Video CodingVideo Coding

TSBK01 Image Coding and Data Compression

Lecture 10

Jörgen Ahlberg

OutlineOutline

I. Colour coding

II. Moving images: From 2D to 3D?

III. Hybrid coding

IV. Video coding standards

Part I:Part I:Colour CodingColour Coding

The base colours of colour television are

– Red: 700 nm

– Green: 546 nm

– Blue: 435 nm

Three base colours enough tosynthesize any visible colour!

B

G

R

The Colour VectorThe Colour Vector

In this plane, theluminance Y = R+G+B = 1

The PAL coloursThe PAL colours

Y = 0.30B + 0.59G + 0.11B

Cr = 0.70R - 0.59G - 0.11B

Cb = - 0.30R - 0.59G + 0.89B

Y luminance; Cr, Cb chrominance

Matrix

R

G

B

Y

R-Y

B-Y

Change basis to YUV (almost the same as YCrCb).

– For more info on color spaces, see colour FAQ at www.poynton.com/Poynton-color.html

The Human Visual System perceives the luminance in higher resolution than the chrominance!

Subsample the colour components.

Digital Colour CodingDigital Colour Coding

YU V

4:2:0

Y U V

4:2:2

Part II:Part II:Coding of Moving ImagesCoding of Moving Images

Principle I - Extend known methods to 3D

Coding MethodCoding Method Prestanda (bpp)Prestanda (bpp) ComplexityComplexity Decoding Decoding complexitycomplexity

PCM 6 – 8 Low Low

VQ 0.5 – 2 Very high Low

Predictive 2 – 5 Low Low

Transform 0.5 – 1.5 High High

Subband/Wavelet

0.1 – 1.0 High High

Fractal 0.1 - 0.5 Very high Low

Extending 2D MethodsExtending 2D Methods

Predictive coding

– 3D predictors

– Motion compensated predictors

Transform coding

– 3D transforms

Subband coding

– 3D subband filters

BUT! The properties of the image signal are different BUT! The properties of the image signal are different in the temporal and the spatial domain!in the temporal and the spatial domain!

Thus:Thus:

Principle II:

Hybrid methods

Hybrid predictive/transform coding popular++

Part III:Part III:Hybrid CodingHybrid Coding

Combine predictive coding and transform coding.

Use predictive coding to predict the next frame in the sequence.

Use transform coding to code the prediction error.

Transform CodingTransform Coding

T Q VLC

T: TransformQ: QuantizerVLC: Variable Length Coder

Predictive CodingPredictive Coding

Q

Q-1

VLC

P

Q: QuantizerQ-1: Inverse quantizer (reconstructor)P: Predictor

Hybrid CodingHybrid Coding

T

T-1

Q

Q-1

VLC

P

Frame PredictionFrame Prediction

Intra-codedI-frame

Predictivelycoded

P-frames

Better prediction if it can compensate for motion!

Motion CompensationMotion Compensation

Motion Compensated Motion Compensated Hybrid CodingHybrid Coding

VLCME

ME: Motion estimation

TQ-1

TQ

P

VLC

TQ: Transform+ quantization

Motion CompensationMotion Compensation

Typically one motion vector per macroblock (4 transform blocks)

Motion estimation is a time consuming process

– Hierarchical motion estimation

– Maximum length of motion vectors

– Clever search strategies

Motion vector accuracy:

– Integer, half or quarter pixel

– Bilinear interpolation

Part IV:Part IV:Video Coding StandardsVideo Coding Standards

8 16 64 384 1.5 5 20

kbit/s Mbit/s

Very low bitrate Low bitrate Medium bitrate High bitrate

Mobilevideophone

Videophoneover PSTN

ISDNvideophone

Digital TV HDTVVideo CD

MPEG-4 MPEG-1 MPEG-2H.261H.263

StandardsStandards

H.26x

– Standards for real time communication like video telephony and video conferencing.

– Standardized by ITU.

MPEG

– Standards for stored video data like movies on CDs, DVDs, etc.

– Standardized by ISO.

H.261H.261 Standard for ISDN picture phones in 1990.

Motion compensation:

– One motion vector per macroblock.

– One macroblock = four 8£8 luminance blocks + two chrominance blocks (one U and one V).

– Motion vectors max 15 pixels long in each direction.

Format:

– CIF (352£288) or QCIF (176£144)

– 7.5 – 30 frames/s.

Bitrate: Multiple of 64 kbit/s (=ISDN) including audio.

Quality: Acceptable for small motion at 128 kbit/s.

H.263H.263

Standard for picture telephones over analog subscriber lines in 1995.

Format:

– CIF, QCIF or Sub-QCIF.

– Usually less than 10 frames/s.

Bitrate: Typically 20 – 30 kbit/s.

Quality: With new options as good as H.261 (at half the bitrate).

MPEGMPEG

Moving Pictures Expert Group – a committee under ISO and IEC.

Original plan:

– MPEG-1 for 1.5 Mbit/s (VideoCD)

– MPEG-2 for 10 Mbit/s (Digital TV)

– MPEG-3 for 40 Mbit/s (HDTV)

What happened:

– MPEG-1 for 1.5 Mbit/s (Video CD)

– MPEG-2 for 2 – 60 Mbit/s (TV and HDTV)

– MPEG-4, -7 and -21 for other things.

MPEG-1MPEG-1

ISO/IEC standard in 1991.

Target bitrate around 1.5 Mbit/s (Video CD).

Properties:

– Bi-directionally predictively coded frames (”B-frames”, see next slide).

– More flexible than H.261.

– Almost JPEG for intra frames.

Format:

– CIF

– No interlace.

– 24 – 30 frames/s.

MPEG Frame TypesMPEG Frame Types

I B PB B PB B PB B IB

Intra-codedI-frame

Predictivelycoded

P-frames

Bi-directionallypredictively

codedB-framesGroup of frames (GOF)

MPEG-coding of I-framesMPEG-coding of I-frames

Intracoded

8£8 DCT

Arbitrary weighting matrix for coefficients

Predictive coding of DC-coefficients

Uniform quantization

Zig-zag, run-level, entropy coding

MPEG-coding of P-framesMPEG-coding of P-frames

Motion compensated prediction from I- or P-frame.

Half-pixel accuracy of motion vectors, bilinear interpolation.

Predictive coding of motion vectors.

Prediction error coded as I-frame.

MPEG-coding of B-framesMPEG-coding of B-frames

Motion compensated prediction from two consecutive I- or P-frames.

– Forward prediction only (1 vector/macroblock).

– Backward prediction only (1 vector/macroblock).

– Average of fwd and bwd (2 vectors/macroblock).

Otherwise as P-frames.

MPEG-2MPEG-2 ISO/IEC standard in 1994.

Properties:

– Handles interlace (optimized for TV)

– Even more flexible than MPEG-1

Format:

– 352£288

– 704£576 (25 frames/s) or 720£480 (30 frames/s)

– 1440£1152 or 1920£1080 (HDTV)

Bitrate:

– 2 – 60 Mbit/s

– ~4 Mbits/s: Image quality similar to PAL / NTSC / SECAM.

– 18 – 20 Mbit/s: HDTV.

MPEG-2 (cont.)MPEG-2 (cont.)

Profiles:

– Simple profile without B-frames.

– Scaleable profiles.

Experience tells that:

– At 1.5 – 2 Mbit/s MPEG-2 is not better than MPEG-1.

– With manual interaction at the coding, good quality can be achieved at 3 – 4 Mbit/s.

– Problems with implementing the full standard has caused compatibility problems.

– Buffering and rate control hard problems.

MPEG-4MPEG-4 ISO/IEC standard in 1998, version 2 in 1999

Instead of frames as coding units, MPEG-4 use audio-visual objects

Focus is not primarily on compression, but on content-based functionality

Contains definitions of:

– Media object types (video, audio, text, graphics, ...)

– Parameters for describing the objects

– Bitstream syntax for the (compressed) parameters

– Scene description, file format, streaming, synchronization, ...

Allows mixing of media objects.

Parts of the MPEG-4 Parts of the MPEG-4 standardstandard

Part 1, Systems, contains

– The bitstream syntax and the the binary ”language” for scene description

– Computer graphics object descriptions

– Multiplexing, transport, ...

Part 2, Visual, contains

– Video coding

– Still image coding

– Texture coding, ...

Part 3, Audio, contains a toolbox of audio coders for different applications

...

Structure of an MPEG-4 Structure of an MPEG-4 DecoderDecoder

A/Vobject

Decoder

MUX

Com

posito

r

Bitstream Audio/Video scene

A/Vobject

Decoder

A/Vobject

Decoder

A video frame

Background VOP

VOP

VOP

MPEG-4 (Natural) VideoMPEG-4 (Natural) Video

Instead of frames: Video Object Planes

Coded with Shape Adaptive DCT

Alpha map

SA DCT

TQ: Transform+ quantization

TQ-1

TQ VLC

Predictor

MPEG-4 Video CodingMPEG-4 Video Coding

Motionestimation

Mux

VLC

VLCShapecoding

Synthetic/Natural Synthetic/Natural Hybrid CodingHybrid Coding

Mix traditional video with 2D/3D graphics

– Compose virtual environments

– Easy to add text, graphs, images, etc

High compression

Receive object from separate sources

– Use predefined or locally defined objects

Scaleability

– Progressive decoding

– Better terminal gives better quality.

Synthetic ObjectsSynthetic Objects

2D/3D graphics

– Lines, polygons

– Still images

– Image/video mapping on polygon meshes

VRML scenes and objects

Animated people

More on animation and virtual characters in Lecture 12!

Synthetic audio

More on natural and synthetic audio in Lecture 11!

Computer graphics generatedvirtual environment

Natural video object

Natural video objectmapped on 2D mesh

Still image or natural video objectmapped on animated 3D mesh

All mixed inthe decoder!!!

Virtual EnvironmentsVirtual Environments

Downloaded virtual environment

Different environments for different users

Simple change between environments

Synthetic environments are cheaper than real ones

Tools for Synthetic ObjectsTools for Synthetic Objects

Wavelet-based still image compression

– Scaleable quality and resolution

– Progressive decoding

– Can be mapped on 2D or 3D meshes

Compression of 2D and 3D meshes

– Mesh geometry and animation

– Transmit vertex coordinates and let the receiving terminal calculate the polygons

– A moving or still image can be mapped on the mesh (texture mapping).

More Tools for Synthetic More Tools for Synthetic ObjectsObjects

Face and Body Animation

Text-to-speech (TTS) interface

View-dependent scaleable texture

– Information about the users view position in a 3D scene is transmitted on a back-channel

– Only the necessary texture information is transmitted to the user

View-dependent Scaleable View-dependent Scaleable TextureTexture

Original texture

The texture is mapped on a surface

What the user sees

Other formatsOther formats

Microsoft, RealVideo, QuickTime, ...

All are variations of the hybrid coder used in MPEG-coders, with some extra features.

New StuffNew Stuff

ITU and ISO in cooperation:

H.264H.264==

MPEG-4 part 10MPEG-4 part 10

Finished in 2003.

H.264 / MPEG-4 part 10H.264 / MPEG-4 part 10

4£4 integer transform (approximating DCT).

Prediction of blocks of sizes up to 16£16.

Motion vectors for blocks of sizes 4£4 up to 16£16.

Up to 5 reference images for prediction.

Non-uniform qunatization.

Arithmetic coding of run-level pairs.

What about the sound?What about the sound?

MPEG-1

– Audio layer I, II and III (mp3).

MPEG-2

– Four channels, same codec as in MPEG-1.

– AAC (Advanced Audio Codec) added later.

MPEG-4

– AAC

– Two speech coders

– Structured audio

– And more...

More on audio codingin Lecture 11.

ConclusionConclusion

Color coding

– Change basis from RGB to YUV

– Colour components are compressed harder than the luminance

Moving image coding

– Hybrid coding: Motion compensated predictive coding and transform coding of the prediction error

– I-, P-, and B-frames

– Object-based coding (MPEG-4) mixing synthetic and natural audio & video

Conclusion (cont)Conclusion (cont)

Standards

– MPEG-1: Video CD

– MPEG-2: Digital TV

– MPEG-4: Multimedia

– H.261: ISDN videophone

– H.263: PSTN videophone

– H.264 / MPEG-4 part 10: Universal video

That was the last slide!