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CS1102 Lec07 – Digital Media Semester B, 2012-13 Computer Science Department City University of Hong Kong.

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Presentation on theme: "CS1102 Lec07 – Digital Media Semester B, 2012-13 Computer Science Department City University of Hong Kong."— Presentation transcript:



3 Jean Wang / CS1102 - Lec07 2 Objectives  Describe the various types of digit media  Identify hardware/software that can be used to work with various types of digital media  Identify digit media files by their file extensions  Compare waveform audio and MIDI music  Compare bitmap images and vector images, and describe procedures required to convert vector graphics to into bitmaps, and convert bitmaps to vector graphics  Discover how resolution, image size, color depth and compression affect the file size of a bitmap image

4 3 Objectives (cont'd)  Illustrate some commonly used image compression techniques  Use storage calculation to justify the reason why compression is essential for digital video  Discover how window size, frame rate, and compression affect file size for a desktop video  Describe how video streaming works Jean Wang / CS1102 - Lec07

5 4 Digital Media  Digital media  Pictures, sound, video and animation stored/processed in computers in digital format (i.e., binary numbers)  Typical hardware requirements work with digital media  A powerful microprocessor  Large memory and storage capacity  A high-quality monitor and a display card  May also need some input devices  A microphone or keyboard for audio input  A digital camera or scanner for graphics input  An analog video camera for video input and a video capture card to convert analog video to digital video  Or a digital video camera Jean Wang / CS1102 - Lec07

6 Lec07 - Digital Media - Digital Sound

7 Jean Wang / CS1102 - Lec07 6 Digital Audio Basics  Audio is originally an analog waveform signal, and it must be converted into discrete digital signal in order to be stored in and processed by computers  Example converting devices  Analog->digital : sound cards, display/video cards, scanners, digital cameras, video capture cards  Digital->analog : sound cards, display card Analog (waveform) data signal Digital (discrete) data signal

8 7 Digital Audio Basics (cont'd) Jean Wang / CS1102 - Lec07

9 8 Analog to Digital  Two steps to convert analog signals to digital signals:  Sampling - to slice the analog wave into small segments regularly at uniform intervals (called samples)  Quantization - the height (amplitude) of each sample is converted into a binary number Sampling rate - the number of times per second that a sound is measured during the converting (recording) process - Measured in hertz (Hz) Jean Wang / CS1102 - Lec07

10 9 Sound Quality  Sampling – how many times per second?  Human ear can hear sound from 20Hz-22000Hz  Audio CD uses a sampling rate of 44100Hz  Some very high quality (studio quality) audio uses a sampling rate of 48000Hz – 96000Hz  For voice/speech signal, a smaller sampling rate is good enough (e.g. 11000Hz)  Quantization – how many bits are needed for a single sample?  CD uses 16 bits per sample (65536 values, high quality)  Lower quality music uses 8 bits (256 values) per sample  Number of channels  Mono – only one channel  Stereo – 2 channels (left, right)  Surround - 5.1 channels (left, right, center, left surround, right surround, and a base enhancement channel) Jean Wang / CS1102 - Lec07

11 10 Audio Compression  Uncompressed audio data need a large amount of storage space  Data compression (or encoding) is the technique of reducing the number of binary digits required to represent data  E.g., a 5-minute.mp3 file takes up less than 5MB  An audio/video compression algorithm is typically referred to as audio/video codec (coding & decoding)  Compressed video/audio data must be decompressed (or decoded) before being viewed/heard Q: For a 5-minute stereo song with a sampling rate of 44100 Hz and 16 bits per sample, how many bytes are needed? Jean Wang / CS1102 - Lec07

12 11 Data Compression  Data compression can be lossless or lossy  Lossless – the compressed-then-decompressed version is 100% identical to the original one  Often used in file compression, such as ZIP, RAR, …  Lossy – the compressed-then-decompressed version is not identical to the original one, but the difference are almost undetectable by human  Often used in audio, image and video compression  Sacrifice some quality for smaller file size  In audio/video compression  Compression ratio measures reduction in data quantity produced by a compression algorithm  Expressed as :  Bit rate measures the amount of data (no. of bits) per second video/audio  Expressed in bps (bit per second) Jean Wang / CS1102 - Lec07

13 12 Waveform Audio File Format Audio Format File Extension AdvantageDisadvantage Wave.wavGood sound quality Supported in browsers without plug-in Audio data is stored in raw, uncompressed format, so files are very large MP3 (also called MPEG-1 Layer 3).mp3Good sound quality even though the file is compressed Can be streamed over the Web Requires a stand-alone player or browser plug-in RealAudio.ra.ram.rx High degree of compression produces small files Can be streamed over the Web Sound quality is not up to the standards of other formats Requires a player or plug-in WMA (Windows Media Audio).wmaCompressed format, very good sound quality Requires Windows Media Player AAC (Advanced Audio Coding) (the successor to the MP3).aac.m4p.mp4 Lossy compressed format with good quality and small file size Default audio format of Apple's iPhone, iPod, iTunes Heavily patent, may limit the usage potential

14 13 MIDI Music  MIDI stands for Musical Instrument Digital Interface  specifies a standard way to record, store and play back music on digital sound synthesizers  Unlike waveform audio files, MIDI files (.mid) do not represent sound directly, but contain instructions on how the sound should be created  Information includes the pitch of a musical note, note-on time, note-off time, note volume, the type of the instrument  Most sound cards are equipped to generate music from MIDI files  Advantage  MIDI files are very small  Disadvantage  Does not produce high quality “real” sound Jean Wang / CS1102 - Lec07

15 Lec07 - Digital Media - Digital Graphics

16 15 Bitmap Image  Bitmap image (also called raster image)  Images are digitized/sampled by a grid of dots, each dot is called a pixel  Each pixel is assigned a binary color value  Resolution : the number of horizontal and vertical pixels  Bitmaps do not have a fixed physical size  Higher monitor resolution -> smaller image size  Color depth : the number of bits to represent the color intensity  Determines the number of colors available for use  Bitmaps does not scale well; resizing bitmap image will reduce quality Jean Wang / CS1102 - Lec07

17 16 Color Depth  RGB representation  Color is commonly represented according to the proportions of red, green and blue that constitutes the color  True color (24-bit for RGB)  32-bit for RGBA (A stands for Alpha, used to create transparency effect)  The colors used in web pages are based on RGB, often written in hexadecimal values  E.g.,  The intensity of colors are: Red = 0F 16, Green = 69 16, Blue = F3 16  #000000 is black; #ffffff is white; gray colors have equal RGB values CMYK color printer

18 Jean Wang / CS1102 - Lec07 17 Image Compression  Similar to audio, image compression can be lossless or lossy  Lossless – the decompressed image is 100% identical to the original one  E.g., RLE (run-length encoding), compresses a sequence of same color pixels into a "Color + Number of Pixel" format  E.g., use color indexes to represent color values using less number of bits  Lossy – the decompressed image is not identical to the original one, but the minor loss of quality is acceptable  E.g., we can reduce the color space to the most common colors in the image Original Quality (424KB)50% Quality (15KB)10% Quality (5KB)

19 18 Vector Image  Vector images (also called object-oriented images)  Images are represented by a collection of points, lines and other geometric objects  Instead of storing the pixel color values, a vector graphic file contains the instructions for the computer to create the shape, size, position, and color for each object in an image  Compared to bitmap images  Vector images resize better; the objects change proportionally and maintain their smooth edges  Vector images require less storage space  It is easier to edit an object in a vector graphic  BUT, vector images are not usually as realistic as bitmaps  Vector images need a rasterization step for output to monitor or printer  Tracing software is needed to convert bitmaps to vector graphics (very hard!) Jean Wang / CS1102 - Lec07

20 19 Graphics File Formats File FormatImage TypeDescription BMPUncompressed bitmap format A graphics format native to Microsoft Windows. BMP is widely used on PCs for icons, buttons and other controls. The BMP file format supports up to 24-bit depth color. TIFFUncompressed bitmap format TIFF is bitmap format defined in 1986 by Microsoft and Aldus (now part of Adobe) and widely used on both Macs and PCs for high-resolution scanned images and digital photos for desktop publishing JPEG or JPGLossy compressed bitmap format JPEG is bitmap format popularly used on the Web. It can compress TrueColor bitmap images with various compression ratio GIFLossless compressed bitmap GIF is also a bitmap format popularly used on the Web. GIF images can only contain 256 or fewer colors. GIF format also supports transparency and animation. PNGLossless compressed bitmap PNG is a format designed to improve the GIF format, supporting 24-bit depth color and transparency. Unlike JPEG, PNG compressed bitmap data without losing any data. PNG itself does not support animation. WMF (Windows MetaFile) Vector graphic formatThe native vector graphics file format in Windows. They can also hold bitmaps and text. WMF files are made up of actual Windows drawing commands which results in an efficient format that renders illustrations very quickly. SVG (Scalable Vector Graphics ) Vector graphic formatA vector graphics format from the W3C for the Web that is expressed in XML. Introduced in 2001, SVG was designed to become the standard vector format just as GIFs and JPEGs have become the standard bitmaps for the Web. Jean Wang / CS1102 - Lec07

21 20 Jean Wang / CS1102 – Lec07 photo.jpg, 2.4M.bmp, mono, 1.7M.bmp, 8-bit colors, 13.7M.gif, 3.7M.png, 32-bit colors, 22.9M.tif, 35.7M

22 Jean Wang / CS1102 - Lec07 21 3D Graphics Basics  2D v.s. 3D pictures  Like vector graphics, 3D graphics are stored as a set of instructions for computers to create geometric objects at run-time  A picture that has or appears to have height, width and depth is three- dimensional (or 3-D).  A picture that has height and width but no depth is two-dimensional (or 2-D). Image from

23 Jean Wang / CS1102 - Lec07 22 3D Graphics Basics (cont'd)  To compose a 3-D graphics, thousands or millions of small shapes are put together into a structure called a wireframe to make the images look as though they have the smooth curves  Rendering covers a wireframe with surface color and texture  Ray tracing adds light and shadows to a 3-D image

24 Make 3D Movies -two synchronized video streams

25 3D (stereo) images Left image Right image

26 A B - Basic principle of 3D from stereo Find correspondence! Construct 3D models: Stereo Vision Parallel cameras

27 Triangulation. - Basic principle of 3D from stereo A B => Problem: it is hard to find the correspondence!! Find correspondence Passive: Stereo Vision


29 Lec07 - Digital Media - Digital Video

30 29 Digital Video  A video is just a sequence of images (frames) showing one after another very quickly  Two most common video signal standards  PAL (Phrase Alternating Line)  25 frames per second (fps)  Frame resolution: VCD (352x288), DVD (704x576) pixels  NTSC (National Television System Committee)  30 frames per second (fps)  Frame resolution: VCD (352x240), DVD (704x480) pixels  For VCD/DVD video, color depth typically is 24 bits  HDTV provides even higher frame resolution:  720p: (1280x720)  1080p: (1920x1080) (so-called “ Full High-Definition ” )  Ultra-HDTV: (7680x4320) (experimental, begin in 2015) Jean Wang / CS1102 - Lec07

31 30 Video Compression  Video compression is similar to repeating image compression on each frame.  Additionally, video compression software (video codec) can also makes use of the similarities between frames, or reducing the frame rate  The video compression is usually lossy  E.g., MPEG video/audio encoding standards

32 31 Streaming Video  Streaming is the transfer of video data in a continuous stream over the Web that allows the user to play it before the entire file is transferred The Streaming Concept The client side stores a few seconds of video in the memory buffer before it starts sending it to the screen and speakers. Throughout the session, it continues to receive video data ahead of time from the server. Jean Wang / CS1102 - Lec07

33 32 Video Format File Extension PlatformDescription AVI (Audio Video Interleave).aviPCA common format for storing digital clips from video cameras; used for desktop video on PC platform QuickTime Movie.movPC, Mac, Unix, Linux One of the most popular formats for desktop and streaming Web videos, requiring Apples Quicktime Movie Player MPEG (Moving Pictures Experts Group).mpg or.mpeg PC, Mac, Unix, Linux Versions include MPEG1, MPEG2, and MPEG4; used for desktop video, PDA video, and streaming Web video RealMedia.rmPC, Mac, Unix, Linux Produced by RealNetworks company, a popular format for streaming Web videos WMV (Windows Media Video).wmvPCOffers different compression options for high- quality videos; used for desktop video, PDA video, and streaming Web video VOB (Video Object).vobStandalone DVD player, PC, Mac, Linux Industry-standard format for standalone DVD players Flash Video.flv or.swfPC. MacPopular for Web-based video; requires Adobe Flash Player

34 Lesson Summary  Music, voice and sound effects can all be recorded and digitally stored as waveform audio, where the amplitude of the sound waves are sampled at small intervals and stored as binary numbers. Popular waveform audio file formats include WAV, MP3, RA, WMA and AAC  Unlike waveform digital audio, MIDI music is artificially created and contains information about how the sound should be created. MIDI files usually has an extension of.mid.  A bitmap image is composed of a grid of pixels, and the color of each pixel is stored as binary number. Popular bitmap file formats include BMP, TIFF, GIF, JPEG and PNG  A vector image consists of a set of instructions for creating the picture. Popular vector file formats include WMF, and SVG 33 Jean Wang / CS1102 - Lec07

35 34 Lesson Summary (continued)  Like vector images, 3D graphics are also stored as a set of instructions  A video is composed of a series of still images, each of which is called a frame. Popular video file formats include AVI, MOV, MPEG, RM, WMA, VOB, FLV and SWF.  Videos can be transferred and played on the Web using streaming technique. The idea is to transfer the first segment of a video, begin to play it and then continue to transfer additional segments.  Data compression is the general term used to describe the process of compacting original data so that it requires less storage space.  Compression algorithms are design to compress text files, audio, image, and video  To measure the data reduction produced by a compression algorithm, compress ratio and bit rate are used.  Compression can be either lossless or lossy Jean Wang / CS1102 - Lec07

36 35 Reference [1] Wikipedia – Audio data compression  [2] HowStuffWorks – How MP3 Files Work?  [3] Wikipedia – Windows bitmap  [4] Understanding image file formats – by Bryan Chamberlain  [5] W3C - SVG Talk  [6] Wikipedia - Digital video  Jean Wang / CS1102 - Lec07

37 36 For you to explore after class  Lec07-Q1: what is the compression ratio when compressing a stereo CD song into the MP3 format with 128 kbit/s bit rate? (assuming 1k=10 3 )  Lec07-Q2: similar to audio, uncompressed video data need a large amount of storage space. F or a 30 seconds clip in PAL DVD format with stereo sound in a sample rate of 48 kHz, how many bytes are needed? (assuming 1k=10 3 )  Lec07-Q3: similar to audio, video compression can also be described in bit rate - the number of data bits per second. If we know that MPEG-2 codec in a DVD player has a bit rate up to 9.8 Mb/s (assuming 1M = 10 6 ), what is the compression ratio if the clip in above question is compressed in MPEG-2? Jean Wang / CS1102 - Lec07

38 Baseline Focal length Depth (in z direction)


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