When you use your mobile phone or computer to watch movies, do you watch them in HD or SD? I think as long as the internet speed is good enough, no one will want to watch SD again! After all, the high resolution and clear picture quality of high-definition video can always make people have a better viewing experience.
With the increase of users' demand for high-definition video, the amount of video data of video multimedia is also increasing. If uncompressed, these videos are difficult to use for practical storage and transmission. Video compression and decoding technology can help effectively remove redundant information in video data, and realize fast transmission and offline storage of video data in the Internet. Therefore, video compression and decoding technology is a key technology in video applications.
Background on Video Coding
In the past few decades, a series of video coding standards have been widely used. There are many existing video compression standards, including the MPEG-1, MPEG-2, and MPEG-4 standards formulated by the International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC); H.261 and H.263 formulated by the International Telecommunication Union-Telecom (ITU-T).
In March 2003, ITU-T and ISO/IEC officially announced the H.264/MPEG-4 AVC video compression standard . As the most widely used video coding standard at present, H.264 has achieved great success in improving coding efficiency and flexibility, making digital video effectively applied in various network types and engineering fields. In order not to be restrained by foreign countries in key technologies, and at the same time without paying a lot of patent fees, China has also formulated the AVS series of standards, which can provide coding efficiency comparable to H.264/AVC.
In recent years, with the continuous upgrading of user experience, high-definition (1920x1080) and ultra-high-definition (3840x2160) videos have become more and more widely used. Compared with standard definition video, high-definition video resolution is larger and clearer, but the corresponding data volume also increases. In the case of limited storage space and network bandwidth, the existing video compression technology can no longer meet the actual application requirements. In order to solve the impact on network transmission and data storage brought about by the sharply increasing data rate of high-definition and ultra-high-definition video, ITU-T and ISO /IEC jointly developed a new generation of video compression standard HEVC ( High Efficiency Compression) with higher compression efficiency. Video Coding ).
Jane Says HEVC
HEVC is based on the traditional hybrid video coding framework, and adopts more technological innovations, including flexible block division, finer intra-frame prediction, newly added Merge mode, tile division, adaptive sample compensation, etc. Flexible block partitioning, which includes coding units (CUs), prediction units (PUs), and transform units (TUs), improves coding performance the most. These techniques double the encoding performance of HEVC compared to H.264/AVC. However, these technologies also greatly increase the complexity of the HEVC encoder, which is not conducive to the real-time application and promotion of the HEVC encoder.
The high performance of HEVC comes at the cost of huge computational complexity: HEVC selects the optimal mode partition by recursively performing a rate-distortion optimization process (RDO) on each coding unit, which is very complex. . Therefore, the research work on reducing HEVC coding complexity has important practical application value and broad application prospects.
The meaning of video decoding
Video transcoding technology is a technology that solves the compatibility problem between the video sending end and the receiving end. It can realize the mutual conversion between different video standards, video resolutions, video frame rates and video bit rates. Adopting the video transcoding technology only needs to change the system structure of the sending end or add a corresponding transcoder at the gateway without any modification to the receiving end.
Video transcoding technology can not only adjust the resolution, frame rate, bit rate and other attributes of the video stream to adapt to different end users and network bandwidth, but also convert the compression format and syntax structure of the video stream. Transcoding technology has been widely used in practical applications.
In general, this technology research is of great practical value, and it will be used to meet the needs of digital video conversion in more fields, not only covering commercial products including media network management multi-conference units, video surveillance, video broadcast transcoding and medical equipment, etc. It can also be used in products including high-definition videoconferencing endpoint digital media adapters, high-definition network cameras, video phones and advanced digital set-top boxes.
Research on key technologies of HEVC
In recent years, with the step-by-step improvement of the HEVC coding standard in the development process, relevant organizations and scientific research institutions at home and abroad have put forward a lot of effective work on the HEVC coding algorithm. Including IEEE Trans. CSVT, IEEE Trans. Multimedia, ICIP, ICME, PCS, VCIP, etc.
HEVC inter-coding optimization
The flexible data division method is one of the very significant features of the HEVC standard. The determination of coding parameters such as CU, PU and TU division methods is performed by traversal search, which makes the rate-distortion performance of HEVC significantly better than its previous coding standards. . However, this also introduces a lot of computational complexity. The inter-frame CU fast selection algorithm is based on the depth information of adjacent CUs or coding intermediate parameters, etc., to realize the advance judgment of the CU size and the inter-frame PU mode, thereby reducing the coding complexity. Process. According to different information used, fast selection methods for inter-frame CUs can be divided into fast selection methods based on adjacent CU depth information, coding intermediate parameters, and rate-distortion cost-based fast selection methods.
HEVC Intra Coding Optimization
The fast selection algorithm for intra-frame CU is a process of evaluating the texture complexity of the CU or according to the relevant CU depth information, etc., to realize the fast selection of the CU scale and the intra-frame prediction mode, thereby reducing the coding complexity. According to different information used, fast selection methods for intra CUs can be divided into fast selection methods based on texture complexity, based on adjacent CU information, based on sub-CU information, and based on rate-distortion cost.
Research on HEVC Rate Control
In practical applications, the transmission bandwidth is usually limited. How to ensure the video quality as much as possible under the effective bandwidth, that is, rate control, is an important problem that needs to be solved in the video coding technology. For the HEVC standard with high-definition and ultra-high-definition video as the main encoding object, how to reasonably allocate the bit rate under limited bandwidth resources to achieve the best video quality has become an important issue to be considered when encoding.
Currently rate control models can be divided into three categories: Q domain, ρ_domain and λ_domain. In addition, there are many related rate control algorithms, including an improved algorithm for the bit allocation problem at the frame layer; an improved rate control algorithm by adjusting the Lagrangian factor; a new initial Quantization parameter prediction algorithm; propose new algorithm for scene switching problem; consider image complexity to establish more accurate rate control and so on. There are also some rate control algorithms proposed for specific applications, such as wireless applications, high-definition applications, and so on. For the new generation of H.265/HEVC, how to effectively improve the rate control from all aspects has gradually become a research hotspot.
Summary and Outlook
With the rapid development of the Internet and mobile networks, the market has an increasing demand for higher-resolution video, and high-quality video services account for an increasing proportion of network data. The demand for video quality makes the coding technology with higher compression efficiency become a research hotspot in industry and academia.
In this context, HEVC came into being. Although HEVC maintains a high compression ratio, its complex operation also limits its promotion and application. Therefore, under the premise of maintaining the quality of transcoded video, greatly optimizing the complexity of the transcoding process is a topic worthy of joint research in industry and academia.
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