Introduction to H.264 / AVC
Video Coding Standard
Multimedia Systems
Sharif University of Technology
November 2008
Scope of video coding standardization
Evolution of video coding standards of ITU-T
VCEG and ISO/IEC MPEG.
H.264/AVC
• H.264 exploits the same video compression principles as its
predecessors
• Simulation results have shown that it has substantial superiority of
video quality over that achieved by H.263++ and MPEG-4.
• H.264 offer significantly higher quality levels for the same bit rates
Rate-distortion curve of H.264 codec compared with its predecessors,
Foreman QCIF at 10 fps
Selected new features in H.264
• Advanced intra-coding
• Enhanced motion estimation with variable block
size
• Multiple reference picture
• Integer block transform
• Improved deblocking filter
• Entropy coding
• H.264 Error Resilient Coding
H.264 advanced intra-coding (Cont.)
• Efficient video encoders mainly use inter-frame prediction,
• Use of intra-frame coding for parts of the picture is necessary to
prevent error propagation
• But intra-frame coding generates a large bit rate, and hence in order
for H.264 to be efficient, special attention is paid to intra-frame
coding.
• H.264 takes advantage of correlations between
neighboring blocks to achieve better compression in
intra-coding.
H.264 advanced intra-coding (Cont.)
• Every intra 16 X 16 pixel MB in a picture is first predicted in an
appropriate mode from the already coded and re-constructed
samples of the same picture.
H.264 advanced intra-coding (Cont.)
• There are nine advanced intra-prediction modes for the samples
when the MB is partitioned into 4 X 4 blocks
Advanced inter-coding
• Inter-frame predictive coding is where H.264 makes most of its gain
in compression efficiency.
• Motion compensation (MC) on each 16 X 16 MB can be performed
with various block sizes and shapes
Multiple reference picture
• The H.264 standard also offers the option of using several previous
pictures for prediction.
• Every MB partition can have a different reference picture that is
more appropriate for that particular block.
– Increases the coding efficiency and produces a better subjective quality
– Improve the robustness of the bitstream to channel errors
Deblocking filter
• In H.264 codec every reconstructed picture, is filtered by default
using an adaptive deblocking filter.
• The filter removes visible block structures on the edges of the 4 X 4
blocks caused by block-based transform coding and motion
estimation
Transformation, Quantization, and entropy
coding
• H.264 employs a 4X4 integer transform
• The transform is an approximation of the DCT
– It has a similar coding-gain to the DCT transform.
– Since the integer transform has an exact inverse operation, there is no
mismatch between the encoder and the decoder which was a problem
in all DCT based codecs
Transformation, Quantization, and entropy
coding (Cont.)
• A quantization parameter is used for determining the quantization of
transform coefficients in H.264/AVC. The parameter can take 52
values.
• The quantized transform coefficients of a block generally are
scanned in a zig-zag fashion and transmitted using entropy coding
methods.
Transformation, Quantization, and entropy
coding (Cont.)
• Before transmission, the generated data of all types are entropy
coded.
• H.264 supports two different methods of entropy coding
– context adaptive variable length coding (CAVLC)
– context adaptive binary arithmetic coding (CABAC)
Transformation, Quantization, and entropy
coding (Cont.)
• Their statistical distribution typically shows large values
for the low frequency part decreasing to small values
later in the scan for the high-frequency part.
• CAVLC
– The number of nonzero quantized coefficients (N) and the actual
size and position of the coefficients are coded separately.
• CABAC
– The efficiency of entropy coding can be improved further if the
Context-Adaptive Binary Arithmetic Coding (CABAC) is used
– Allows the assignment of a non-integer number of bits to each
symbol of an alphabet
H.264 Error Resilient Coding
• Spatial Error Propagation.
– The use of entropy coding means that every coded bit within a
slice requires the previous bits for its decoding. Hence, a single bit
error in the transmitted stream may destroy the whole remaining
coded bits of one slice
• Temporal error propagation
– Dependencies of consecutive pictures, as a result of inter-coding.
– Damage in one frame may propagate into many future frames,
even if their information is received without error.
H.264 Error Resilient Coding (Cont.)
• First, the coded video data are grouped in network
abstraction layer (NAL) units
– Each NAL unit can be considered as a packet that contains an integer
number of bytes including a header and a payload
– The header specifies the NAL unit type and the payload contains the
related data.
• Data Partitioning
H.264 Error Resilient Coding (Cont.)
• In a video sequence, each frame can be divided into several slices;
each containing a flexible number of MBs.
– In each slice, the arithmetic coder is aligned and the spatial predictions
are reset.
– Every slice in the frame is independently decodable, and therefore can
be considered as a re-synchronization point that prevents spatial
propagation of a probable error to the next slice.
H.264 Error Resilient Coding (Cont.)
• Intra-updates
• Multiple-reference selection
References
•
[1] Thomas Wiegand, Gary J. Sullivan, Gisle Bjontegaard, and Ajay Luthra “Overview of
the H.264 / AVC Video Coding Standard”, IEEE TRANSACTIONS ON CIRCUITS AND
SYSTEMS FOR VIDEO TECHNOLOGY, JULY 2003.
•
[2] Gary J. Sullivan, Pankaj Topiwala, and Ajay Luthra, “The H.264/AVC Advanced Video
Coding Standard: Overview and Introduction to the Fidelity Range Extensions”, Presented
at the SPIE Conference on Applications of Digital Image Processing XXVII Special Session
on Advances in the New Emerging Standard: H.264/AVC, August, 2004.