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, additiona[......]

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Permanent Link: Two Loop Filters in KTA

Why use interpolation in video coding?  

Motion-compensated prediction (MCP) is the key to the success of the modern video coding standards, as it removes the temporal redundancy in video signals and reduces the size of bitstreams significantly. With MCP, the pixels to be coded are predicted from the temporally neighboring ones, and only the prediction errors and the motion vectors (MV) are transmitted. However, due to the finite sampling rate, the actual position of the prediction in the neighboring frames may be out of the sampling grid, where the intensity is unknown, so the intensities of the positions in between the integer pixels, called sub-positions, must be interpolated and the resolution of MV is increased accordingly.  

Interpolation in H.264/AVC  

In H.264/AVC, for the resolution of MV is quarter-pixel, the reference frame is interpolated to be 16 times the size for MCP, 4 times both sides. As shown in Fig. 1(a), the interpolation defined in H.264 includes two stages, inter[......]

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Permanent Link: Adaptive Interpolation Filter for Video Coding

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[......]

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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[......]

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Permanent Link: Adaptive Post/Loop Filters in JM/KTA – Part 1

The technique of 1/8-pel interpolation [AD09] was proposed for motion-compensated prediction (MCP) and adopted in KTA software. Three types of interpolation filters are used for 1/2-, 1/4-, and 1/8-pel sub positions, respectively.

• [-3, 12, -39, 158, 158, -39, 12, -3]/256 for 1/2-pel sub positions.
• [-3, 12, -37, 229, 71, -21, 6, -1]/256 and [-1, 6, -21, 71, 229, -37, 12, -3]/256 for 1/4-pel sub positions.
• Bilinear filter for 1/8-pel sub positions.

  The frequency response of the interpolation filter is shown in the following figure. As can be seen, it is almost an ideal low-pass filter with a gain of 8 and a cutoff frequency π/8.

  According to the performance reported in the proposal, the gain on CIF/QCIF sequences is quite significant, i.e., up to 14% bit-rate reduction. I tested this technique based on a set of HD sequences. As shown in Table 1, the R-D performance is measured by BDPSNR [1], i.e., PSNR improvement at the same bit-rate or bit-rate reduction at the same PSNR.

 

 

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Permanent Link: R-D Performance of 1/8-pel MCP on HD Sequences