3. Adaptive Loop Filter
As far as adaptive loop filter (ALF) is concerned, there are three types of ALF: frame-based, block-based and quadtree-based ALFs. All of them are based on wiener filter, but with different filtering control basis. In frame-based ALF [VCEG-C437/AI14, C402], only one picture level flag is used to signal the decision of filtering or non-filtering.
Although wiener filter can restore the reconstructed picture to the original picture globally, there are degraded pixels locally. Since the degraded area reduce the filtering efficiency, if these areas are not filtered, the capabilities of picture restoration and loop filtering are improved. Therefore, block-based ALF [VCEG-AI18/AJ13] use explicit flags for filtering on-off on block by block basis, while quadtree-based ALF [VCEG-C181/AK22] introduces a quadtree data structure to carry out the variable-size block filtering.
3.1 Block-based Adaptive Loop Filter
Block-based ALF is an improvement of frame-based ALF. Figure 2[......]
Permanent Link: Adaptive Post/Loop Filters in JM/KTA – Part 2
1. Introduction
The basic idea of adaptive post/loop filter is the same. Both of them use adaptive wiener filtering technique to improve the quality of reconstructed picture which is degraded by compression. The difference between them is whether the filtering process is applied in or out of the core coding loop, as shown in Figure 1, to improve the quality of reconstructed picture or just displayed picture.
Figure 1. Block diagram of JM/KTA
2. Adaptive Post Filter
In H.264/AVC, there is already an existing post-filter hint SEI message [JVT-S030/T039/U035] which provides the coefficients of a post-filter or correlation information for the design of a post-filter for potential use in post-processing of the output decoded pictures to obtain improved displayed quality.
To find the coefficients of adaptive wiener filter, the following cost function based on the whole frame is minimized:
(1)
where R is the reconstructed picture, R’ is the filtered picture, and I is the original pic[......]
Permanent Link: Adaptive Post/Loop Filters in JM/KTA – Part 1
The 90th MPEG/50th JPEG Meeting, hosted by Xidian University, will be held at Sheraton Hotel on 26-30 October 2009, in Xian, China.
The official web site is accessible now at http://see.xidian.edu.cn/conference/mpegjpeg/, and the early bird registration date is 30 September 2009.
Permanent Link: The 90th MPEG Meeting in Xi’an, China
As well-known, JVT (a joint video team between MPEG and VCEG) has achieved great success in the standardization of the state-of-the-art video compression, including H.264/AVC, SVC, and MVC. The academic and industry want to continue the success in the collaboration between MPEG and VCEG. Some proposals from VCEG suggest the H.NGVC (or H.265) standardization should be collaboratively done with MPEG to produce a significantly advanced video coding standard and to avoid producing multiple similar standards, which confuse the market. VCEG agrees that conducting this work jointly with MPEG will be beneficial to avoid duplication of efforts in this area. Therefore, in this MPEG London meeting, a joint meeting between MPEG and VCEG discusses the future collaboration. However, the collaboration method arouses a lot of controversy, focusing on which one of the following practical experiences should be chosen:
- Practical experience 1: MPEG-2, a integrate part of video subgroup when under WG 11 a[......]
Permanent Link: Future Collaboration between MPEG and VCEG
Current Status of HVC (High-Performance Video Coding) in MPEG
2009-07-03 News 11 Comments Views(16,171)In the last MPEG meeting, MPEG issued a Call for Evidence (CfE) on High-performance Video Coding (HVC). Nine responses to the CfE are received in this meeting (89th MPEG London). Those reponse proposals adopt typical coding tools in KTA, such as adaptive loop filter (ALF), extended macroblock size (EMS), larger transform size (LTS), internal bit depth increasing (IBDI), adaptive quantization matrix selection (AQMS), as well as new tools, such as modified intra prediction, modified de-block filter, decoder-side motion vector derviation (DMVD).
The objective experimental results show that 20% average bit reduction is achieved compared with H.264/AVC High Profile for all classes of test video sequences (Class A: 19%, Class B:25%, Class C:22%, Class D: 15% bit reductions, respectively). Subjective evalution is also conducted during this meeting. The purpose of subjective evaluation is identifying examples that give the best evidence and assessing whether the evidence is large enough. The s[......]
Permanent Link: Current Status of HVC (High-Performance Video Coding) in MPEG
4. Rate-Distortion Optimized Quantization
Previously, adaptive rounding was proposed to improve quantization, which captures the statistics of the incoming residual signal and adjusts the rounding offsets accordingly. However, the adaptive rounding quantization is still based on the criterion which minimizes the mean-squared quantization error between the original signal and the quantization reconstructed signal. From the sense of rate-distortion optimization, the cost from the rate should also be considered.
The basic idea underlying the rate-distortion optimized quantization is to minimize a cost function D+ λR such that both the rate R and the distortion D are considered in coding decisions. For quantization case, the RD optimal coding is to solve a minimization problem of
                                                  (7)
where S is the original signal, and T-1 denotes the inverse transform operation. Consider that the DCT is a unitary transform, which maintains the Euclidean d[......]
Permanent Link: Quantization Techniques in JM/KTA – Part 4
3. Adaptive Rounding Encoding Technique using an Equal Expected-Value Rule
As discussed above, if the input p.d.f. is Laplacian distributed and if we can estimate λ, then the optimal f can be found analytically. But, usually the estimate of input p.d.f. is not available, then, how to select the rounding offset f?
In order to select rounding offset f adaptively, an adaptive quantization encoding technique using an equal expected-value rule is proposed by Gary Sullivan from Microsoft. The adaptive adjustment of the rounding offset f occurs only in the encoding quantization process, which tries to select f without using any priori model knowledge on the input W. The aim is to make that the mean of the absolute value of the input, |W|, is equal to its expected reconstruction value |W’|, i.e.,
                                                                                         (5)
Any values in an interval would be reconstructed to some W’, so the distribution of W’ is a probability ma[......]
Permanent Link: Quantization Techniques in JM/KTA – Part 3
2. Principle of H.264/AVC Normal Quantization Scheme
2.1. Scalar dead-zone quantization
In this section the principle of H.264/AVC normal quantization scheme is described in a generalized form.
A scalar quantizer for input signal W can be decomposed into a function Z=C[W] called a classification rule that selects an integer-valued class identifier called the quantization index at the encoder, and a reconstruction rule that produces a real-valued output W’=R[Z] at the decoder. Video encoder applies entropy coding to the quantization indices and communicates to the decoder. Although H.264/AVC JM reference software implements some classification functions, only reconstruction function is standardized.
In the quantization step of the encoder, the transform coefficients of the prediction error are quantized. This quantization is used to reduce the precision of the coefficients. Furthermore, the quantizer is designed to map insignificant coefficient values to zero whilst retaining a reduced [......]
Permanent Link: Quantization Techniques in JM/KTA – Part 2