ORIGINAL RESEARCH PAPER Search range reduction for uni-prediction and bi-prediction in HEVC K. C. Ravi Chandra Varma 1 • M. Venkata Phani Kumar 1 • Sudipta Mahapatra 1 Received: 28 June 2016 / Accepted: 1 September 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract The motion estimation block of the high-effi- ciency video coding (HEVC) standard is highly complex. This paper proposes two algorithms, namely external search range reduction (ESR) and internal search range reduction (ISR) to reduce the motion estimation complex- ity of HEVC in uni-prediction and bi-prediction both in the fast search mode. The proposed ESR algorithm for uni- prediction motion estimation reduces the search range adaptively for Test Zonal Search (TZS) motion estimation with negligible loss in coding efficiency. The proposed ISR algorithm for uni-prediction, namely ISR-R, limits the search range further in the raster search stage of the TZS algorithm. Moreover, a fast bi-prediction motion estima- tion algorithm is proposed which includes both ESR and ISR to reduce the motion estimation complexity in bi- prediction. Our algorithms are implemented in the HM- 16.6 encoder in the fast search mode. The performance of the proposed algorithms are tested individually and then by combining all the algorithms. When the algorithms are combined, the number of search points and motion esti- mation time are reduced by 60.58 and 59.58 %, respec- tively in the fast search mode with a BD-Rate of 0.193 % and BD-PSNR of -0.005 % in the LD-B main profile. The number of search points and motion estimation time are reduced by 59.55 and 57.71 %, respectively in the fast search mode with a BD-Rate of 0.265 % and BD-PSNR of -0.008 % in the RA main profile. Keywords High-efficiency video coding (HEVC) Á Search range reduction algorithm Á Uni-prediction Á Bi-prediction 1 Introduction The high-efficiency video coding (HEVC) standard is developed by the Joint Collaborative Team on Video Coding (JCT-VC) to increase the coding efficiency [1]. HEVC reduces the bit rate by nearly 50 % compared to its previous standard, H.264/AVC [2]. On the other hand, the complexity of the encoder is greatly increased. In HEVC, the use of high- performance coding tools has increased the complexity while improving the video quality and obtaining a higher compression ratio. HEVC uses larger block sizes which enables higher compression especially for high-resolution sequences. A Coding tree unit (CTU) is the basic block of size L Â L, where L is 64, 32 or 16. Large sized CTUs yield higher compression. A CTU is divided into Coding Units (CUs) in a quad tree structure. These CUs are further divided into Prediction Units (PUs) and Transform Units (TUs) in a tree structure. Each of the CTUs, CUs, PUs and TUs has one luma component and two chroma components and they are called as Coding Tree Blocks (CTBs), Coding Block (CBs), Prediction Block (PB) and Transform Block (TB), respec- tively. For deciding the block size, rate-distortion (R-D) optimization is done for all the possible combinations of CUs, PUs and TUs. Due to the recursive tree-structured partitioning of CTUs to CUs and then CUs to PUs and TUs, the complexity of motion estimation in HEVC is increased tremendously. & K. C. Ravi Chandra Varma kcravi912@gmail.com M. Venkata Phani Kumar venkataphanikumarm@gmail.com Sudipta Mahapatra sudipta.mahapatra@gmail.com 1 Department of Electronics and Electrical Communication Engineering, IIT Kharagpur, Kharagpur, West Bengal 721302, India 123 J Real-Time Image Proc DOI 10.1007/s11554-016-0636-5