PAL uncompressed. 768x576 pixels per frame. 31 MB per second 1.85 GB per minute. x 3 bytes per pixel (24 bit colour) x 25 frames per second

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2 PAL uncompressed 768x576 pixels per frame x 3 bytes per pixel (24 bit colour) x 25 frames per second 31 MB per second 1.85 GB per minute

3 NTSC uncompressed 640x480 pixels per frame x 3 bytes per pixel (24 bit colour) x 30 frames per second (approx) 26 MB per second 1.6 GB per minute

4 In the camera DV + Firewire In the computer vi ideo capture card Digitization in camera (DV) means less noise Less noise allows better compression

5 197 Play back a video stream as it arrives over a network (like broadcast TV), instead of downloading an entire video clip and playing it fr rom disk (like renting a DVD)

6 198 Start playing a downloaded clip as soon as enough of it has arrived Starts when the (estimated) time to download the rest is equal to the duration of the clip

7 199 Digital video devices must conform to standards Digital standards must maintain compatibility with o lder analogue standards for broadcast TV

8 200 Required for TV, so encountered in captured footage Each frame is divided into two fields Field 1: odd lines; Fi ield 2: even lines Fields are transmitted one after the other Frame is built out of the interlaced fields

9 199 PAL (Phase Alternating Line) Western Europe, Australia & New Zealand, China, NTSC (National Tele evision Standards Committee) North America, Japan, Taiwan, parts of South America,

10 199 SECAM (Séquential Couleur avec Mémoire) France and former Soviet Union Standard only used for transmission Uses PAL cameras etc

11 202 PAL Frame has 625 lines, 576 are picture 25 frames (50 fields) per second NTSC Frame has 525 lines, 480 are picture frames ( fields) per second (Often quoted as 30 frames per second)

12 Digital video standard, properly called Rec. ITU-R BT luminance samples (Y), 2x360 colour difference samples line PAL 720x576 pixels; pixels Pixels are not square (B Y and R Y) per NTSC 720x480

13 203 Used in CCIR 601 Twice as many Y samples as each of the colour difference samples Co-sited: same pixel is used for all three samples Reduces data rate to j ust over 20MB per second

14 204 DV Consumer format, also known as mini- DVCAM, DVPRO use different tape formats, but generat te the same data stream 4:1:1 chrominance sub-sampling Data rate constant 25Mbits per second Compression ratio 5:1

15 210 PAL DV 4:2:0 chrominance sub-sampling

16 210 NTSC DV 4:1:1 chrominance sub-sampling

17 ISO/IEC Motion Picture Experts Group Series of standards including MPEG-1 intended for video CD MPEG-2 used in DVD and broadcast MPEG-4 for low bitrate multimedia

18 Profiles define subsets of the features of the data stream Levels define parameters such as frame size and data rate Each profile may be implemented at one or more levels Notation: e.g.

19 205 MPEG-2 Main Profile at Main Level used for DVD video CCIR 601 scanning 4:2:0 chrominance sub-sampling 15 Mbits per second Most elaborate represe entation of MPEG-2 compressed data

20 Designed to support a range of multimedia data at bit rates from 10kbps to >1.8Mbps Applications from mobile phones to HDTV Video codec becoming popular for Internet use, is incorporated in QuickTime, RealMedia and DivX

21 205 Visual Simple Profile (SP), suitable for low bandwidth strea aming over Internet Visual Advanced Simple Profile (ASP) suitable for broadba and streaming (Level 1 of Simple Profile), 64 kbps, 176x144 pixel frame ASP@L5, 8000 kbps, full CCIR 601 frame

22 Spatial (intra-fram e) compression Compress each frame in isolation, treating it as a bitmapped image Temporal (inter-frame) compression Compress sequences of frames by only storing differences between them Always some compression because of sub-sampling

23 207 Image compression applied to each frame Can therefore be lossless or lossy, but lossless rarely produces sufficiently high compression ratios for volume of data Lossless compression implies a loss of quality if decompress ed then recompressed Ideally, work with uncompressed video during post-productio on

24 Key frames are spatially compressed only Key frames often regul larly spaced (e.g. every 12 frames) Difference e e frames on nly store the differences between the frame and the preceding frame or frame Difference frames can be efficiently i spatially compressed most recent key

25 Purely spatial compression Apply JPEG to each Used by most analogue capture cards No standard, but MJP PEG-A format widely supported frame

26 Starts with chrominance sub-sampling of CCIR 601 frame Constant data rate 25Mbits per second Higher quality than MJPEG at same rate Apply DCT, quantization, run-length and Huffman coding on zig-zag sequence like JPEG to 8x8 blocks of pixels

27 If little or no difference between fields (almost static frame) ), apply DCT to block containing alternatee lines from odd and even fields If motion between fields, apply DCT to two 8x4 blocks (one from each field) separately, leading to more efficient compression of frames with motion

28 Shuffling Construct video segme ents by taking 8x8 blocks from five different areas of the frame, to average amount of detail Calculate coefficients for whole video segment, making more efficient use of available bytes

29 Spatial compression based on quantization and cod ding of DCT coefficients Temporal compress ion based on motion compensation Record displacement of object plus changed pixels in area exposed by its movement

30 I-pictures purely intra-frame compressed P-pictures predictiv ve Difference frames based on earlier I- or P- pictures B-pictures bi-directionally predictive Difference frames base ed on preceding and following I- or P-pictures

31 Group of Pictures (GOP) Repeating sequence of I-, P- and B-pictures Always begins with an I-picture Display order frames in order they will be displayed Bitstream order re-ordered so that every P- or B-picture comes after frames it depends on, allowing reconstruction of the complete frames

32 214 Source Input Format (SIF) 4:2:0 chroma sub-sampled 352x240 pixel frame MPEG-1 compressed SIF video at 30 frames per second has data rate of 1.86Mbits per second (CD video 40mins of video at that rate) MPEG-1 can be scaled up to larger frames, but cannot handle interlacing

33 215 Standard defines an encoding for multimedia streams made up of different sorts of object video o, still images, animation, 3-D models Higher profiles divide a scene into arbitrarily shaped video objects each one may be compressed and transmitted separately, scene is composed at the receiving end by combining them SP and ASP restricted to rectangular objects, usually complete frames

34 215 Refinement of MPEG-1 compression I-pictures compressed by yquantizing and Huffman coding DCT coefficients Improved motion comp pensation leads to better quality than MPEG-1 at same bit rates

35 Simple Profile P-pictures only Efficient decompression, suitable for PDAs etc Advanced Simple Pro ofile (ASP) B-pictures Global Motion Compe nsation Sub-pixel motion compensation

36 Cinepak Longest established, high compression ratio, takes much longer to compress than to decompress Intel Indeo Similar to Cinepak, but roughly 30% faster compression Sorenson More recen nt, higher quality and better compression ratios than other two All three based on vec ctor quantization Quality of all three inferior to MPEG-4

37 216 Divide each frame into small rectangular blocks ( vectors ) Code Book collection of constant vectors representing typical patterns (edges, textures, flat colour, ) Compress by replacing each vector in image by index of vector from code book that most closely resembles it

38 Making a constructed whole from a collection of parts Selection, trimming and organization of raw footage Apply transitions (e.g. dissolves) between shots Combination of picture with sound No changes made to the footage itself

39 Changing or adding to the material Most changes are gene eralizations of image manipulation operations (e.g. colour correction, blurring and sharpening, ) Compositing combining elements from different shots into a composite sequence Animating elements and combining animation with live action

40 Compromises required to bring resource requirements of vide eo within capabilities of delivery media (e.g. networks) and low-end machines Reduce frame size (e.g. downsample to quarter frame) Reduce frame rate (12fps is OK for smooth motion, flicker not a pr roblem on computer) Reduce colour depth