Ivan Zupancic

Inter-Prediction Optimizations for Video Coding Using Adaptive Coding Unit Visiting Order

The flexible partitioning scheme and increased number of prediction modes in the high efficiency video coding (HEVC) standard are largely responsible for both its high compression efficiency and computational complexity. In typical HEVC encoder implementations, coding units (CUs) in a coding tree unit (CTU) are visited from top to bottom at each level of recursion to select the optimal coding configuration. In this paper, a novel approach is presented in which CUs in a CTU can be adaptively visited also ina reverse, bottom to top visiting order. This reverse CU (RCU) visiting order allows for different algorithmic optimizations for further complexity reduction of many HEVC encoding steps, especially under challenging conditions, such as highly textured or fast moving content. In particular, algorithms to reduce complexity of HEVC depth selection, mode decision, and inter-prediction are presented here based on the coding information obtained from higher depths when using the RCU visiting order. Experimental results show that enabling different stages of the proposed algorithm can achieve average speedups from 16.3% to 36.6% compared to fast reference HEVC implementation with pre-built speed-ups enabled (up to 51.2% in some cases), for 0.3% to 2.2% BD-rate penalty.

HEVC coding optimisation for Ultra High Definition television services

Ultra High Definition TV (UHDTV) services are being trialled while UHD streaming services have already seen commercial débuts. The amount of data associated with these new services is very high thus extremely efficient video compression tools are required for delivery to the end user. The recently published High Efficiency Video Coding (HEVC) standard promises a new level of compression efficiency, up to 50% better than its predecessor, Advanced Video Coding (AVC). The greater efficiency in HEVC is obtained at much greater computational cost compared to AVC. A practical encoder must optimise the choice of coding tools and devise strategies to reduce the complexity without affecting the compression efficiency. This paper describes the results of a study aimed at optimising HEVC encoding for UHDTV content. The study first reviews the available HEVC coding tools to identify the best configuration before developing three new algorithms to further reduce the computational cost. The proposed optimisations can provide an additional 11.5% encoder speed-up for an average 3.1% bitrate increase on top of the best encoder configuration.

Multiple Early Termination for fast HEVC coding of UHD content

The recently ratified High Efficiency Video Coding (HEVC) standard is significantly outperforming previous video coding standards in terms of compression efficiency. However, this comes at the cost of very high computational complexity, which may limit its real-time usage, particularly when targeting Ultra High Definition (UHD) applications. In this paper, an analysis of HEVC coding on UHD content is presented, showing that on average more than 18% of the total encoding time is spent performing uni-directional Motion Estimation (ME) even when using fast algorithms such as Enhanced Predictive Zonal Search (EPZS). In order to speed up the ME process, a novel approach for fast inter prediction is proposed in this paper based on a Multiple Early Termination (MET) decision process. EPZS is only performed in blocks in which it is needed based on local features of the encoded content, or it is skipped otherwise. Experimental results show that the algorithm achieves on average 9.3% speed-ups over conventional HEVC, at the cost of very small BD-rate losses.

Fast HEVC coding using reverse CU visiting

The High Efficiency Video Coding (HEVC) standard makes use of flexible partitioning to achieve high compression ratios. Each frame is divided in Coding Tree Units (CTUs) of fixed size, which are further partitioned into Coding Units (CUs) following a recursive quadtree structure. Typically, CUs at each level of recursion are tested to select the optimal coding configuration, hence this process is extremely demanding in terms of computational complexity. In this paper, a method to reduce complexity of HEVC quadtree configuration selection and mode decision is presented, based on a reverse bottom-to-top visiting order of CUs in the quadtree. By visiting smallest CUs first, information can be extracted to make decisions on larger CUs. The encoder adaptively selects whether a CTU is encoded using the reverse CU visiting, allowing for considerably faster encoding under all conditions. Experimental results show that the algorithm achieves on average 21% speedups over previous state-of-the-art fast HEVC algorithms, and up to 36% for some sequences, at very limited efficiency losses.

Adaptive precision motion estimation for HEVC coding

Most video coding standards, including the state-of-the-art High Efficiency Video Coding (HEVC), make use of sub-pixel Motion Estimation (ME) with Motion Vectors (MV) at fractional precisions to achieve high compression ratios. Unfortunately, sub-pixel ME comes at very high computational costs due to the interpolation step and additional motion searches. In this paper, a fast sub-pixel ME algorithm is proposed. The MV precision is adaptively selected on each block to skip the half or quarter precision steps when not needed. The algorithm bases the decision on local features, such as the behaviour of the residual error samples, and global features, such as the amount of edges in the pictures. Experimental results show that the method reduces total encoding time by up to 17.6% compared to conventional HEVC, at modest efficiency losses.

Context adaptive mode sorting for fast HEVC mode decision

Typical H.265/High Efficiency Video Coding (HEVC) encoder implementations test a variety of prediction modes and select the optimal configuration for each block in terms of Rate-Distortion (RD) cost. A fast HEVC mode decision algorithm is proposed here referred to as Context Adaptive Mode Sorting (CAMS). The frequency of selection of modes and their RD costs are collected while encoding the training frames based on local parameters (the context). This information is then used to sort and restrict the prediction modes to test for each context, while the optimal mode found using CAMS on each CU is validated based on the RD cost distributions found during the training. Experimental results show that the method reduces total encoding time of fast HEVC implementations on average by 29.3%, at modest efficiency losses.

Studying Rate Control Methods for UHDTV Delivery Using HEVC

Since the early video coding standardisation efforts, rate control has been considered essential for almost any application. With the advent of improved video coding standards and the introduction of advanced flexible coding tools, previous Rate-Distortion (RD) models used for rate control have become obsolete. To address this issue, some rate control methods have been recently proposed specifically for the current state-of-the-art High Efficiency Video Coding (HEVC) standard which introduce many useful features, such as a robust correspondence between the rate and Lagrange multiplier λ. However, when applying these rate control methods on sequences in the new Ultra High Definition Television (UHDTV) format, degraded coding performance was observed. In this paper, an analysis of the state-of-the-art HEVC rate control method was performed and two directions for its improvement were evaluated. These improvements target frame-level bit-allocation and model parameter initialisation. When compared to the rate control method implemented in the HEVC reference software, these improvements result in reduced BD-rate losses of 3.1% and 2.1%, versus the 8.8% provided by the reference algorithm. Moreover, the proposed improvements increase the accuracy in hitting the target bit-rate.

Inter-prediction optimisations for fast HEVC encoding of ultra high definition content

Thanks to technological advances in devices for video signal capturing and displaying, the usage of large resolution video formats is becoming more and more popular in consumer and professional applications. The new Ultra High Definition (UHD) format aims to provide enhanced viewing experience with its spatial resolution of at least 3840 × 2160 luma samples per frame and temporal resolution of up to 120 frames per second. Such a large amount of raw data requires extremely high levels of compression efficiency. The latest state-of-the-art High Efficiency Video Coding (HEVC) standard was developed also to respond to these needs. HEVC outperforms previous video coding standards in terms of coding efficiency, especially while encoding high resolution video content. However, this comes at very high computational costs, making current HEVC encoder implementations of very limited usage. In this paper, a fast inter-prediction scheme is proposed targeting fast mode decision based on reverse Coding Unit (CU) visiting order, and early termination of both the integer- and fractional-pel motion estimation processes. Experimental results show that the proposed algorithm achieves on average 72.9% time savings, for limited coding losses.

HEVC encoder optimisations using adaptive coding unit visiting order

The flexible partitioning scheme and increased number of prediction modes in the High Efficiency Video Coding (HEVC) standard are largely responsible for both its high compression efficiency and computational complexity. Each frame in HEVC is partitioned in Coding Tree Units (CTUs) of fixed size, which are then recursively partitioned in Coding Units (CUs). In typical implementations, CUs in a CTU are visited from top to bottom at each level of recursion. In this paper, a different approach is used in which CUs in a CTU can be adaptively visited also in reverse order from bottom to top. Three novel algorithms to reduce complexity of HEVC depth selection, mode decision and inter-prediction are presented, based on this adaptive visiting order. Experimental results show that the proposed encoder achieves on average 38.2% speed-ups compared to fast reference HEVC implementations with pre-built speed-ups enabled, for very limited efficiency losses.

Two-pass rate control for UHDTV delivery with HEVC

Rate control has been regarded as an indispensable video coding tool for virtually any application involving video transmission. With the advent of many flexible tools introduced in the current state-of-the-art High Efficiency Video Coding (HEVC) standard, previous Rate-Distortion (RD) models used for rate control become insufficiently accurate. To overcome this issue, a new RD model has been recently proposed based on a robust correspondence between the rate and Lagrange multiplier λ. However, existing methods based on this model tend to perform sub-optimally after the scene change. In this paper, a two-pass rate control method is proposed, targeting Ultra High Definition Television (UHDTV) applications. In the first pass, a fast encoder with a reduced set of coding tools is used to obtain the data used for rate allocation and model parameter initialisation utilised during the second pass. Multiple encoding steps required to derive this information are avoided with the proposed variable quantization parameter framework. Experimental evaluation showed that the proposed two-pass rate control method achieves on average 4.4% BD-rate loss, compared with variable bit-rate encoding. That significantly outperforms the state-of-the-art HEVC rate control method with an average BD-rate loss of 8.8%.