HEVC标准中的内循环去块滤波器详解：降低边界artifact

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本文主要探讨的是HEVC（高效视频编码）标准中的内循环去块（in-loop deblocking filter），这是一种旨在减少视频块边界处可见artifact的技术。HEVC是H.264/AVC编码技术的后续升级，它在提高编码效率的同时，对视频处理的性能进行了优化。与H.264/AVC标准中的去块滤波器相比，HEVC的内循环去块滤波器具有更低的计算复杂度和更好的并行处理能力。这意味着在编码过程中，它能更有效地利用硬件资源，减少了在解码时的延迟，提高了编码速度。这一改进对于现代多媒体应用，如高清视频流、虚拟现实和增强现实等，尤其重要，因为它们对视频质量和处理效率有极高的要求。 HEVC的内循环去块滤波器工作原理包括以下几个关键步骤： 1. **检测艺术**：滤波器首先对编码后的视频块边界进行分析，识别可能存在的边缘模糊或块效应（artifact）问题。这些问题是由于块状编码方式可能导致的，即视频信号被分割成多个固定大小的块进行处理，从而造成图像的不连续性。 2. **滤波器选择**：基于检测到的艺术效果，滤波器会选择适当的算法来处理这些问题。这通常涉及到平滑或锐化操作，目的是尽可能地使相邻块之间的过渡更加平滑，减少视觉上的锯齿和方块效应。 3. **应用滤波**：选择的滤波器会在编码块的边界区域执行，通过调整像素值，减小艺术效果的明显程度。这一步骤通常是在线性空间进行，以保持颜色一致性。 4. **并行处理**：HEVC的内循环去块滤波器设计考虑到了硬件并行性的优势，使得多个滤波器可以同时处理不同的视频块，进一步提升了处理速度。 5. **降低视觉影响**：尽管引入了额外的处理步骤，HEVC的内循环去块滤波器能够在不显著增加码率的前提下，显著改善视频的观看体验，提供更平滑、自然的图像。总结来说，HEVC的内循环去块滤波器是H.265标准的重要组成部分，它在提升视频编码效率和图像质量上做出了关键贡献，是现代视频压缩和处理技术中不可或缺的一部分。对于开发支持HEVC的编解码器、软件和硬件平台来说，理解这种滤波器的工作原理和技术细节至关重要。

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Abstract—This paper describes the in-loop deblocking filter

used in the upcoming high efficiency video coding (HEVC)

standard to reduce visible artifacts at block boundaries. The

deblocking filter performs detection of artifacts at the coded block

boundaries and attenuates those by applying a selected filter.

Compared to the H.264/AVC deblocking filter, the HEVC

deblocking filter has lower computational complexity and better

parallel processing capabilities while still achieving significant

reduction of the visual artifacts.

Index Terms—deblocking, video coding, video processing, video

filtering, block-based coding.

I. INTRODUCTION

IGH efficiency video coding (HEVC) [1] is a new video

coding standard that is currently being developed jointly

by ITU-T SG 16 Q.6, also known as VCEG (Video Coding

Experts Group), and by ISO/IEC JTC 1/SC 29/WG 11, also

known as MPEG (Moving Picture Experts Group) in the joint

collaborative team on video coding (JCT-VC). The first version

of the HEVC standard is planned to be finalized in January

2013, while the development of scalable and 3D extensions of

HEVC is expected in the following years. Similar to previous

video coding standards, such as H.264/AVC, the upcoming

HEVC standard is based on a hybrid coding scheme using

block-based prediction and transform coding. First, the input

signal is split into rectangular blocks that are predicted from the

previously decoded data by either motion-compensated

prediction [3] or intra prediction. The resulting prediction error

is coded by applying block transforms based on an integer

approximation of the discrete cosine transform, which is

followed by quantization and coding of the transform

Manuscript received April 15, 2012.

A. Norkin and K. Andersson are with Ericsson Research, 164 89,

Stockholm, Sweden (phone: +46 10 714 84 02, e-mails:

andrey.norkin@ericsson.com, kenneth.r.andersson@ericsson.com).

G. Bjøntegaard and A. Fuldseth are with Cisco Systems, Oslo, Norway

(e-mails: arilfuld@cisco.com, gbjonteg@cisco.com)

M. Narroschke is with Panasonic R&D Center, 63255 Langen, Germany

(e-mail: matthias.narroschke@eu.panasonic.com).

M. Zhou is with Systems and Applications R&D Center, Texas Instruments

Inc., Dallas, TX-75243, USA (e-mail: zhou@ti.com).

M. Ikeda is with Technology Development Group, Sony Corporation, Tokyo,

141-8610, Japan (e-mail: masaru.ikeda@jp.sony.com)

G. Van der Auwera is with Qualcomm Technologies Inc., San Diego,

CA-92121, USA (e-mail: geertv@qualcomm.com)

coefficients. While H.264/AVC [2] divides a picture into fixed

size macroblocks of 16 × 16 samples, HEVC divides a picture

into coding tree units (CTU) of 16 × 16, 32 × 32 or 64 × 64

samples. The coding tree units can be further divided into

smaller blocks using a quad-tree structure. Such a block, called

a coding unit (CU), can be split further into prediction units

(PUs) and is also a root for the transform quad-tree. Each of the

child nodes of the transform quad-tree defines a transform unit

(TU). The size of the transforms used in prediction error coding

can vary from 4 × 4 to 32 × 32 samples, thus allowing larger

transforms than in H.264/AVC, which uses 4 × 4 and 8 × 8

transforms. As the optimal size of the above mentioned blocks

typically depends on the picture content, the reconstructed

picture is composed of blocks of various sizes, each block being

coded using an individual prediction mode and the prediction

error transform.

In a coding scheme that uses block-based prediction and

transform coding, discontinuities can occur in the reconstructed

signal at block boundaries. Visible discontinuities at block

boundaries are known as blocking artifacts. A major source of

blocking artifacts is the block-transform coding of the

prediction error followed by coarse quantization. Moreover, in

the motion-compensated prediction process, predictions for

adjacent blocks in the current picture might not come from

adjacent blocks in the previously coded pictures, which creates

discontinuities at the block boundaries of the prediction signal.

Similarly, when applying intra prediction, the prediction

process of adjacent blocks might be different causing

discontinuities at the block boundaries of the prediction signal.

Two approaches to reducing blocking artifacts are

post-filtering and in-loop filtering. Post-filtering is not specified

by the video coding standard and can be performed, e.g., in the

display buffer. The implementer has the freedom to design an

algorithm driven by application-specific requirements. In-loop

filters operate within the encoding/decoding loop. Therefore,

they need to be normative in order to avoid drift between the

encoder and the decoder.

The HEVC draft standard defines two in-loop filters, which

can be applied sequentially to the reconstructed picture. The

first one is the deblocking filter and the second one is the sample

adaptive offset filter (SAO), which are currently included into

the main profile. This paper describes the first of these two

in-loop filters, the deblocking filter. Depending on the

configuration, SAO can be applied to the output of the

deblocking filtering process.

The deblocking filter in HEVC has been designed to improve

Andrey Norkin, Gisle Bjøntegaard, Arild Fuldseth, Matthias Narroschke, Masaru Ikeda, Kenneth

Andersson, Minhua Zhou, and Geert Van der Auwera

HEVC Deblocking Filter

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