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

TMuC is the initial test model of JCT-VC, but it is not formally adopted as a test model of the draft standard, as no thorough testing has been performed for such a possible combination of tools. The coding tools in TMuC will be further tested to confirm their effectiveness, before adopted in a formal test model.

TMuC provides more flexibility than H.264/AVC. The  basic coding unit, called coding tree block (CTB), which has a similar role to the macroblocks in H.264/AVC, can have variable sizes (a power of 2). The sizes of the largest and smallest CTBs are specified in the sequence parameter set (SPS). A frame is divided into non-overlapped largest CTBs (LCTB), e.g., 128×128, and then each LCTB can be further divided in a recursive tree representation.

Each CTB has its own prediction type (intra/inter) and prediction partition. The partition can be symmetric, just as in H.264/AVC, or asymmetric, e.g., 64×64 block can be partitioned into 64×16/64×48 or 16×64/48&[......]

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Permanent Link: Introduction to Test Model under Consideration (TMuC)

During Apr. 15-23, 2010, the first meeting of JCT-VC was held in Dresden, DE. In the meeting, 27 responses to the Call for Proposal, issued in Jan. 2010, were reviewed and the associated video material was evaluated in extensive subjective tests.

Tentative Conclusions from CfP Responses Reviews

Coding tools used in respective proposals are listed in JCTVC-A203. High-level noteworthy aspects from the review of the proposals and the outcome of the subjective tests are outlined:

• Substantial progress in coding efficiency has clearly been demonstrated, compared with H.264/AVC
• There is no indication of a need to change the fundamental architecture of “conventional” hybrid video coding designs to achieve a substantial improvement
• Inclusion of support of larger block sizes in a highly variable (typically tree-structured) block segmentation approach is a major common theme, although large block sizes were not found in all proposals that did well subjectively (and objectively)
• Modif[......]

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Permanent Link: The First JCT-VC Meeting, Dresden, DE

The intra prediction in H.264/AVC is a type of spatial domain directional prediction, which means different intra prediction modes represent different prediction directions, such as horizontal, vertical, and diagonal. An intra-coded MB can be partitioned into 4×4, 8×8, or 16×16 intra prediction blocks. The 4×4 and 8×8 intra prediction blocks have nine prediction directions, respectively, and the 16×16 block has four. Hence, totally 22 (9+9+4) intra prediction modes are used in H.264/AVC. The residue usually has high energy along the direction of prediction, as edges are more difficult to be predicted than smooth areas.

Mode-dependent directional transform (MDDT) was proposed to compact the residue produced by intra prediction. It consists of a series of pre-defined separable transforms; each transform is efficient in compacting energy along one of the prediction directions, thus favoring one of the intra modes. The type of MDDT is coupled with the selected[......]

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Permanent Link: Mode-Dependent Directional Transform (MDDT) in JM/KTA

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

In the Geneva meeting held in Feb. 2009, a proposal with the title “Video Coding Using Extended Block Sizes” was adopted by KTA, where the MB size is extended up to 64×64 and the motion partitions are scaled accordingly. At the same time, a 2D order-16 transform was also proposed for transforming the residual blocks with the size larger than or equal to 16×16. The transformation matrix of the proposed 2D order-16 transform is given as below, which is obtained by scaling the transformation matrix of 2D order-16 DCT by the factor 128 and rounding, and is non-orthogonal.

  Non-orthogonality will inevitably introduce transform error. Before analyzing the transform error quantitatively, let’s recall two properties of orthogonal transforms. Firstly, signals can be reconstructed perfectly if no quantization is performed in the transform domain. Secondly, if quantization is performed in the transform domain, the average variance (or energy) of the reconstruction er[......]

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Permanent Link: Transform Error Introduced by Non-orthogonality

The latest KTA software is JM11KTA2.3, which integrates the coding tools adopted in the Geneva meeting (Jan. 2009) and before.  

1. Inter prediction
   1. Adaptive interpolation filter (AIF)
      • 2-D non-separable AIF (AD08, AE16)
      • Separable AIF (COM16-C219, AG10)
      • Directional AIF (DAIF) (AG21, AG22, AH17, AH18)
      • Enhanced DAIF (E-DAIF) (AI12,  COM16-C126)
      • Enhanced DAIF 2 (E-DAIF2) (COM16-C125)
      • Enhanced AIF (EAIF) (COM16-C464, AI38, AJ30)
      • Switch interpolation filters with offsets (SIFO) (COM16-C463, AI35, AJ29, COM16-C126)
      • High precision filter (HPF) (AI33)
      • Single-Pass Encoding (AJ29, AK26)
   2. 1/8-pel MCP (AD09)
   3. Extended MCP block size (COM16-C123)
   4. Competition-based MV prediction (AC06r1)
2. Transform and quantization
   1. Mode-dependent directional transform (MDDT) (AF15, AG11, AH20, AJ24, AI36)
   2. Very large block transform (COM16-C123)
   3. Adaptive prediction error coding (APEC) (AB06, AD07, AE15)
   4. Adaptive quantization matrix selection (AQMS) (AC07, AD06, AF08, AI19)
   5. Rate-distortion optimized quantization (RDO[......]

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Permanent Link: KTA Software JM11KTA2.3