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Adaptive Post/Loop Filters in JM/KTA – Part 1

2009-08-22 KTA View Comments Views(7,495)

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.

kta_diagram

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:

Eq1 (1)

where R is the reconstructed picture, R’ is the filtered picture, and I is the original picture. The optimal filter coefficients can be obtained by solving the Wiener-Hopf Equation:

Eq2 (2)

The rate overhead associated with transmitting the filter coefficients is further reduced by predicting filter coefficients or using coefficient symmetry.

In VCEG-AH26, filter coefficients f(k.l) with indices k and l such that k>0 or l>0 are predicted from f(k,l) for which k<=0 and l<=0:

Eq3 (3)

In VEG-AI34, both complexity and number of bits required to transmit the post-filter are reduced by using coefficient symmetry as follows:

Eq4 (4)

According to the experimental results of improved post-filter, the average bit rate reduction obtained for 720p sequences is 8.3% for IPP configuration, 7% for IBBP configuration and 7.8% for hierarchical B configuration. It is also found out that if the overhead is neglected, the gain brought by the non-separable 2D filter does not seem to strongly depend on rate (QP=22,27,32,37) or resolution (CIF and 720p).

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