The unique feature of MPEG-4 is that it supports content-based encoding and decoding. For this reason, the concept of audiovisual object AVO (Audio / Video Object) is introduced. AVO can be a violin or piano sound in an image. Each AVO can be encoded independently, but there is a spatial-temporal structure relationship between each other. Therefore, when encoding, the composition structure of the encoded object must be transmitted. To represent the spatiotemporal structure relationship between AVOs in the scene. According to this "scene description", the relevant contents of the image and sound are edited and operated during decoding. The audiovisual object can also be a rectangular frame, thus making MPEG-4 compatible with the original MPEG standard. The video object VO in the audiovisual object is usually described by three types of information, namely motion, shape and texture information.
1 Coding principle
The MPEG-4 encoder is mainly composed of three parts: shape coding, texture coding and motion coding. The frame format is divided into I-VoP, P-VOP and B-VOP. Only I-VOP and P-VOP are discussed here. I-VOP uses texture coding to eliminate the spatial redundancy between image data in one frame; P-VOP refers to the previous frame image and uses two parameters to encode. One is the difference between the current image to be encoded and the reference image; the other is the motion vector. The specific structural module of the encoder is shown in Figure 1.
1.1 Shape coding
Shape coding is mainly used to record the shape information of the VOP extracted from the image sequence, which is divided into binary shape information and gray-scale shape information. Binary shape information uses two values ​​of 0 and 1 to represent the shape of the VOP; gray-scale shape information uses 0 to 255 to represent the different transparency of the VOP area. When encoding, when the extracted VOP has a non-rectangular shape, it is necessary to expand the boundary to make the rectangular boundary be a multiple of 16, while ensuring the smallest area after expansion, and then shape coding; when the extracted VOP is rectangular When (the length and width of the rectangle are multiples of 16), the shape coding will be masked.
1.2 Texture encoding
Texture coding mainly encodes image pixels in I-VOP or difference pixels in P-VOP, including DCT, quantization, DC and AC prediction, entropy coding and other processes to maximize the removal of the pixels between the current VOP Spatial redundancy.
The first frame of the video is coded in I-VOP format, and the remaining frames are in I-VOP or P-VOP format, which is constrained by two factors. The user will set the format of the current frame according to IPPPIPPPI; the second is that the current frame has been artificially set to P-VOP, and the value of mad_P is calculated through motion estimation. If mad_P meets one of the following two conditions, the current frame P-VOP encoding format is used, otherwise I-VOP encoding format is used.
Condition 1: mad P <50/3;
Condition 2: mad P <50, and IntraMBRaTIo <0.4. Among them, IntraMBRaTIo is the proportion of macroblocks using the MBM_INTRA prediction mode in the total macroblocks in the current frame.
The DCT and quantization modules in texture coding are relatively simple. I will skip them here and focus on the remaining texture coding modules.
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