Video (Fundamentals, Compression Techniques & Standards)
Hamid R. RabieeMostafa Salehi, Fatemeh Dabiran, Hoda Ayatollahi
Spring 2011
Digital Media Lab - Sharif University of Technology2
Outlines
² Frame Types
² Color
² Video Compression Techniques
² Video Coding Standards² H.261
² H.263
² MPEG Family
Digital Video
² Video; a multi-dimensional signal
² Video is a sequence of 2D images called frames. Digital video is digitized
version of a 3D function f(x,y,t) .
3
Frame N-1
Frame 0
time
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Frame Types
² I-frames are coded without reference to other frames. Serve as reference
pictures for predictive-coded frames.
² P-frames are coded using motion compensated prediction from a past I-
frame or P-frame.
² B-frames are bidirectionally predictive-coded. Highest degree of
compression, but require both past and future reference pictures for motion
compensation.
² D-frames are DC-coded. Of the DCT coefficients only the DC coefficients
are present. Used in interactive applications like VoD for rewind and fast-
forward operations.
4
Frames display order : I B Frames display order : I B BB P B P B BB P B P B BB I I ……A GroupA Group--OfOf--PicturePicture
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I-frames Compression
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Motion Compensation
² Exploits temporal redundancy in video frames
² Prediction:² Assumes that “locally” current frame can be modeled as a translation of a previous
frame
² Displacement need not be the same everywhere in the frame à encode motion
information properly for accurate reconstruction
² Bi-directional Interpolation:² High degree of compression
² Areas just uncovered are not predictable from the past, but can be predicted from the future
² Effect of noise and errors can be reduced by averaging between past and future references
² Frequency of B-frames: increasing the frequency of B-frames
² Improves compression efficiency, but …
² Decreases the correlation between the B-frame and the references, as well as between the references
à reasonable to space references by 1/10th of a second
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Motion Compensation
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Motion Compensation
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P-frame Compression
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Why do we need B-frames?
² Bi-directional prediction works better than only using previous frames
when occlusion occurs.
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For this example, the prediction from nextframe is used and the prediction from previousframe is not considered.
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B-frames Compression
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B-frame Advantage
² B-frames increase compression.
² Typically use twice as many B frames as I+P frames.
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B-frame Disadvantages
² Computational complexity.
² More motion search, need to decide whether or not to average.
² Increase in memory bandwidth.
² Extra picture buffer needed.
² Need to store frames and encode or playback out of order.
² Delay
² Adds several frames delay at encoder waiting for need later frame.
² Adds several frames delay at decoder holding decoded I/P frame, while
decoding and playing prior B-frames that depend on it.
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Color
² Human Eye has receptors for brightness (in low light), and separate
receptors for red, green, and blue.
² Can make any color we can see by mixing red, green and blue light in
different intensities
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Color TV
² Original TV standards were black and white.
² AM: Amplitude of signal determines brightness.
² How to add color without changing TV transmitters, and in such a way that
it’s compatible with existing B&W TVs?
² Add a high frequency subcarrier in band within B&W TV signal.
² Not noticeable on B&W TV - would show as high frequency pattern, but human eye
can’t really see this well.
² Modulate the phase of the sub carrier to indicate the color.
² Problem: how to calibrate the absolute phase.
² Get this wrong, and the colors display incorrectly.
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NTSCNational Television Standard Committee
² Introduced in 1953 (in US)
² Used in US, Canada, Japan
² 30 frames per second (Actually 29.97)² Interlaced (even/odd field lines), so 60 fields per second.
² Same as 60Hz AC power in these countries
² 525 lines² Picture only on 480 of these => 640x480 monitors
² Rest are the vertical rescan.
² Aspect ratio is 4:3
² Needs color calibration
² Uses a color burst signal at start of each line, but needs TV to be adjusted relative to
this. “NTSC = Never Twice Same Color”
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PALPhase Alternating Line
² Introduced in 1967 (by Walter Bruch in Germany)
² PAL-I(UK), PAL-B/G (much of Europe), PAL-M (Brazil) …² Differ mainly in audio subcarrier frequency.
² 25 frames per second² Interlaced (even/odd field lines), so 50 fields per second.
² Same as 50Hz AC power in these countries
² 625 lines² Only 576 lines used for picture.
² Rest are vertical retrace, but often carry teletext information.
² Color phase is reversed on every alternate line.² Originally human eye would average to derive correct color.
² Now TV sets auto calibrate to derive correct color.
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SÉCAMSéquentiel Couleur Avec Mémoire
² Introduced in 1967 (in France)
² “System Essentially Contrary to American Method”
² Used in France, Russia, Eastern Europe…
² 625 lines, 25 fps interlaced, like PAL
² Uses FM modulation of subcarrier.
² Red-Luminance difference on one line
² Blue-Luminance difference on next line
² Uses a video line store to recombine the two signals
² Vertical color resolution is halved relative to NTSC and PAL.
² Human eye is not sensitive to lack of spatial color information.
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Colorspace Representations
² RGB (Red, Green, Blue)
² Basic analog components (from camera/to TV tube)
² YPbPr (Y, B-Y, R-Y)
² Color space derived from RGB used in component video. Y= Luminance, B =
Blue, R = Red
² YUV
² Similar to YPbPr but scaled to be carried on a composite carrier.
² YCbCr Digital representation of YPbPr colorspace (8 bit, twos complement)
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Color System in Video
² YUV was used in PAL (an analog video standard) and also for digital
video.
² Y is the luminance component (brightness)
Y = 0.299 R + 0.587 G + 0.144 B
² U and V are color components
U = B – Y
V = R - Y
Y U V21 Digital Media Lab - Sharif University of Technology
RGB vs. YUV
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YUV Formats
² YUV 4:4:4
² 8 bits per Y,U,V channel (no chroma down sampling)
² YUV 4:2:2
² 4 Y pixels sample for every 2 U and 2V
² 2:1 horizontal down sampling, no vertical down sampling
² YUV 4:2:0
² 2:1 horizontal down sampling
² 2:1 vertical down sampling
² YUV 4:1:1
² 4 Y pixels sample for every 1 U and 1V
² 4:1 horizontal down sampling, no vertical down sampling
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Color System in Video
² YIQ is the color standard in NTSC.
I Q
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Digital Video Formats
² Common Intermediate Format (CIF):
² This format was defined by CCITT (TSS) for H.261 coding standard
(teleconferencing and videophone).
² Several size formats:
² SQCIF: 88x72 pixels.
² QCIF: 176x144 pixels.
² CIF: 352x288 pixels.
² 4CIF: 704x576 pixels.
² Non-interlaced (progressive), and chrominance sub-sampling using 4:2:0.
² Frame rates up to 25 frames/sec
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Digital Video Formats
² Source Input Format (SIF):
² Utilized in MPEG as a compromise with Rec. 601.
² Two size formats (similar to CIF):
² QSIF: 180x120 or 176x144 pixels at 30 or 25 fps
² SIF: 360x240 or 352x288 pixels at 30 or 25 fps
² Non-interlaced (progressive), and chrominance sub-sampling using 4:2:0.
² High Definition Television (HDTV):
² 1080x720 pixels.
² 1920x1080 pixels.
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Uncompressed Video Data Rate
² Examples (CCIR 601)
² PAL signal: 864x625, YUV 4:2:2 20 bits/pixel, 25fps. 270Mb/s
² PAL signal: 864x625, YUV 4:2:2 16 bits/pixel, 25fps. 216Mb/s
² PAL video: 720x576, YUV 4:2:2 16 bits/pixel, 25fps. 166Mb/s (~1GByte/min)
² Firewire: 400Mb/s (800Mb/s)
² USB 2.0: 480Mb/s
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VIDEO COMPRESSION REVIEW
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Need for Compression
² Large data rate and storage capacity requirement
² Compression algorithms exploit:
² Spatial redundancy (i.e., correlation between neighboring pixels)
² Spectral redundancy (i.e., correlation between different frequency spectrum)
² Temporal redundancy (i.e., correlation between successive frames)
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600 MB/image180x180 km230 m2 resolution
Satelliteimagery
30 Mbytes/s30 frames/s,640x480 pixels,3 bytes/pixel
NTSC video
Requirement for Compression Algorithms
² Objectives² Minimize the complexity of the encoding and decoding process
² Ensure a good quality of decoded images
² Achieve high compression ratios
² Other general requirements² Independence of specific size and frame rate
² Support various data rates
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Classification of Compression Algorithms
² Lossless compression² Reconstructed image is mathematically equivalent to the original image
(i.e., reconstruction is perfect)
² Drawback: achieves only a modest level of compression (about a factor of
5)
² Lossy compression² Reconstructed image demonstrates degradation in the quality of the image
àthe techniques are irreversible
² Advantage: achieves very high degree of compression (compression ratios
up to 200)
² Objective: maximize the degree of compression while maintaining the
quality of the image to be virtually lossless
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Compression Techniques: Fundamentals
² Entropy encoding
² Ignores semantics of input data and compresses media streams by regarding
them as sequences of bits
² Examples: run-length encoding, Huffman encoding, ...
² Source encoding
² Optimizes the compression ratio by considering media specific characteristics
² Examples:
² Predictive coding: e.g., DPCM
² Layered coding: e.g., bit-plane coding, sub-sampling
² Transform coding: e.g., DCT, FFT, Wavelet, ...
² Most compression algorithms employ a hybrid of the above techniques
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Entropy Coding
² Run-length encoding
α α α α →4α
² Huffman encoding
² Employ variable length codes
² Assign fewer bits to encode more frequently occurring values à exploit the
statistical distribution of the values within an data sequence
² Share codebook between encoder and decoder
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Source Coding: Predictive Coding
² Basic technique
² Predict the value at a pixel by using the values of the neighboring pixels; and
² Encode the difference between the actual value and the predicted value
² Predictor
² Dimension of the predictor
² Order of the predictor: number of pixels used
² Example of a third order predictor:
² Huffman encoding of differential images
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Source Coding: Bit-plane Encoding
² An N *N image with k bits per pixel can be viewed as k N *N bit planes
² Encode each bit plane separately
² Advantages:
² Permits progressive transmission of encoded images (most significant bit plane
first - since it generally contains more information)
² Encoding should be carried out such that separate encoding yields better
performance than jointly encoding the bit Planes
² Gray codes are better suited as compared to binary encoding
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Source Coding: Transform Coding
² Subdivide an individual N x N image into several n x n blocks
² Each n x n block undergoes a reversible transformation
² Basic approach:
² De-correlate the original block à radiant energy is redistributed amongst only a
small number of transform coefficients
² Discard many of the low energy coefficients (through quantization)
² General requirements:
² Image independence
² Should be computationally efficient
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VIDEO CODING STANDARDS
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Video Coding Standards
² H.261
² H.263
² H.263+
² MPEG Family
² MPEG-1
² MPEG-2
² MPEG-4
² MPEG-7
² H.264
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H.261
² H.261 is an ITU video compression standard finalized in 1990.
² The basic scheme of H.261 has been retained in the newer video standards.
² H.261 supports bit rates at p*64 kbps (p=1..30).
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H.261
² In H.261, motion vectors are in the range [-15,15]x[-15,15]
² H.261 uses a constant step-size for different DCT coefficients.
² For DC coefficients
² For AC coefficients
Where scale = 1 .. 31
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Group of macroBlocks (GOB)
² To reduce the error propagation problem, H.261 makes sure that a
“group” of Macro-Blocks can be decoded independently.
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H.261 Bit Stream Syntax
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H.263
² H.263 is an improved video coding standard for video conferencing
through PSTN (public switching telecommunication network).
² Apart from QCIF and CIF, it supports SubQCIF, 4CIF and 16CIF.
² H.263 has a different GOB scheme.
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H.263 Motion Compensation
² The motion compensation in this standard is a bit different from the
MPEG method!
² The motion compensation in the core H.263 is based on one motion vector per
macroblock of 16 × 16 pixels, with half pixel precision.
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Motion vector prediction
Motion vector prediction for the border macroblocks
H.263+
² H.263 Ver. 2 (H.263+), ITU-T² Additional negotiable options for H.263.
² New features include: deblocking filter, scalability, slicing for network
packetization and local decode, square pixel support, arbitrary frame size,
chromakey transparency, etc…
² Arbitrary frame size, pixel aspect ratio (including square), and picture clock
frequency
² Advanced INTRA frame coding
² Loop de-blocking filter
² Slice structures
² Supplemental enhancement information
² Improved PB-frames
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MPEG
² Moving Picture Experts Group
² JPEG does not exploit temporal (i.e., frame-to-frame) redundancy present in all video
sequences
² MPEG exploits temporal redundancy
² MPEG requirements:² Random access
² Fast searches - both forward and reverse
² Reverse playback
² Audio-video synchronization
² Robustness to errors
² Low encoding/decoding delay
² Editability
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MPEG Family
² MPEG-1
² Similar to H.263 CIF in quality
² MPEG-2
² Higher quality: DVD, Digital TV, HDTV
² MPEG-4/H.264
² More modern codec.
² Aimed at lower bitrates.
² Works well for HDTV too.
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MPEG-1 Video
² MPEG-1 was approved by ISO and IEC in 1991 for “Coding of Moving
Pictures and Associated Audio for Digital Storage Media at up to about
1.5Mbps”.
² MPEG-1 standard is composed of
² System
² Video
² Audio
² Conformance
² And Software
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MPEG-1 Standard - An Overview
² Two categories: intra-frame and inter-frame encoding
² Contrasting requirements: delicate balance between intra- and inter-
frame encoding
² Need for high compression à only intra-frame encoding is not sufficient
² Need for random access à best satisfied by intra-frame encoding
² Overview of the MPEG algorithm:² DCT-based compression for the reduction of spatial redundancy (similar to
JPEG)
² Block-based motion compensation for exploiting the temporal redundancy
² Motion compensation using both causal (predictive coding) and non causal
(interpolative coding) predictors
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Exploiting Temporal RedundancyThree types of frames in MPEG-1
² I-frames:
² Intra-coded frames, provide access
points for random access – yield
moderate compression
² P-frames:
² Predicted frames are encoded with
reference to a previous I or P frame
² B-frames:
² Bi-directional frames encoded using
the previous and the next I/P frame
² Achieves maximum compression
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Example
² The figure illustrates the relationship between these three types of picture.
Since B-pictures use I and P-pictures as predictions, they have to be coded
later. This requires reordering the incoming picture order, which is
carried out at the preprocessor.
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Motion Representation
² 16 x 16 blocks used as motion-compensation units (referred to as macro blocks)² Macro block size selected based on the tradeoff between the gain due to motion compensation and
the cost of coding motion information
² Types of macro blocks:² Intra, forward-predicted, backward-predicted, average
² Two types of information are maintained:² Motion vector:
² The difference between the spatial locations of the macro blocks
² One motion vector for forward/backward predicted blocks, and two vectors for average blocks
² Adjacent motion vectors typically differ only slightly encode them using differential encoding
techniques (e.g., DPCM)
² Difference between the macro block being encoded and its predictor block(s) - encode the difference
using DCT-based transform coding techniques
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Motion Estimation
² Block-matching techniques employed for motion estimation
² Motion vector obtained by minimizing the mismatch between the block
being encoded and its predictor
² Exhaustive search for such a block yield good results – but the complexity
can be prohibitive
² Tradeoff between the quality of the motion vector versus the complexity of
the motion estimation process is left to the implementer
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Difference of MPEG-1 with H.261
² Picture formats (SIF vs. CIF)
² GOB structure
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Slices in MPEG-1
Difference of MPEG-1 with H.261 (cont)
Intra-coding quantization table Inter-coding quantization table
Intra mode:
Inter mode:
Scale=1..31
(the prediction error is like noise and their DCT coefficients are quite “flat”. We can use a uniform quantization table.)
MPEG-1 uses different quantization tables for I and P or B frames.
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Difference of MPEG-1 with H.261 (cont)
² Sub pixel motion estimation in MPEG-1.
² Motion range up to 512 pixels.
² MPEG adds another layer called “Group Of Pictures” (GOP) to allow
random video access.
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MPEG-1 Video Stream
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MPEG-2
² MPEG-2 profiles and levels:
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Profiles and Levels in MPEG-2
Scalable Layered Coding
² Need for Hierarchical Coding/Scalable Compression
² Facilitate access to images at different quality levels or resolutions
² Progressive transmission:
² Transmit image information in stages; at each stage, the reconstructed image is
progressively improved
² Motivated by the need for transmitting images over low bandwidth channels
² Permits progressive transmission to be stopped either if an intermediate version is of
satisfactory quality or the image is found to be of no interest
² Examples: multimedia databases, tele-browsing, etc.
² Multi-use environments:
² Support a number of display devices with differing resolutions
² Optimizes utilization of storage server and network resources
² Example: video-in-a-window archical Coding/Scalable Compression
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Scalability
² SNR scalability² Base layer uses rough quantization, while enhancement layers encode the
residue errors.
² Spatial scalability² Base layer encodes a small resolution video; enhancement layers encode the
difference of bigger resolution video with the “un-sampled” lower resolution
one.
² Temporal scalability² Base layer down-samples the video in time; enhancement layers include the rest
of the frames.
² Hybrid scalability
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Scalability Example
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Spatial Scalability
SNR Scalability
MPEG-2 vs. MPEG-1
² Sequence layer:
² progressive vs. interlaced
² More aspect ratios (e.g. 16x9)
² Syntax can now signal frames sizes up to 16383x16383
² Pictures must be a multiple of 16 pixels
² MPEG-2 can use a modified zig-zag for run-length encoding of the
coefficients:
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MPEG-2 vs. MPEG-1
² Picture Layer:
² All MPEG-2 motion vectors are always half-pixel accuracy
² MPEG-1 can opt out, and do one-pixel accuracy.
² DC coefficient can be coded as 8, 9, 10, or 11 bits.
² MPEG-1 always uses 8 bits.
² Optional non-linear macroblock quantization, giving a more dynamic step size
range:
² 0.5 to 56 vs. 1 to 32 in MPEG-1.
² Good for high-rate high-quality video.
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Interlacing
² Although MPEG-2 only codes full frames (both fields), it support both
field prediction and frame prediction for interlaced sources.
² The current uncompressed frame has two fields.
² Can do the motion search independently for each field.
² Half the lines use one motion vector and half use the other to produce the
reference block.
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MPEG-4
² ISO/IEC designation 'ISO/IEC 14496’: 1999
² MPEG-4 Version 2: 2000
² Aimed at low bitrate (10Kb/s)
² Can scale very high (1Gb/s)
² Based around the concept of the composition of basic video objects into a
scene.
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MPEG-4
² Initial goal of MPEG-4
² Very low bit rate coding of audio visual data.
² MPEG-4 (at the end)
² Officially up to 10 Mbits/sec.
² Improved encoding efficiency.
² Content-based interactivity.
² Content-based and temporal random access.
² Integration of both natural and synthetic objects.
² Temporal, spatial, quality and object-based scalability.
² Improved error resilience.
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Audio-Video Object
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MPEG-4 Standard
² Defines the scheme of encoding audio and video objects
² Encoding of shaped video objects.
² Sprite encoding.
² Encoding of synthesized 2D and 3D objects.
² Defines the scheme of decoding media objects.
² Defines the composition and synchronization scheme.
² Defines how media objects interact with users.
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MPEG-7 Standard
² (2001) MPEG7, ISO “Content Representation for Info Search”
² Specify a standardized description of various types of multimedia
information. This description shall be associated with the content itself, to
allow fast and efficient searching for material that is of a user’s interest.
² Mpeg-7
² Independent of the coding format of the media & physical location of the
media
² Facilitates searching for media content with ease
² Mpeg-7 supports
² text-based queries
² complex content-based queries.
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Structure of the standard
² Mpeg-7 standardizes a representation of meta-data
² Media content has metadata
² Metadata provides context for data
² Normative representations and semantics of metadata is common in
MPEG-7
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MPEG-7 Applications
² Storage and retrieval of audiovisual databases (image, film, radio archives)
² Broadcast media selection (radio, TV programs)
² Surveillance (traffic control, surface transportation, production chains)
² E-commerce and Tele-shopping (searching for clothes / patterns)
² Remote sensing (cartography, ecology, natural resources management)
² Entertainment (searching for a game, for a karaoke)
² Cultural services (museums, art galleries)
² Journalism (searching for events, persons)
² Personalized news service on Internet (push media filtering)
² Intelligent multimedia presentations
² Educational applications
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Why do we need MPEG-7 ?
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H.264 (MPEG-4, Part 10)
² MPEG-4, Part 10 is also known as H.264.
² Advanced video coding standard, finalized in 2003.
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H.264 vs. MPEG-2
² Multi-picture motion compensation.² Can use up to 32 different frames to predict a single frame.
² B-frames in MPEG-2 only code from two.
² Variable block-size motion compensation² From 4x4 to 16x16 pixels.
² Allows precise segmentation of edges of moving regions.
² Quarter-pixel precision for motion compensation.
² Weighted prediction (can scale or offset predicted block)² Useful in fade-to-black or cross-fade between scenes.
² Spatial prediction from the edges of neighboring blocks for "intra“ coding.
² Choice of several more advanced context-aware variable length coding schemes (instead
of Huffman).
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H.264 Performance
² Typically half the data rate of MPEG-2.
² HDTV:
² MPEG-2: 1920x1080 typically 12-20 Mbps
² H.264: 1920x1080 content at 7-8 Mbps
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H.264 Usage
² Pretty new, but expanding use.
² Included in MacOS 10 (Tiger) for iChat video conferencing.
² Used by Video iPod.
² Adopted by 3GPP for Mobile Video.
² Mandatory in both the HD-DVD and Blu-ray specifications for High Definition
DVD.
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Video Standards Applications
² H.261, ITU-T
² Designed to work at multiples of 64 kb/s (px64).
² MPEG-1, ISO “Storage & Retrieval of Audio & Video”
² Main application is CD-ROM based video (~1.5 Mb/s).
² MPEG-2, ISO “Digital Television”
² Main application is video broadcast (DirecTV, DVD, HDTV).
² Typically operates at data rates of 2-3 Mb/s and above.
² H.263, ITU-T
² Evolution of all of the above.
² Targeted low bit rate video <64 kb/s. Works well at high rates, too.
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Video Standards Applications
² H.263 Ver. 2 (H.263+), ITU-T
² Additional negotiable options for H.263.
² MPEG-4, ISO “Multimedia Applications”
² Support for multi-layered, non-rectangular video display
² MPEG7, ISO “Content Representation for Info Search”
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Next Session
NGN
79
80
Any Question
Thank you!Winter 2011
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