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0 PrefaceWith the advent of wireless multimedia applications and the proliferation of multi-data services, wireless services are widely used in various aspects, and it is more important to use wireless networks to transmit audio and video information in multimedia communications. This requires dynamic, real-time processing of sound, animation, and video signals in multimedia information processing, but it is difficult for a computer to transfer massive amounts of data generated during image information processing from the memory to the central processing unit in real time, so if the video is not Data compression is almost impossible to achieve real-time requirements. This requires secure low bit rate encoding and transmission in a wireless environment that is prone to error, and MPEG-4 technology provides the possibility of implementation.
MPEG-4 image compression technology emphasizes content-based interactivity for video and audio coding at a certain bit rate. Therefore, the use of MPEG-4 encoding and decoding technology to achieve video data compression has become a key technology for wireless video transmission in multimedia system development. The multimedia video conferencing system is a combination of computer technology and communication technology. It provides a cross-regional environment for people to communicate, discuss problems and work together (CSCW) through the support of multimedia technology and communication technology, and generate "you See the effect of seeing me."
The multimedia video conferencing system will greatly improve people's work efficiency and will change people's traditional work and communication modes, with far-reaching and profound social significance. Video medical system is a multimedia video conferencing system
1 MPEG-4 key technology
MPEG-4 uses object-based second-generation compression coding technology and uses AV objects (Audio / VisualObject) for visual coding. It also provides many content-based interactive video service functions, and also supports traditional streaming video. In addition to the core technologies of first-generation video coding (such as motion estimation and compensation, entropy coding), MPEG-4 also proposes some new key technologies:
1) Video Object Segmentation MPEG-4's content-based interactivity is primarily achieved by segmentation of video objects. The so-called video object segmentation is to divide the video and image into different objects or separate the moving objects from the background, and then use different encoding methods for different objects to achieve high-efficiency compression. For example, the compression of moving objects is relatively low. The method of less loss, as much as possible to preserve the details and smoothness of the video object; the background object uses a large compression ratio encoding strategy to achieve a better balance between compression efficiency and decoded image. Basically, it can be divided into three steps: simplification of image data by filtering; feature extraction of video and image data, such as color, texture, motion, semantics, etc. Finally, related post-processing operations to achieve denoising and accurate extraction The purpose of the border.
It can be seen that the MPEG-4 encoding technology requires more analysis and understanding of images and videos, and unlike the past compression algorithms, only the intraframe and interframe redundancy is removed. Therefore, object-based coding can improve the compression ratio of data, and the segmentation (extraction) technology of video objects has become a key technology in MPEG-4 video coding.
2) Video coding Scalable technology The purpose of video grading coding technology is to solve the need for video transmission on heterogeneous networks with different transmission characteristics. The scalability of video coding refers to the adjustability of the code rate. That is to say, video encoding is only compressed once, but can be decoded at multiple frame rates, spatial resolutions or video quality to support different application needs of different users. The grading technique of video coding is implemented by the data structure of the video object layer (VOL). There are two basic grading tools, time domain grading and airspace grading, as well as mixed grading of time domain airspace. Each level encodes at least two layers of VOLs, the lower layer (base layer) is used to provide basic information about the video sequence, and the higher layer (enhancement layer) is used to provide higher resolution and detail of the video sequence.
3) Motion estimation and compensation techniques In video coding, motion estimation is quite time consuming and has a great impact on the real-time performance of the code. In the motion estimation section, it is necessary to repeatedly calculate to determine the correlation of macroblocks in adjacent frames. Estimates are typically made using block matching criteria, such as absolute error sum (SAD) criteria and mean square error (MSE) criteria, to select a best matching operation to produce a motion estimate for the best matching point. For example, the bitstream can be optimized to develop fixed fault tolerance by developing MPEG-4-specific syntax units for adaptive resynchronization, data segmentation, and data recovery. In most cases, high compression ratio video data cannot avoid the impact of errors caused by channel degradation. However, as long as the effects of bit errors are reduced by employing a range of appropriate methods and tools, high quality wireless video transmission is entirely possible.
2 MPEG-4 technology application - video medical system
Through the above analysis of the key technologies of MPEG-4 video coding, it can be seen that MPEG-4 video coding has its unique advantages in video transmission: (1) content-based interactivity; (2) efficient coding method; (3) Universal accessibility.
The application of video conferencing systems to medical video processing systems is a new application area for video conferencing. In the video medical system, one-to-one diagnosis and expert consultation can be conducted in the form of point-to-point or point-to-multipoint application, and medical teaching practice can also be performed. The video system mainly has the functions of real-time video capture, synchronous compression, real-time transmission and playback. That is to say, while transmitting local video, local video information can be played back and monitored, and video snapshot and storage functions are also provided. Figure 1 is a structural model diagram of the system.
As can be seen from Figure 1, video capture and compression based on MPEG-4 is a key step in video medical systems. In the design process of video medical system, because the software video compression algorithm is extremely time-consuming, in order to improve the compression efficiency, the MPEG-4 compression algorithm plug-in is introduced in JMF, and the video acquisition and transmission technology based on MPEG-4 is adopted. Video compression, supplemented by double buffering and multi-threading.
An important premise of the interaction and coding of video objects is to use the segmentation technology of video objects to obtain a tracking based on motion information and visual characteristics through motion estimation and compensation techniques, and finally obtain the tracking results of video objects. Each video image collected by the video capture thread is stored in the video source buffer 1 and the video source buffer 2, and then the images in the frequency source buffer 1 and the video source buffer 2 are processed by the software compression thread. Compress and save the compressed results to compressed video buffer 1 and compressed video buffer 2, respectively. At least in two video source buffers
The MPEG4 compression algorithm has a high compression ratio and has good network adaptability. The use of MPEG-4 compression algorithm while using double buffering and multi-threading technology, can make image acquisition, compression and transmission concurrent execution, greatly improve the CPU efficiency, so that the overall performance of video capture, compression and transmission is very large Improvement.
The flow chart of video capture compression is shown in Figure 2.
The image obtained after video capture and compression is shown in Figure 3.
3 Conclusion
Due to the scalability and flexibility of MPEG-4 and the emergence of broadband wireless networks, it is foreseeable that wireless multimedia services will become the trend of the times in the near future. MPEG-4 video transmission in broadband wireless networks will become an important part of the emerging multimedia applications. The low storage capacity and high definition of MPEG-4 determine that MPEG-4 is a superior video compression method in low bandwidth networks. It can be seen that when video compression is performed in a video medical system, one-to-one video transmission using a plug-in based on the MPEG-4 compression algorithm can obtain a higher quality video image and can basically achieve system real-time performance. The requirement is that this is not possible with traditional multipoint transmission video systems.
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