Multimedia Systems and Applications Lecture 6 - Video and Animation PDF
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Lecture notes on multimedia systems and applications, focusing on video and animation topics. Covers the visual effect of motion, persistence of vision, phi phenomenon, display technologies, frame-based video, and video formats.
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Multimedia Systems and Applications Lecture 6 - Video and Animation Introduction to Multimedia 1 Motion Both video and animation give us a sense of motion. They exploit some properties of human eye’s ability of viewing pictures. Motion video is the element of multimedia that...
Multimedia Systems and Applications Lecture 6 - Video and Animation Introduction to Multimedia 1 Motion Both video and animation give us a sense of motion. They exploit some properties of human eye’s ability of viewing pictures. Motion video is the element of multimedia that can hold the interest of viewers in a presentation. Introduction to Multimedia 2 Visual Representation The visual effect of motion is due to a biological phenomenon known as persistence of vision. An object seen by the human eye remains mapped on the eye’s retina for a brief time after viewing (approximately 25 ms). Another phenomenon contributing to the vision of motion is known as phi phenomenon. When two light sources are close by and they are illuminated in quick succession, what we see is not two lights but a single light moving between the two points. Introduction to Multimedia 3 Visual Representation Due to the above two phenomena of our vision system, a discrete sequence of individual pictures can be perceived as a continuous sequence. Temporal aspect of Illumination —To represent visual reality, two conditions must be met: The rate of repetition of the images must be high enough to guarantee smooth motion from frame to frame. The rate must be high enough so that the persistence of vision extends over the interval between flashes. Introduction to Multimedia 4 Critical Fusion Frequency Critical fusion frequency is the rate at which stimuli can be presented and still be perceived as a separate stimuli. Stimuli presented at a higher rate than CFF are perceived as continuous stimuli. Motion pictures move because the frames are presented at higher rate than CFF. This depends on the brightness of the light source. The brighter the light source the higher the fusion frequency. Introduction to Multimedia 5 Display Technologies Cathode Ray Tubes (CRTs) Most common display device. Evacuated glass bottle. Extremely high voltage. Introduction to Multimedia 6 Cathode Ray Tubes (CRTs) Introduction to Multimedia 7 Basic design of a Magnetic deflection CRT Magnetic Focusing Deflection Coils Base System Electron Gun Electron Connector Beam Phosphor Pins Coated Screen Introduction to Multimedia 8 Electron Gun Contains a filament that, when heated, emits a stream of electrons. Electrons are focused with an electromagnet into a sharp beam and directed to a specific point of the face of the picture tube. The front surface of the picture tube is coated with small phosphor dots. When the beam hits a phosphor dot it glows with a brightness proportional to the strength of the beam and how long it is hit. Introduction to Multimedia 9 Display Technologies: CRTs Raster Displays Raster: A rectangular array of points or dots Pixel: One dot or picture element of the raster Scan line: A row of pixels Introduction to Multimedia 10 Display Technologies: CRTs Phosphors Fluorescence: Light emitted while the phosphor is being struck by electrons. Phosphorescence: Light emitted once the electron beam is removed. Persistence: The time from the removal of the excitation to the moment when phosphorescence has decayed to 10% of the initial light output. Introduction to Multimedia 11 Display Technologies: CRTs Refresh Frame must be “refreshed” to draw new images or repaint the current image. As new pixels are struck by electron beam, others are decaying Electron beam must hit all pixels frequently to eliminate flicker. Critical fusion frequency Typically 60 times/sec Varies with intensity, individuals, phosphor persistence, lighting... Introduction to Multimedia 12 Display Technology: Color CRTs Color CRTs are much more complicated. Requires manufacturing very precise geometry. Uses a pattern of color phosphors on the screen: Delta electron gun arrangement In-line electron gun arrangement Why red, green, and blue phosphors? Introduction to Multimedia 13 Display Technology: Color CRTs Color CRTs have Three electron guns A metal shadow mask to differentiate the beams Introduction to Multimedia 14 Frame-based Video It is known that we perceive a continuous motion to happen at any frame rate faster than 15 frames per second. PAL television system has a frame rate of 25 frames/s. It will look bad lower than this frame rate. To playback the video, we just display one image after the other, e.g. at 30 images per second. This is known as frame-based video (aka progressive scan). Introduction to Multimedia 15 Flicker/Field-based Video To overcome the problem of flickering that occurs due to fading between image frames displayed on cathode ray tube (CRT) based monitor. Flicker occurs when the monitor's CRT is driven at a low refresh rate, allowing the screen's phosphors to lose their excitation (afterglow) between sweeps of the electron gun A technique known as field-based (interleaving /interlacing) systems that improves the view by: Dividing a frame into two fields, each contains the alternative scan lines, and, Displaying the field in twice of the frames rate Introduction to Multimedia 16 Introduction to Multimedia 17 Progressive vs. Interlacing Stream of three progressive frames Stream of six interlaced fields Introduction to Multimedia 18 Interlacing Imagine you have a film camera that can take 60 pictures per second. Imagine you have a video camera and shoot the same sequence. Introduction to Multimedia 19 Interlacing (Capturing) The odd-numbered images contain one set of lines, The even-numbered images contain the other set of lines, and, No two images are captured at the same instant of time. Introduction to Multimedia 20 Interlacing - Still Frames(1) To show a still frame of this sequence, much of our software choose two adjacent fields to complete the missing set of lines. This problem, known as "tearing" or "fingering. Introduction to Multimedia 21 Interlacing - Still Frames(2) Another possible technique for producing still-frames is to choose some field and double the lines in that field: Introduction to Multimedia 22 Playback If you viewed a playback that used the simple frame- based technique, you'd see the entire video image (the lines from both fields) light up and fade out uniformly. Introduction to Multimedia 23 Broadcast Television There are mainly three conventional broadcast television systems: NTSC (National Television Systems Committee) developed in the U.S. and used in North and South America, Japan A picture consists of 525 lines and frame rate is 30Hz. SECAM (SEquential Couleur Avec Memoire) is a standard used in France and Eastern Europe A picture consists of 625 lines and frame rate is 25Hz. PAL (Phase Alternating Line) is used in most part of western Europe, most part of Asia (including China and Hong Kong) and other countries A picture consists of 625 lines and frame rate is 25Hz. Introduction to Multimedia 24 Video Resolution The picture width, horizontal resolution and the total detail content of the image can be calculated Introduction to Multimedia 25 Video Resolution The smallest detail that can be reproduced in the image is a pixel. Practically, some of the scene inevitably fall between scanning lines, so that two lines are required for such picture elements. Only about 70% of the vertical detail is presented by the scanning lines. Aspect ratio is the ratio of the picture width to height. It is 4:3 for conventional TV. Introduction to Multimedia 26 Video Resolution Conventional video systems have relative low resolution compare to computer screens: typical resolution of 640480, even up to 1024768. One consequence of this low resolution is that video played on computer screen are usually in a small window. On the other hand, even with this low resolution, the amount of data in video is huge. Consider PAL TV at 25 frames per second, if we sample at 425290 with 16 bits per pixel, the raw video size is 4252901625 = 47 Mbit/s = 5.9 Mbytes/s Compare this with a typical Ethernet bandwidth of 10Mbit/s or a double speed CD-ROM drive of 300Kbyte/s Therefore, we need to compress the video data Introduction to Multimedia 27 Video Sampling Rate We need to capture or digitize video for playing back on computers or integrating into multimedia applications We need to take a lot of samples (PAL system) At 25 frames per second (fps), each frame requires 1/25 = 40 ms There 625 scan lines in each frame, giving each scan line is 40 ms/625 = 64 s At a horizontal resolution of 425 pixel, the time for sampling each pixel is 64 s/425 = 0.15 s i.e., sampling rate is at least 7MHz This requires very fast hardware Hardware required to capture video: Video sources: TV, VCR, LaserDisc player, Camcorder Video capture card Storage space: large hard disk to Multimedia Introduction 28 Time Metric fractions of a second metric name 0.000 000 000 000 000 000 000 001 yoctosecond [ ys ] 0.000 000 000 000 000 000 001 zeptosecond [ zs ] 0.000 000 000 000 000 001 attosecond [ as ] 0.000 000 000 000 001 femtosecond [ fs ] 0.000 000 000 001 [ trillionth ] picosecond [ ps ] 0.000 000 001 [ billionth ] nanosecond [ ns ] 0.000 001 [ millionth ] microsecond [µs ] 0.001 [ thousandth ] millisecond [ ms ] 0.01 [ hundredth ] centisecond [ cs ] 1.0 second [ s ] Introduction to Multimedia 29 Video Formats AVI (Audio Video Interleaved) format was defined by Microsoft for its Video for Windows systems It supports video playback at up to 30 frames per second on a small window (typical size 300200 with 8 or 16 bit color) It is a software-only system It supports a number of compression algorithms QuickTime was originally developed by Apple for storing audio and video in Macintosh systems It supports video playback at up to 30 frames per second on a small window (typical size 300200 with 8 or 16 bit color) It is a software-only system It supports a number of compression algorithms Introduction to Multimedia 30 Video Formats MPEG (Motion Picture Expect Group) is a working group under ISO There are several versions of mpeg standard. The most commonly used now is mpeg-1 It requires hardware support for encoding and decoding (on slow systems) The maximum data rate is 1.5Megabit/sec The next generation mpeg-2 is now getting popular Mpeg-2 improves mpeg-1 by increasing the maximum data rate to 15Mbit/sec It can interleave audio and video Introduction to Multimedia 31 Animation To animate something is, literally, to bring it to life. An animation covers all changes that have a visual effect. Visual effect can be of two major kinds: motion dynamic— time varying positions update dynamic— time varying shape, color, texture, or even lighting, camera position, etc. The visual effects is the result of exploiting the properties of human vision system as described above. A computer animation is an animation performed by a computer using graphical tools to provide visual effects. Introduction to Multimedia 32 Input Process The first step in producing computer animation is input process. Key frames have to be created and input into the computer. Key frames are the frames in which the objects being animated are at extreme or characteristic positions. They can be drawn using traditional artistic tools, such as pen and brush, and then digitized. In composition stage, the foreground and background figures are combined to generate the individual frames. Introduction to Multimedia 33 In-between Process The animation of movement from one position to another needs a composition of frames with intermediate positions in between the key frames. The process of inbetweening is performed in computer animation through interpolation. The system is given the starting and ending positions It calculates the positions in between. Introduction to Multimedia 34 Introduction to Multimedia 35 In-between Calculations The easiest interpolation is linear interpolation It has many limitations: the object does not move smoothly, look unreal. Spline interpolation can make object move more smoothly. Inbetweening also involves interpolating the shapes of objects. Some animation involves changing the color of objects This is usually done using color look-up table (CLUT). By cycling through the colors in the CLUT, the objects colors will change. Morphing is a popular effect in which one image transforms into another. Introduction to Multimedia 36 Displaying Animation The rules governing the showing of video apply to animation as well. The frame rate should be at least 10, preferably 15 to 20, to give a reasonably smooth effect. There are many common ways to display animation: Generate a digital video clip: Many Animation tools will export an animation in common digital video format, e.g., QuickTime Show the animation in the animation tool. Introduction to Multimedia 40 Animation Tools Macromedia Director and Flash It is one of the most popular interactive animation tool for generating interactive multimedia applications MetaCreations Poser It understands human motion and inverse kinematics, e.g., move an arm the shoulders will follow. Discreet 3D Studio Max Very popular for creating 3D animations Animation language—VRML (Virtual Reality Modeling Language) Introduction to Multimedia 41 Report .asf: Advanced Systems Format File .avi: Audio Video Interleave File .qt: Apple QuickTime Movie .wmv: Windows Media Video File Introduction to Multimedia 42
