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Abstract: H.26L is the next generation video coding standard. Its coding exceeds all existing standards, including H.263+ and MPEG-4 (SP). This paper analyzes a variety of new coding features introduced by H.26L, focuses on 4×4 point integer transform, and proposes a fast transform algorithm implemented on TM1300.Keywords: H.26L UVLC 4×4 point integer transform TM1300
introduction
H.26L is the next generation video coding standard. Initially, H.26L was developed by the ITU-T VCEG team. In November 2001, MPEG and VCEG jointly established the JVT team to participate in the development of H.26L. Also because of the addition of MPEG, H.26L will be included in the tenth part of MPEG-4. Since the H.26L standard is still in the process of development, this article is temporarily based on the test model TML8 provided by JVT.
The basic coding framework of H.26L source coding is similar to the currently popular video coding standard, using a hybrid coding technique combining transform coding and predictive coding. Its outstanding performance mainly comes from the introduction of new coding features: 4 × 4 point integer transform, entropy coding using UVLC, run vector with 1/4 to 1/8 pixel precision, motion estimation with multiple block sizes, and so on. These new coding techniques improve compression and fault tolerance from different sides. In particular, the 4x4 point integer transform is unique among all video compression protocols.
Although the H.26L standard is still under development, in the initial test, its coding performance surpassed all existing standards, including H.263+ and MPEG-4 (Simple profile). The results of these tests show that H.26L can save 20% to 50% bit rate compared to H.263+ and achieve up to 50% code rate over MPEG-4 (SP) with the same objective video quality. As a next-generation video coding standard, H.26L demonstrates its huge development prospects.
1 H.26L 4×4 point integer transform
1.1 Introduction to transformation
In the H.26L coding technique, the 4×4 point integer transform can be regarded as an integer version of the DCT transform, which mainly completes the spatial correlation of the removed image, and has the same properties as the 4×4 point DCT transform. First consider the one-dimensional integer transformation: Let a, b, c, d be 4 points to be transformed, A, B, C, D are the corresponding 4 transform coefficients, then you can use the following formula to represent a, b, c , the positive transformation of point d:
A=13a+13b+13c+13d
B=17a+7b-7c-17d
C=13a-13b-13c+13d
D=7a-17b+17c-7d
The inverse transformation formula is as follows:
a'=13A+17B+13C+7D
b'=13A+7B-13C-17D
c'=13A-7B-13C+17D
d'=13A-17B+13C-7D
The relationship between a and a' is a'=676a. That is to say, after the inverse transformation, the normalization operation is also required, so that the positive transformation and the transformation scale are consistent.
The transform kernel of the same two-dimensional 4×4 integer transform is separable. The separated transformation reduces the computational complexity from O(N4) to O(N3).
1.2 Comparison with 8×8 point DCT transform
Compared with the traditional DCT transform, H.26L adopts 4×4 point integer transform to bring the following advantages to video coding:
1 helps to reduce block and ring spots and improves image quality. Since the transform coefficients are quantized, the high frequency coefficients are lost, so there are block shifts and loop classes in the restored images. In H.26L, a smaller 4×4 point transform is used, which can effectively suppress block and ring spots.
2 integer transformation reduces the accumulation error. The traditional accumulation error comes from two aspects: the error caused by the mismatch of the positive and negative transforms and the error caused by the quantization. In order to achieve the purpose of compression, the second error is unavoidable. However, since H.26L uses an accurate integer transform, the forward transform and the inverse transform do not generate errors, which effectively reduces the accumulation error.
3 operation speed is fast. Because the transformation formula used by H.26L is a simple integer equation, that is to say, the calculation is based on integers, not floating-point numbers, so it reduces the computational complexity of a single transformation, and is also advantageous for fixed-point DSP implementations.
2 Implementation in TM1300
The TM1300 is a 32-bit ultra-high performance multimedia processor. Its core processor uses VLIW's long instruction word structure, which can perform 5 operations simultaneously in each clock cycle. Supports highly parallel custom operations, which can greatly accelerate the special operations commonly seen in digital signal processing and multimedia applications. Performance, while custom operations are similar in use to C language function calls, which facilitates the design of the program.
In this paper, for the characteristics of 4×4 point integer transformation and the characteristics of TM1300's custom operation instruction, the following adjustments are made to the integer transformation: row transformation first, then column transformation. Since the result of the row transformation does not exceed the representation range of 16 bits, the data is recombined and column transformed before the column transformation, which is based on the following two considerations.
First, since the video input data is an unsigned byte type, and the TM1300 is a 32-bit processor, accessing the memory in word units can improve access efficiency. The data of the current 4x4 data block (the pointer is P1) and the reference frame 4x4 data block (the pointer is P2) are organized as follows. The point to be transformed is the difference between the value of the current data block and the value corresponding to the reference frame data block.
P1: cal, cb1, cc1, cd1 P2: ra1, rb1, rc1, rd1
Ca2,cb2,cc2,cd2 ra2,rb2,rc2,rd2
Ca3,cb3,cc3,cd3 ra3,rb3,rc3,rd3
Ca4,cb4,cc4,cd4 ra4,rb4,rc4,rd4
Second, you can use 8-bit multiply/accumulate custom operations, one operation can complete four 8-bit multiply/accumulate, and one machine cycle (CLK) can perform up to five operations. Compared with the non-customized multiply/accumulate, the number of operations is reduced, and the parallelism of the program operation is improved.
  Figure 1 shows the function of the ifir8ui custom operation.
3 Experimental results
The fast algorithm based on TM1300 for 4×4 integer transform proposed in this paper uses parallel computing technology to greatly reduce the amount of computation. Experiments show that it takes 80 machine cycles to perform a 4×4 point integer transformation, and only 28 machine cycles are needed for the multiplication and addition. The 8×8-point fixed-point DCT transformation is needed for the TM1300. The 180 machine cycles are also significantly larger than the four 4 x 4 point integer conversion times. In the aspect of transform, the transform coding operation complexity of H.264 is smaller than other coding methods.
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