Compression Video and Standards
Hao Jiang Computer Science Department Boston College Oct. 23, 2007
H261
H.261 is an ITU video compression standard finalized in 1990. The basic scheme of H.261 has been retained in the newer video standards. H.261 supports bit rates at p*64 kbps (p=1..30).
Video Formats Supported by H261
I frames and P frames
In H.261, there are two types of compressed video frames. The first type of compressed frames are like JPEG compressed images. Such frames are denoted as Iframes (Intra-frames). The second type of frames are compressed using motion compensation schemes. These frames are denoted as P-frames (Predictive-frames).
Compression of I-frames
Motion Compensation
In H.261, motion vectors are in the range [-15,15]x[-15,15], e.g, p = 15.
P-frame Compression
Quantization
H.261 uses a constant step-size for different DCT coefficients. For DC coefficients
For AC coefficients
Where scale = 1 .. 31
The Encoder Diagram
Local Decoder 6 : Decoded video
The Decoder
Group of macroBlocks (GOB)
To reduce the error propagation problem, H.261 makes sure that a "group" of Macro-Blocks can be decoded independently.
H.261 Bit Stream Syntax
H.263
H.263 is an improved video coding standard for video conferencing through PSTN (public switching telecommunication network). Apart from QCIF and CIF, it supports SubQCIF, 4CIF and 16CIF. H.263 has a different GOB scheme.
H.263 Motion Compensation
The difference of MV with the median of surrounding MVs is encoded.
Supports sub-pixel motion estimation.
MPEG-1 Video
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". MPEG-1 standard is composed of
System Video Audio Conformance And Software
MPEG-1's video format is called SIF(Source Input Format)
352x240 for NTSC at 30f/s 352x288 for PAL at 25f/s
MPEG-1 Motion Compensation
MPEG-1 introduces a new type of compressed frame: the B-frame.
Why do we need B-frames?
Bi-directional prediction works better than only using previous frames when occlusion occurs.
For this example, the prediction from next frame is used and the prediction from previous frame is not considered.
Compression of B-frames
Difference of MPEG-1 with H.261
Picture formats (SIF vs. CIF) GOB structure
Slices in MPEG-1
Difference of MPEG-1 with H.261 (cont) MPEG-1 uses different quanzation tables for I and P or B frames.
Intra-coding quantization table
Inter-coding quantization table
Intra mode:
Scale=1..31
Inter mode:
(the prediction error is like noise
and their DCT coefficients are quite "flat". We can use a uniform quantization table.)
Difference of MPEG-1 with H.261 (cont)
Sub pixel motion estimation in MPEG-1. Motion range up to 512 pixels. MPEG adds another layer called "Group Of Pictures" (GOP) to allow random video access.
MPEG-1 Video Stream
MPEG-2
MPEG-2 profiles and levels:
Profiles and Levels in MPEG-2
Interlace Video Compression
Scalability
SNR scalability
Base layer uses rough quantization, while enhancement layers encode the residue errors.
Spatial scalability
Base layer encodes a small resolution video; enhancement layers encode the difference of bigger resolution video with the "un-sampled" lower resolution one.
Temporal scalability
Base layer down-samples the video in time; enhancement layers the include rest of the frames.
Hybrid scalability Data partitioning
MPEG-4
Initial goal of MPEG-4
Very low bit rate coding of audio visual data.
MPEG-4 (at the end)
Officially up to 10 Mbits/sec. Improved encoding efficiency. Content-based interactivity. Content-based and temporal random access. Integration of both natural and synthetic objects. Temporal, spatial, quality and object-based scalability. Improved error resilience.
Audio-Video Object
MPEG4 is based on the concept of media objects.
Audio Video Objects
A media object in MPEG4 could be
A video of an object with "shape". The speech of a person. A piece of music. A static picture. A synthetic 3D cartoon figure.
In MPEG4, a scene is composed of media objects based on a scene graph: Video background Music The bull scene video The walking person audio The car Video
MPEG-4 Standard
Defines the scheme of encoding audio and video objects
Encoding of shaped video objects. Sprite encoding. Encoding of synthesized 2D and 3D objects.
Defines the scheme of decoding media objects. Defines the composition and synchronization scheme. Defines how media objects interact with users.
Composition and Interaction
Video Coding in MPEG4
Support for 4 types of video coding:
Video Object Coding
For coding of natural and /or synthetic originated, rectangular or arbitrary shaped video objects.
Mesh Object Coding
For visual objects represented with a mesh structure.
Model-based Coding
For coding of a synthetic representation and animation of a human face and body.
Still Texture Coding
For wavelet coding of still textures.
Video Object Coding
Video Object (VO)
Arbitrarily shaped video segment that has a semantic meaning.
Video Object Plane (VOP)
2D snapshot of a VO at a particular time instance.
Coding of VOs: 3 "elements"
Shape
Rectangularly shaped VO. Arbitrarily shaped VO.
Motion Texture
Shape Coding
Transparent block Boundary block Shape coding: Bitmap image of a shape alpha plane Binary alpha plane. Gray...