tailieunhanh - H.264 and MPEG-4 Video Compression phần 4

Chương 3 bao gồm các nguyên tắc cơ bản của nén video, tập trung vào các khía cạnh của quá trình nén được phổ biến cho cả hai tiêu chuẩn và giới thiệu bộ giải mã dựa trên biến đổi 'mô hình' là trung tâm của tất cả các chuẩn video mã hóa. Chương 4 xem xét các tiêu chuẩn bản thân và kiểm tra rằng các tiêu chuẩn đã được định hình và | ENTROPY CODER 69 can be generated automatically on the fly if the input symbol is known. Exponential Golomb codes Exp-Golomb fall into this category and are described in Chapter 6. Arithmetic Coding The variable length coding schemes described in Section share the fundamental disadvantage that assigning a codeword containing an integral number of bits to each symbol is sub-optimal since the optimal number of bits for a symbol depends on the information content and is usually a fractional number. Compression efficiency of variable length codes is particularly poor for symbols with probabilities greater than as the best that can be achieved is to represent these symbols with a single-bit code. Arithmetic coding provides a practical alternative to Huffman coding that can more closely approach theoretical maximum compression ratios 8 . An arithmetic encoder converts a sequence of data symbols into a single fractional number and can approach the optimal fractional number of bits required to represent each symbol. Example Table lists the five motion vector values -2 -1 0 1 2 and their probabilities from Example 1 in Section . Each vector is assigned a sub-range within the range to depending on its probability of occurrence. In this example -2 has a probability of and is given the subrange . the first 10 of the total range 0 to . -1 has a probability of and is given the next 20 of the total range . the subrange . After assigning a sub-range to each vector the total range has been divided amongst the data symbols the vectors according to their probabilities Figure . Table Motion vectors sequence 1 probabilities and sub-ranges Vector Probability log2 1 P Sub-range -2 -1 0 1 2 Total range x_ 0 II I - -2 -1 0 1 1 2 Figure Sub-range example 70 VIDEO CODING CONCEPTS Encoding Procedure for Vector