Context-based Adaptive Binary Arithmetic Coding (CABAC) is the entropy coding module in the HEVC/H video coding standard. As in its predecessor. High Throughput CABAC Entropy Coding in HEVC. Abstract: Context-adaptive binary arithmetic coding (CAB-AC) is a method of entropy coding first introduced . Context-based Adaptive Binary Arithmetic Coding (CABAC) is a method of entropy coding which is widely used in the next generation standard of video coding.
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The design of CABAC has been highly inspired by our prior work on wavelet-based image and video coding. Each probability model in CABAC can take one out of different states with associated probability values p ranging in the interval [0.
Since the encoder can choose between the corresponding three tables of initialization parameters and signal its choice to the decoder, an additional degree of pre-adaptation is achieved, especially in the case of using small slices at low to medium bit rates. As a consequence of these important criteria within any standardization effort, additional constraints have been imposed on the design of CABAC with the result that some of its original algorithmic components, like the binary arithmetic coding engine have been completely re-designed.
CABAC is notable for providing much better compression than most other entropy encoding algorithms used in video encoding, and it is one of the key elements that provides the H. The design of CABAC involves the key elements of binarization, context modeling, and binary arithmetic coding.
Support of additional coding tools such as interlaced coding, variable-block size transforms as ccabac for Version 1 of H. However, in cases where the amount of data in the process of adapting to the true underlying statistics is comparably small, it is useful to provide some more appropriate initialization values for each probability model in order to better reflect its typically skewed nature.
CABAC has multiple probability modes for different contexts. Pre-Coding of Transform-Coefficient Levels Coding of residual data in CABAC involves specifically designed syntax elements czbac are different from those used in the traditional run-length pre-coding approach.
Context-Based Adaptive Binary Arithmetic Coding (CABAC) – Fraunhofer Heinrich Hertz Institute
The definition of the decoding process is designed to facilitate low-complexity implementations of arithmetic caabc and decoding. These aspects are mostly related to implementation complexity and additional requirements in terms of conformity and applicability. Other components that are needed to alleviate potential losses in coding efficiency when using small-sized slices, as further described below, were added at a later stage of the development.
For the latter, a fast branch of the coding engine with a considerably reduced complexity is used while for the former cwbac mode, encoding of the given bin value depends on the actual cabc of the associated adaptive probability model that is passed along with the bin value to the M coder – a term that has been chosen for the novel table-based binary arithmetic coding engine in CABAC. The other method specified in H. The arithmetic decoder is described in some detail in the Standard.
Context-adaptive binary arithmetic coding – Wikipedia
Usually the addition of syntax elements also affects the distribution hefc already available syntax elements which, in general, for a VLC-based entropy-coding approach may require to re-optimize the VLC tables of the given syntax elements rather than just adding a suitable VLC code for the new syntax element s. These elements are illustrated hev the main algorithmic building blocks of the CABAC encoding block diagram, as shown above.
CABAC is also difficult to parallelize and vectorize, so other forms of parallelism such as spatial region parallelism xabac be coupled with its use. In general, a binarization scheme defines a unique mapping of syntax element values to sequences of binary decisions, so-called bins, which can also be interpreted in terms of a binary code tree. By decomposing each syntax element value into a sequence of bins, further processing of each bin value in CABAC depends on the associated coding-mode decision, which can be either chosen as the regular or the bypass mode.
Context-adaptive binary arithmetic coding
From that time until completion of the first standard specification of H. However, in comparison to this research work, additional aspects previously largely ignored have been taken into account during the development of CABAC. The design of these four prototypes is based on a priori knowledge about the typical characteristics of the source data to be modeled and it reflects the aim to find a good compromise between the conflicting objectives of avoiding unnecessary modeling-cost overhead and exploiting the statistical dependencies to a large extent.
It generates an initial state value depending on the given slice-dependent hevx parameter SliceQP using a pair of so-called initialization parameters for each model which describes a modeled linear relationship between the SliceQP and the model probability p.
Context-Based Adaptive Binary Arithmetic Coding (CABAC)
Update the context models. Since CABAC guarantees an inherent adaptivity to the actually given conditional probability, there is no need for further structural adjustments besides the choice of a binarization or context model and associated initialization values which, as a first approximation, can be chosen in a canonical way by using the prototypes already specified in the CABAC design.
Video Coding for Next-generation Multimedia. Utilizing suitable context models, a given inter-symbol redundancy can be exploited by switching between different probability models according to already-coded symbols cavac the neighborhood of the current symbol to encode.
In this way, CABAC enables selective context modeling on a sub-symbol level, and hence, provides an efficient instrument for exploiting inter-symbol redundancies at significantly reduced overall modeling or learning costs. Choose a context model for each bin.
The remaining bins are coded using one of 4 further context models:. From Wikipedia, the free encyclopedia. Context-modeling for coding of binarized level magnitudes are based on the number of previously transmitted level magnitudes greater or equal to 1 within the reverse scanning path, which is motivated by the observation that levels with magnitude equal to 1 are statistical dominant at the end of the scanning path. For each block with at least one nonzero quantized transform coefficient, a sequence of binary significance flags, indicating the position of significant i.
The design of binarization schemes in CABAC is based on a few elementary prototypes whose structure enables simple online calculation and which are adapted to some suitable model-probability distributions. Binarization The coding strategy of CABAC is based on the finding that a very efficient coding of syntax-element values in a hybrid block-based video coder, like components of motion vector differences or transform-coefficient level values, can be achieved by employing a binarization scheme as a kind of preprocessing unit for the subsequent stages of context modeling and binary arithmetic coding.
Redesign of VLC tables is, however, a far-reaching structural change, which may not be justified for the addition of a single coding tool, especially if it relates to an optional feature only. Circuits and Systems for Video TechnologyVol.