Video
description
Transcript of Video
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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
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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
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In the camera – DV + Firewire In the computer – video capture card Digitization in camera (DV) means
less noise Less noise allows better compression
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Play back a video stream as it arrives over a network (like broadcast TV), instead of downloading an entire video clip and playing it from disk (like renting a DVD)
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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
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Digital video devices must conform to standards
Digital standards must maintain compatibility with older analogue standards for broadcast TV
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Required for TV, so encountered in captured footage
Each frame is divided into two fields Field 1: odd lines; Field 2: even lines Fields are transmitted one after the
other Frame is built out of the interlaced
fields
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PAL (Phase Alternating Line)• Western Europe, Australia & New Zealand,
China,… NTSC (National Television Standards
Committee)• North America, Japan, Taiwan, parts of
South America,…
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SECAM (Séquential Couleur avec Mémoire)• France and former Soviet Union• Standard only used for transmission• Uses PAL cameras etc
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PAL• Frame has 625 lines, 576 are picture• 25 frames (50 fields) per second
NTSC• Frame has 525 lines, 480 are picture• 29.97 frames (59.94 fields) per second
(Often quoted as 30 frames per second)
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Digital video standard, properly called Rec. ITU-R BT.601
720 luminance samples (Y), 2x360 colour difference samples (B−Y and R−Y) per line
PAL 720x576 pixels; NTSC 720x480 pixels
Pixels are not square
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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 just over 20MB per second
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Consumer format, also known as mini-DV
DVCAM, DVPRO use different tape formats, but generate the same data stream
4:1:1 chrominance sub-sampling Data rate constant 25Mbits per
second Compression ratio 5:1
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PAL DV 4:2:0 chrominance sub-sampling
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NTSC DV 4:1:1 chrominance sub-sampling
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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
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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: profile@level, e.g. MP@ML
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MPEG-2 Main Profile at Main Level (MP@ML) used for DVD video• CCIR 601 scanning• 4:2:0 chrominance sub-sampling• 15 Mbits per second• Most elaborate representation of MPEG-2
compressed data
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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
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Visual Simple Profile (SP), suitable for low bandwidth streaming over Internet
Visual Advanced Simple Profile (ASP) suitable for broadband streaming
SP@L1 (Level 1 of Simple Profile), 64 kbps, 176x144 pixel frame
ASP@L5, 8000 kbps, full CCIR 601 frame
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Spatial (intra-frame) 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
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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 decompressed then recompressed
Ideally, work with uncompressed video during post-production
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Key frames are spatially compressed only• Key frames often regularly spaced (e.g. every
12 frames) Difference frames only store the
differences between the frame and the preceding frame or most recent key frame
Difference frames can be efficiently spatially compressed
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Purely spatial compression Apply JPEG to each frame Used by most analogue capture
cards No standard, but MJPEG-A format
widely supported
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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
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If little or no difference between fields (almost static frame), apply DCT to block containing alternate 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
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Shuffling• Construct video segments 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
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Spatial compression based on quantization and coding of DCT coefficients
Temporal compression based on motion compensation• Record displacement of object plus changed
pixels in area exposed by its movement
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I-pictures purely intra-frame compressed
P-pictures ‘predictive’• Difference frames based on earlier I- or P-
pictures B-pictures ‘bi-directionally predictive’
• Difference frames based on preceding and following I- or P-pictures
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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
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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
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Standard defines an encoding for multimedia streams made up of different sorts of object – video, 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
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Refinement of MPEG-1 compression• I-pictures compressed by quantizing and
Huffman coding DCT coefficients• Improved motion compensation leads to
better quality than MPEG-1 at same bit rates
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Simple Profile• P-pictures only• Efficient decompression, suitable for PDAs
etc Advanced Simple Profile (ASP)
• B-pictures• Global Motion Compensation• Sub-pixel motion compensation
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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 recent, higher quality and better compression ratios than other two
All three based on vector quantization Quality of all three inferior to MPEG-4
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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
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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
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Changing or adding to the material• Most changes are generalizations 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
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Compromises required to bring resource requirements of video 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 problem on computer)• Reduce colour depth
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