Game Development Reference
In-Depth Information
(a)
(b)
Fig. 7.1
One frame of
akiyo
compressed by AVS-2 at QP
=
42,
a
in-loop filters off,
b
in-loop
filters on
In order to alleviate the compression artifacts, a variety of approaches have been
proposed in the literatures. There are primarily two categories: post-processing and
in-loop processing. Post-processing methods operate outside of the coding loop,
and thus are not normative in the standardization process, such as filtering tech-
niques (Takeda et al.
2007
), themaximuma
posteriori
probability approaches (Zhang
et al.
2013
), and iterative methods based on the theory of projections onto convex
sets (POCS) (Minami and Zakhor
1995
). These post-processing schemes have good
potential to be integrated into image and video communications, while maintaining
compatibility with the existing different coding standards. However, these methods
can only improve the quality of current image, and sometimes may introduce blurring
by smoothing out some image details. On the contrary, in-loop processing (or in-loop
filtering) methods operate within the coding loop. That is, the processed frames can
be used as reference frames for motion compensation of subsequent frame coding. In
additional, the filtering strength can be well controlled according to coding modes or
rate-distortion cost without deteriorating the quality of the reconstructed images. Due
to the fact that filtered reference frames offer higher quality prediction for motion
compensation, in-loop filters can significantly improve the compression efficiency.
However, due to the in-loop filtering forces all standard conformant codecs to per-
form identical operation both at encoder and decoder, it increases the computation
complexity substantially, especially for the decoder. Thanks to the improvement of
the computation capacity, the first in-loop deblocking filter is adopted into video
coding standard H.263v2. Since that, more in-loop filters are widely discussed and
adopted into latest video coding standards, e.g., an improved deblocking filters in
AVC/H.264 (List et al.
2003
), sample adaptive offset (SAO) in HEVC (Fu et al.
2012
) and adaptive loop filter (ALF) in AVS-2 (Zhang et al.
2014a
). Figure
7.2
is
a block diagram illustrating the three in-loop filters in AVS-2. The three in-loop
filters, deblocking filter, sample adaptive offset and adaptive loop filter, are applied
to the reconstructed frames sequentially to reduce compression artifacts and provide
high-quality reference for further coding frames.
