C. General-purpose Languages Embed animation capability within programming languages. Values of variables as parameters to the routines that perform animation. e.g. ASAS, which is built on top of LISP: (grasp my-cube): cube becomes current object (cw 0.05): spin it clockwise, by a small amount 42 / 48

43 4.3 Computer-based Animation: Animation Languages Graphical Languages (Motivation: Textual languages cannot visualize the actions by looking at the programme code.) Describe animation in a more visual way than textual languages. Express, edit and comprehend the changes in an animation. Explicit descriptions of actions are replaced by a picture of the action. Examples are: GENESYS, DIAL and S-Dynamic System. 43 / 48

44 4.3 Computer-based Animation: Controlling of Animation Techniques for controlling animations (independent of the language which describes the animation): Full Explicit Control: Complete way of control, because all aspects are defined: simple changes (scaling, translation, rotation) are specified or key frames and interpolation methods (either explicit or by direct manipulations by mouse, joystick, data glove) are provided. Procedural Control: Communication between objects to determine properties. Physically-based systems: position of one object may influence motion of another (ball cannot pass a wall). Actor-based systems: actors pass their position to other actors to affect their behavior (actor A stays behind actor B). 44 / 48

45 4.3 Computer-based Animation: Controlling of Animation Constraint-based Systems Natural way of moving from A to B is via a straight line, i.e. linearly. However, very often the motion is more complicated. Movement of objects is determined by other objects, they are in contact with. Compound motion may not be linear and is modeled by constraints (ball follows a pathway). Tracking Live Action Trajectories of animated objects are generated by tracking live action. Rotoscoping: Film with real actors as template, designers draw over the film, change background and replace human actors with animated counterparts. Attach indicators to key points of actor s body. Tracking of indicator positions provides key points in the animation model. Another example: Data glove measures: position and orientation of the hand flexion and extension of fingers and fingerparts From these informations we can calculate actions, e.g. movements 45 / 48

46 4.3 Computer-based Animation: Controlling of Animation Kinematics: Description using the position and velocity of objects. e.g. At time t = 0 the CUBE is at the origin. It moves with the constant acceleration of 0.5 m/s 2 for 2 sec. in the direction of (1,1,4). (0, 0, 0) (1, 1, 4) 2 seconds kinematic description of b = 0.5m/s 2 a motion of a cube Dynamics: Takes into consideration the physical laws that define the kinematics. e.g. At time t = 0 the CUBE is in position (0 meters, 100 meters, 0 meters) and has a mass of 5 Kg. The force of gravity (g) acts on the cube (Result in this case: the ball will fall down). (0, 100, 0) y z x 46 / 48

47 4.3 Computer-based Animation: Display of Animation For the display of animations with raster systems the animated objects have to be scan-converted to their pixmap in the frame buffer. This procedure has to be done at least 10 (better: 20) times per second in order to give a reasonably smooth effect. Problem: Frame rate of 20 pictures/sec. requires manipulation, scan-conversion and display of an object in only 50 msec. Scan conversion should only use a small fraction of these 50 msec since other operations (erasing, redrawing,... etc) have to be done too. Solution: Double-buffering: frame buffer is divided into two images, each with half of the bits of the overall frame buffer ( pipeline ). While the operation (like rotating) and scan-conversion is processed for the second half of the pixmap, the first half is displayed and vice versa. Preparation Display Pic 1 Pic 0 Pic 2 Pic 1 Pic 3 Pic 2 47 / 48 Time

48 4.3 Computer-based Animation: Transmission of Animation Symbolic Representation Graphical descriptions (circle) of an animated object (ball) + operations (roll) Animation is displayed at the receiver by scan-conversion of objects to pixmap Transmission rate depends on (transmission rate is context dependent): size of the symbolic representation structure, size of operation structure, number of animated objects and of commands. (Transmission time is shorter since symbolic representation is smaller than pixmap representation, but display time is longer since scan converting has to be done) Pixmap Representation Longer times for data transmission than with symbolic representation, because of the large data size of pixmap. Shorter display times, because no scan-conversion is necessary at receiver side. Transmission rate = size of pixmap frame rate (fixed transmission rate). 48 / 48