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Two Loop Filters in KTA

2010-08-01 KTA View Comments Views(9,750)

KTA employs two concatenating loop filters: the deblocking loop filter and the adaptive loop filter.

The deblocking loop filter, inherited from H.264/AVC, alleviates the blocking artifacts caused by the block-based DCT+MCP video coding framework. It uses a bank of low-pass filters, which are adaptively applied to block boundaries according to the boundary strength (BS), and provides better visual quality and improved capability to predict other pictures.

Adaptive loop filter (ALF; click here for introduction) is placed in the MCP loop after the deblocking process, and is used to restore the degraded picture (caused by compression) such that the MSE between the reconstructed and source frames is minimized. The coefficients of ALF are calculated and transmitted on a frame basis and the minimum mean squared error (MMSE) estimator is used. For each degraded frame, ALF can be applied to the entire frame or to local areas. The former is known as frame-based ALF. In the latter case, additional side information indicating which areas are to be filtered is transmitted, which can be block-based or quadtree-based.

In short, the deblocking loop filter improves the subjective quality, whereas ALF improves the objective quality, i.e., pixel-wise fidelity. From this perspective, the two techniques can be viewed as mutual complements. However, the joint use of the two techniques has limitations. As pointed out by Ikai et al. (JCTVC-B064), sequentially processing the deblocking filter and ALF is too time-consuming. Another problem is the technical redundancy, which means the information loss caused by the deblocking filter is partially restored by ALF.

In the video coding standards prior to H.264/AVC, quantization is the only cause of information loss.  The quantization noise, usually modeled as Gaussian noise, can be efficiently suppressed by ALF. In H.264/AVC, the deblocking filter, applying a fixed bank of low-pass filters to reconstructed frames, actually does not have the frequency selective properties to suppress the quantization noise; instead, it becomes another cause of information loss, although it improves subjective quality. Placed after the deblocking filter, ALF becomes inefficient to deal with the dual distortion, because the quantization distortion distribution of the reconstructed pictures has been significantly altered by the deblocking filter.

To solve the problem, unified loop filter (ULF) is proposed in [1] to replace the joint use of the deblocking filter and ALF. By unifying the deblocking filtering and ALF into one filtering framework, the causes of information loss are reduced, and therefore the degraded frames can be better restored.  Experimental results show that ULF can suppress the quantization noise optimally and improve the objective and subjective qualities of the reconstructed pictures simultaneously. Interested readers are referred to [2] for in-depth expositions.

[1] Yu Liu, Yan Huo, and Ka-Man Cheng, “Unified Loop Filter for Video Coding”, Hong Kong Applied Science & Technology Research Institute, MPEG M17171, Video Proposal, 91st MPEG Meeting, Kyoto, Japan, Jan. 2010.
[2] Yu Liu, “Unified Loop Filter for Video Compression”, IEEE Transactions on Circuits and Systems for Video Technology, Vol.20, No.10, pp.1378-1382, Oct. 2010.

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