Video coding standards k. R. Rao, Do Nyeon Kim and Jae Jeong Hwang Springer 2014

Analysis of coding tools in HEVC Test Model, HM 1.0 – Intra prediction

In HM 1.0, unified intra prediction provides up to 34 directional prediction modes for different PUs. With the PU size of 4x4, 8x8, 16x16, 32x32 and 64x64, there are 17, 34, 34, 34 and 5 prediction modes available respectively. The prediction directions in the unified intra prediction have the angles of +/- [0, 2, 5, 9, 13, 17, 21, 26, 32] /32. The angle is given by displacement of the bottom row of the PU and the reference row above the PU in case of vertical prediction, or displacement of the rightmost column of the PU and the reference column left from the PU in case of horizontal prediction. Figure 5.4 shows an example of prediction directions for 32x32 block size. Instead of different accuracies for different sizes, the reconstruction of the pixel uses the linear interpolation of the reference top or left samples at 1/32th pixel accuracy for all block sizes.


Figure 5.4. Available prediction directions in the unified intra prediction in HM 1.0

More details on unified intra prediction in HM 1.0 are available in

http://www.h265.net/2010/12/analysis-of-coding-tools-in-hevc-test-model-hm-intra-prediction.html

The working draft (WD) of the HEVC has gone through several updates/revisions and the final draft international standard (FDIS) has come out in January 2013. This refers to Main, Main10 and Main Intra profiles. In August 2013 five additional profiles Main 12, Main 4:2:2 12, Main 4:4:4 10 and Main 4:4:4 12 were released.[E153] Other range extensions include increased emphasis on high quality coding, lossless coding and screen content coding (SCC) [E216, E217, E322]. Scalable video coding (spatial, temporal, quality and color gamut scalabilities) and multiview video coding were finalized in July 2014 and standardized in October 2014. Joint call for proposals for coding screen content is described in [E218] and is to be finalized in 2016.

IEEE Journal on Emerging and Selected Topics in Circuits and Systems (JETCAS) has called for papers for the special Issue on Screen Content Video Coding and Applications. This special issue opens up number of research areas in SCC. (See P.5.85 and P.5.85a).

Scalability extensions [E160, E325, E326] and 3D video extensions which enable stereoscopic and multiview representations and consider newer 3D capabilities such as the depth maps and view-synthesis techniques have been finalized in 2015. For work on 3D video topics for multiple standards including 3D video extensions JCT-VC formed a team known as JCT on 3D Video (JCT-3V) in July 2012 [E158]. See also [E199]. The overall objective is to reduce the bit rate, increase the PSNR significantly compared to H.264/AVC (Chapter 4) with reasonable increase in encoder/decoder complexity. These three overview papers [E325, E326, E160] give detailed development and description of these extensions.

4. HEVC Encoder

IEEE Trans. CSVT vol.22, Dec. 2012 is a special issue on emerging research and standards in next generation video coding [E43]. This special issue provides the latest developments in HEVC related technologies, implementations and systems with focus on further research. As the HEVC development is ongoing, this chapter concludes with a number of projects related to HEVC with appropriate references and the info on the KTA and HEVC software [E95]. Hopefully these projects can provide additional insights to the tools and techniques proposed and provide a forum for their modifications leading to further improvements in the HEVC encoder/decoder. Figure 5.5 describes the HEVC encoder block diagram [E59]. This does not show the coded bit stream representing the various modes (intra/inter, CU/PU/TU sizes, intra angular prediction directions/modes, MV prediction, scaling and quantization of transform coefficients and other modes are shown in the decoder block diagram – see Fig. 5.6).

Figure 5.5 HEVC encoder block diagram [E59] © IEEE 2012.

For directional intra modes, an alternative transform related to DST is applied to 4x4 luma prediction residuals. For all other cases integer DCT is applied (Intra/Inter chroma, and Inter luma). This change was adopted in Stockholm in July 2012.

In entropy coding only the context adaptive binary arithmetic coding (CABAC) [E65, E66] is adopted unlike two (CAVLC and CABAC) in H.264/AVC (Chapter 4). Details on context modeling, adaptive coefficient scanning and coefficient coding are provided in [E58]. Chapter 8 – Entropy coding in HEVC – authored by Sze and Marpe in [E200] describes the functionality and design methodology behind CABAC entropy coding in HEVC . In the conclusions section of this Chapter Sze and Marpe state “The final design of CABAC in HEVC shows that by accounting for implementation cost and coding efficieny when designing entropy coding algorithms results in a design that can maximize processing speed and minimize area cost, while delivering high coding efficiency in the latest video coding standard’. The references cited at the end of this chapter are extensive and valuable.

Mode dependent directional transform [E3, E15] is not adopted. Only INTDCT (separable 2-D) for all cases other than 44 intra luma is used.

Figure 5.6 HEVC decoder block diagram⁺ [E22]

⁺ The decoder block diagram is adopted from C. Fogg, “Suggested figures for the HEVC specification”, ITU-T / ISO-IEC Document: JCTVC-J0292r1, July 2012. [E22]

While the HEVC follows the traditional (also proven) block based motion compensated prediction followed by transform coding, quantization, variable length coding (also adaptive intra mode) it differs significantly by adopting flexible quad tree coding block partitioning structure. This recursive tree coding structure is augmented by large block size transforms (up to 32x32), advanced motion prediction, sample adaptive offset (SAO) [E85, E109] besides the deblocking filter [E71]. The large multiple sizes recursive structure is categorized into coding unit (CU), prediction unit (PU) and transform unit (Fig.5.7) [E42]. For details about transform coefficient coding in HEVC see [E72].

Fig. 5.7 Recursive block structure for HEVC, where k indicates the depth for CU_k and TU_k [E42]

Complexity of the HEVC encoder is further increased by introducing intra adaptive angular direction prediction, mode dependent context sample smoothing, adaptive motion parameter prediction, in loop filtering (deblocking filter and sample adaptive offset -SAO). Details on these two in loop filters and their effectiveness in improving the subjective quality are described in Chapter 7 – In-loop filters in HEVC authored by Norkin et al, see [E200]. These and other tools contribute to 50% improved coding efficiency over H.264 at the cost of substantial increase in encoder complexity. The decoder complexity, however, is similar to that of H.264/AVC [E23, E59, E103]. Several techniques for reducing the intra prediction encoder complexity (see [E42] and various references cited at the end) are suggested. See also [E106]. Zhang and Ma [E42, E147] are also exploring the reduction of inter prediction complexity. They suggest that these two areas (intra and inter prediction modes) can be combined to reduce the overall HEVC encoder complexity. (See projects P.5.16 thru P.5.19). This is a fertile ground for research. A summary of the tools included in main and high efficiency 10 [HE10] is shown in Table 5.2. Details of these tools are described in Test Model encoder description [E57]. See also the review papers [E59, E97, E105]. The paper [E59] states “To assist industry community in learning how to use the standard, the standardization effort not only includes the development of a text specification document (HM8) but also reference software source code (both encoder/decoder)” [E95]. This software can be used as a research tool and as the basis of products. This paper also states “A standard test data suite is also being developed for testing conformance to the standard”.

Main		High efficiency 10 (HE10)
High-level Structure:
High-level support for frame rate temporal nesting and random access
Clean random access (CRA) support
Rectangular tile-structured scanning
Wavefront-structured processing dependencies for parallelism
Slices with spatial granularity equal to coding tree unit
Slices with independent and dependent slice segments
Coding units, Prediction units, and Transform units:
Coding unit quadtree structure square coding unit block sizes 2Nx2N, for N=4, 8, 16, 32 (i.e. up to 64x64 luma samples in size)
Prediction units (for coding unit size 2Nx2N: for Inter, 2Nx2N, 2NxN, Nx2N, and, for N>4, also 2Nx(N/2+3N/2) & (N/2+3N/2)x2N; for Intra, only 2Nx2N and, for N=4, also NxN)
Transform unit tree structure within coding unit (maximum of 3 levels)
Transform block size of 4x4 to 32x32 samples (always square)
Spatial Signal Transformation and PCM Representation:
DCT-like integer block transform; for Intra also a DST-based integer block transform (only for Luma 4x4)
Transforms can cross prediction unit boundaries for Inter; not for Intra
Skipping transform is allowed for 4x4 transform unit
PCM coding with worst-case bit usage limit
Intra-picture Prediction:
Angular intra prediction (35 modes including DC and Planar )
Planar intra prediction
Inter-picture Prediction:
Luma motion compensation interpolation: 1/4 sample precision, 8x8 separable with 6 bit tap values for 1/2 precision, 7x7 separable with 6 bit tap values for 1/4 precision
Chroma motion compensation interpolation: 1/8 sample precision, 4x4 separable with 6 bit tap values
Advanced motion vector prediction with motion vector “competition” and “merging”
Entropy Coding:
Context adaptive binary arithmetic entropy coding (CABAC)
Rate-distortion optimized quantization (RDOQ)
Picture Storage and Output Precision:
8 bit-per-sample storage and output	10 bit-per-sample storage and output
In-Loop Filtering:
Deblocking filter
Sample-adaptive offset filter (SAO)

Table 5.2 Structure of tools in HM9 configuration [E57]

1. Intra prediction

Figure 5.8 shows the 33 intra prediction angle directions [E57, E59, E123] corresponding to the VER and HOR described in Fig. 5.4. Figure 5.9 shows the 33 intra prediction mode directions. The mapping between the intra prediction mode directions and angles is shown in Table 5.3 [E123]. See also [E102]. These intra prediction modes contribute significantly to the improved performance of HEVC. Statistical analysis of the usage of the directional prediction modes for all intra case has shown that besides planar (mode 0) and dc (mode 1), horizontal (mode 10) and vertical (mode26) are at the top of this ranking [E102]. The authors in [E102] by developing a new angular table have demonstrated improved coding gains for video sequences with large amounts of various textures. Each intra coded PU shall have an intra prediction mode for luma and another for chroma components. All TUs within a PU shall use the same associated mode for each component. Encoder then selects the best luma intra prediction mode from the 35 directions (33 plus planar and DC). Due to increased number of directions (compared to those in H.264/AVC – Chapter 4) HEVC considers three most probable modes (mpm) compared to one mpm in H.264/AVC. For chroma of intra PU, encoder then selects the best chroma prediction mode from 5 modes including planar, DC, horizontal, vertical and direct copy of intra prediction mode for luma. Details about mapping between intra prediction direction and mode # for chroma are given in [E57, E123].

Detailed description of the HEVC encoder related to slices/tiles, coding units, prediction units, transform units, inter prediction modes, special coding modes, MV estimation/ prediction, interpolation filters for fractional pixel MV resolution, weighted prediction, transform sizes (4x4, 8x8, 16x16 and 32x32) [E72], scanning the transform coefficients – Fig.5.10 - (see [E72] for details), scaling/quantization, loop filtering (deblocking filter [E71, E283]) and SAO [E85, E109]), entropy coding (CABAC [E65, E66]) and myriad other functionalities are provided in [E59, E97, E105]. An excellent resource is V. Sze, M. Budagavi and G.J. Sullivan (Editors), “High efficiency video coding: Algorithms and architectures”, Springer 2014. [E200]. Another excellent resource is the book M. Wien, “High efficiency video coding: Coding tools and specification”, Springer 2015. In [E200], different aspects of HEVC are dealt within different chapters contributed by various authors who have been involved in all phases of the HEVC development as an ITU-T and ISO/IEC standard. [E200] Popular zigzag scan is not adopted.

Fig. 5.8 Intra prediction angle definition [E57, E123]

Fig. 5.9 Intra prediction mode directions [E57, E123]. See also [E102].

Table 5.3 Mapping between intra prediction mode direction (shown in Fig. 5.9) and intra prediction angles (shown in Fig. 5.8) [E123]

Both visual and textual description of these directional modes, besides filtering process of reference samples as predictors, post processing of predicted samples and myriad other details are described in Chapter 4 Lainema and W.-J. Han, “Intra-picture prediction in HEVC”, [E200]

2. Transform coefficient scanning

The three transform coefficient scanning methods, diagonal, horizontal and vertical adopted in HEVC for a 8x8 transform block (TB) are shown in Fig. 5.10 [E72]. The scan in a 4x4 transform block is diagonal. Horizontal and vertical scans may also be applied in the intra case for 4x4 and 8x8 transform blocks.


(a)


(b)
Fig. 5.10 a) Diagonal scan pattern in 8x8 TB. The diagonal scan of a 4x4 TB is used within each 4x4 sub block of larger blocks. b) Coefficient groups for 8x8 TB [E72]. © 2012 IEEE.

Chapter 6, Budagavi, Fuldseth and Bjontegaard, “HEVC transform and quantization”, in [E200] describes the 4x4 to32x32 integer 2-D DCTs including the embedding process (small size transforms are embedded in large size transforms)¸ default quantization matrices for transform block sizes of 4x4 and 8x8 and other details are addressed. The embedding feature allows for different transform sizes to be implemented using the same architecture thereby facilitating hardware sharing.

An alternate 4x4 integer transform derived from DST for intra 4x4 luma blocks is also listed.

3. Luma and chroma fractional pixel interpolation

Integer (A_i,j) and fractional pixel positions (lower case letters) for luma interpolation are shown in Fig.5.11 [E59, E107]. See [E69] for generalized interpolation.

Fig. 5.11 Integer and fractional positions for luma interpolation [E59] ©2012 IEEE

Unlike a two-stage interpolation process adopted in H.264, HEVC uses separable 8-tap filter for ½ pixels and 7-tap filter for ¼ pixels (Table 5.4) [E59, E69, E107]. Similarly 4-tap filter coefficients for chroma fractional (1/8 accuracy) pixel interpolation are listed in Table 5.5. Lv et al [E107] have conducted a detailed study of performance comparison of fractional-pel interpolation filters in HEVC and H.264/AVC and conclude that the filters in HEVC increase the BD rates [E79, E196] by more than 10% compared to those in H.264/AVC (Chapter 4) at the cost of increased implementation complexity.

Table 5.4 Filter coefficients for luma fractional sample interpolation [E59] © 2012 IEEE

Table 5.5 Filter coefficients for chroma fractional sample interpolation [E59] © 2012 IEEE

4. Comparison of coding tools of HM1 and HEVC draft 9

Coding tools of HEVC test model version 1 (HM1) and draft 9 [E57] are summarized in Table 5.6 [E59]. The overview paper on HEVC by Sullivan et al [E59] is an excellent resource which clarifies not only all the inherent functionalities but also addresses the history and standardization process leading to this most efficient standard. In the long run, the HEVC (including the additions/extensions/profiles) has the potential/prospects to overtake all the previous standards including H.264/AVC (Chapter 4).

Table 5.6 Summary of coding tools of high efficiency configuration in HM1 and HEVC [E59] © 2012 IEEE

5.