A High Performance Deblocking Filter Hardware for High Efficiency Video Coding

Transcription

1 714 IEEE Transactions on Consumer Electronics, Vol. 59, No. 3, August 2013 A High Performance Deblocking Filter Hardware for High Efficiency Video Coding Erdem Ozcan, Yusuf Adibelli, Ilker Hamzaoglu, Senior Member, IEEE Abstract The recently developed High Efficiency Video Coding (HEVC) international video compression standard uses adaptive deblocking filter for reducing blocking artifacts. Deblocking filters increase both subjective and objective quality. But, they have high computational complexity. Therefore, in this paper, the first HEVC deblocking filter hardware in the literature is proposed. Two parallel datapaths are used in the proposed hardware in order to increase its performance. The proposed hardware is implemented in Verilog HDL. The Verilog RTL code is verified to work correctly on an FPGA board. The proposed HEVC deblocking filter hardware can code 30 full HD () video frames per second. Therefore, it can be used in consumer electronics products that require a real-time HEVC encoder or decoder. 1 Index Terms HEVC, Deblocking Filter, Hardware Implementation, FPGA. I. INTRODUCTION Joint collaborative team on video coding (JCT-VC) recently developed a new international video compression standard called High Efficiency Video Coding (HEVC) [1, 2, 3]. HEVC has 36% better compression efficiency than H.264 which is the current state-of-the-art video compression standard. HEVC standard achieves this video compression efficiency by using a number of new encoding tools. As it can be seen from the HEVC encoder block diagram shown in Fig. 1, one of these tools is the new deblocking filter algorithm. HEVC, same as the previous video compression standards, divides video frames into blocks and performs transform and quantization for each block separately. This causes correlation loss between blocks and discontinuities on the edges of blocks. Therefore, reconstructed frames suffer from blocking artifacts. Deblocking filter (DBF) improves the visual quality of decoded frames by reducing the visually disturbing blocking artifacts and discontinuities in a frame due to coarse quantization. Since the filtered frame is used as a reference frame for motion-compensated prediction of future frames, DBF also increases coding efficiency resulting in bit rate savings [4, 5, 6]. HEVC DBF algorithm is applied to each edge of all luma and chroma blocks in a Largest Coding Unit (LCU), a 64x64 pixel 1 This work was supported in part by the Scientific and Technological Research Council of Turkey (TUBITAK). E. Ozcan, Y. Adibelli, and I. Hamzaoglu are with Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla, Istanbul, Turkey ( {eozcan, yadibelli, hamzaoglu}@sabanciuniv.edu). Contributed Paper Manuscript received 07/01/13 Current version published 09/25/13 Electronic version published 09/25/ /13/$ IEEE array, after inverse quantization and inverse transform [1, 6]. In order to decide whether DBF will be applied to an edge or not, the related pixels in the current and neighboring 16x16 Coding Units (CU) must be read from memory and processed. H.264 DBF algorithm has high computational complexity. H.264 DBF algorithm accounts for one-third of the computational complexity of an H.264 video decoder [4]. HEVC DBF algorithm also has high computational complexity. HEVC has higher computational complexity than H.264, and HEVC DBF algorithm accounts for one-fifth of the computational complexity of an HEVC video decoder [7]. Therefore, the first HEVC DBF hardware in the literature is proposed in [8]. In this paper, it is explained in more detail, more experimental results are given, and it is compared with H.264 DBF hardware in the literature. The proposed HEVC DBF hardware can be used as part of an HEVC video encoder or decoder. The proposed DBF hardware starts filtering the available edges after a new 64x64 LCU is ready. Two parallel datapaths are used in the proposed DBF hardware in order to increase its performance. The proposed DBF hardware is implemented in Verilog HDL. The Verilog RTL code is verified to work correctly on an FPGA board which includes an FPGA implemented in 40nm CMOS technology, external memory and interfaces such as DVI. The proposed HEVC DBF hardware can code 30 full HD () video frames per second. The rest of the paper is organized as follows. Section II presents a brief overview of HEVC DBF algorithm. Section III describes the proposed HEVC DBF hardware in detail. Section IV presents the implementation results. Finally, Section V presents the conclusions. II. HEVC DBF ALGORITHM HEVC DBF algorithm for an 8x8 block edge consisting of two segments is shown in Fig. 2. In HEVC, there is a quadtree structure [1]. Each video frame is divided into 64x64 LCUs in raster scan order, and each LCU is divided into 16x16 CUs as shown in Fig. 3. DBF is applied to edges of the 8x8 blocks in all 16x16 CUs. Each edge of an 8x8 block consists of 8 consecutive lines which are divided into two independent 4 line segments. Each line has 8 pixels along the edge. DBF can update up to 3 pixels in each direction that the filtering takes place. First, vertical edges are filtered. Then, horizontal edges are filtered. There are several conditions that determine whether a segment will be filtered or not. There are additional conditions that determine the strength of the filtering for 16x16 CU edges

2 E. Ozcan et al.: A High Performance Deblocking Filter Hardware for High Efficiency Video Coding 715 Fig. 1. High Efficiency Video Coding Encoder Block Diagram Fig. 2. HEVC Deblocking Filter Algorithm

3 716 IEEE Transactions on Consumer Electronics, Vol. 59, No. 3, August 2013 Since the decision process needs the first and fourth lines of each segment, input pixel memory is loaded with the pixels along the edge for subsequent filtering process. TABLE I CONDITIONS THAT DETERMINE BS Coding Modes and Conditions BS Fig. 3. Edge Processing Order that will be filtered. Strong or weak filtering can be applied to an edge depending on these conditions. Boundary strength (BS) parameter, quantization parameter (QP), β and tc threshold values and the values of the pixels in the edge determine the outcomes of these conditions, and the values of up to 3 pixels on both sides of an edge can be changed depending on the outcomes of these conditions. Every edge is assigned a BS value depending on the coding modes and conditions of 16x16 CUs. The strength of the filtering done for an edge is proportional to its BS value. BS value can be 0, 1, or 2. No filtering is done for the edges with a BS value of 0, whereas strongest filtering is done for edges with a BS value of 2. BS decision is critical, since excessive filtering may lead to unnecessary smoothing of the picture details whereas lack of filtering may leave blocking artifacts which would reduce visual quality. The conditions used for determining the BS value for an edge between two neighboring 16x16 CUs are summarized in Table I. III. PROPOSED HEVC DBF HARDWARE The proposed DBF hardware architecture is shown in Fig. 4. It includes two parallel datapaths, a control unit, a transpose memory, two input buffers to store the pixels in segment1 and segment2 of a CU, two and four internal s to store partially filtered pixels, and two output buffers to store the filtered output pixels. In order to process full HD video frames in real time, proposed DBF hardware reads 16 pixels in one clock cycle from external memory. Therefore, it fills the input pixel memory in 4 clock cycles. At least one of the blocks is Intra 2 At least one of the blocks has non-zero coded residual coefficient and boundary is a transform boundary Absolute differences between corresponding spatial motion vector components of the two blocks are >= 1 in units of integer pixels Motion compensated prediction for the two blocks refers to different reference pictures or the number of motion vectors is different for the two blocks Otherwise 0 Fig. 5. Pixels Stored in Top and Left Memories Fig. 4. Proposed HEVC DBF Hardware

4 E. Ozcan et al.: A High Performance Deblocking Filter Hardware for High Efficiency Video Coding 717 Fig. 6. Proposed HEVC DBF Datapath The proposed DBF hardware starts filtering as soon as 64x64 LCU is ready. The two datapaths filter two segments, segment1 and segment2, in parallel. Transpose memory is used to transpose the filtered pixels before they are stored to intermediate or output s. This allows accessing 16 pixels in one clock cycle from transpose memory and simplifies reading the pixels from intermediate s. If an LCU is located in the left frame boundary, its left edges are not filtered. This causes an irregularity, and therefore increases the complexity of the control unit. In order to avoid this irregularity and therefore simplify the control unit, frame is extended at left boundary for 4 pixels as shown in Fig. 5. These pixels and the BS values of these edges are assigned as zero in order to avoid filtering these edges without causing an irregularity in the control unit. Top and left memories are used to store the pixels in the leftmost and topmost edges of an LCU as shown in Fig. 5. In the MxN frame shown in Fig. 5, squares represent 64x64 LCUs and each LCU has sixteen 16x16 CUs. In order to filter an LCU, its top and left neighboring 4x64 and 64x4 blocks, shown as shaded small squares in Fig. 5, should be available. In order to reduce the amount of off-chip memory accesses and therefore reduce power consumption of the DBF hardware, top 64x4 blocks of all LCUs in a row of a frame, shown as lightly shaded small squares in Fig. 5, and left 4x64 blocks of the current LCU, shown as darkly shaded small squares in Fig. 5, are stored in on-chip memories. For full HD video frames, 1920x32/2 = 960x32 size 2 memories are used for storing top blocks, and 64x32/2 = 32x32 size 2 memories are used for storing left blocks. The proposed DBF datapath is shown in Fig. 6. It can process 4 pixels, which are selected by the first four multiplexers, in parallel to increase the performance. The proposed datapath implements both the decision and filtering parts of HEVC DBF algorithm. Comparator1 is used for implementing the decision part. Comparator2 is used for implementing Clip3 function. Comparator3 and Comparator4 are used for implementing Clip1Y function. The filtered pixels are stored in outreg register.

5 718 IEEE Transactions on Consumer Electronics, Vol. 59, No. 3, August 2013 IV. IMPLEMENTATION RESULTS The proposed HEVC DBF hardware is implemented in Verilog HDL. The implementation is verified with RTL simulations. RTL simulation results matched the results of a software model of the HEVC DBF algorithm. The Verilog RTL code is mapped to an FPGA implemented in 40nm CMOS technology. The FPGA implementation is verified to work correctly on an FPGA board which includes an FPGA implemented in 40nm CMOS technology, external memory and interfaces such as DVI. The FPGA implementation uses 5236 LUTs, 1547 DFFs and 8 BRAMs. BRAMs are implemented as dual-port block SelectRAMs. The FPGA implementation works at 108 MHz. It takes 7680 clock cycles in the worst-case to process an LCU. The FPGA implementation can process a full HD () video frame in 33.9 ms (480 LCUs x 7680 clock cycles per LCU x 9.2 ns clock cycle = 33.9 ms). Therefore, it can process 1000/33.9 = 30 full HD frames per second. Since HEVC DBF algorithm is highly adaptive, amounts of strong and weak filtering operations performed for block edges differ from frame to frame. The amounts of strong and weak filtering operations performed for five different video sequences are shown in Fig. 7. All video sequences are intra coded and quantization parameter (QP) is 42. An example unfiltered video frame and the same frame filtered by HEVC DBF algorithm are shown in Fig. 8. As it can be seen from Fig. 8, some of the blocking artifacts are reduced and some of them are totally removed. The power and energy consumptions of the FPGA implementation for several full HD () video frames are given in Table II. The power consumption results are estimated using a gate level power estimation tool. Post place & route timing simulations are performed for one frame of each video sequence at 50 MHz, and signal activities are stored in VCD files. These VCD files are used for estimating the power consumption of the FPGA implementation using the gate level power estimation tool. The Verilog RTL code of the proposed HEVC DBF hardware is also synthesized and place & routed to a standard cell library implemented in 90nm CMOS technology. The resulting ASIC implementation works at 86 MHz and its gate count is calculated as 16.4k, excluding on-chip memories, based on NAND (2x1) gate area. In HEVC DBF algorithm, the pixels in the neighboring edges of 8x8 blocks do not overlap. Since the pixels in the neighboring edges can be filtered in parallel, depending on the application requirements, large number of parallel datapaths can be used in an HEVC DBF hardware. The impact of parallel filtering on the proposed HEVC DBF hardware is shown in Table III. The clock frequency for all cases is 108 MHz. As the number of parallel datapaths (PD) in HEVC DBF hardware increases, its performance increases significantly. However, this increases its gate count and on-chip memory usage. 640 byte on-chip memory is used for processing 16x16 CUs, and each parallel datapath uses 32 byte on-chip transpose memory. Since this is the first HEVC DBF hardware in the literature, it is compared with the H.264 DBF hardware in the literature. In order to make a fair comparison, its implementation results for processing 16x16 CUs are given. The comparison results are given in Table IV. However, this comparison is not perfect because of the following differences between HEVC and H.264 DBF algorithms Clock Logic Signal BRAM Total Time (sec) Energy (mj) PD Strong Filter Weak Filter Bdrive Cactus Terrace Tennis Kimono1 Fig. 7. Strong and Weak Filter Amounts TABLE II POWER AND ENERGY CONSUMPTION RESULTS Basketball Drive Video Sequences Cactus Terrace Tennis Kimono TABLE III HEVC DBF HARDWARE SCALABILITY RESULTS Cycles/CU (worst case) Through put (CU/sec) k k k k k Through put (fps) 30 fps 43 fps 57 fps 72 fps 86 fps On-Chip Memory (Byte) Gate Count k k k k k

6 E. Ozcan et al.: A High Performance Deblocking Filter Hardware for High Efficiency Video Coding 719 (a) (b) Fig. 8. (a) Unfiltered Tennis () Video Frame (b) The Same Frame Filtered by HEVC DBF Algorithm DBF Hardware Proposed (2 parallel datapaths) Proposed (6 parallel datapaths) Technology FPGA (40 nm) FPGA (40 nm) Memory Type Cycles/MB (worst case) TABLE IV DBF HARDWARE COMPARISON Frequency (MHz) Throughput (MB/sec) k k Throughput (fps) 30 fps 86 fps On-Chip Memory (Byte) = = 832 Gate Count 16.4k (ASIC) 36.8k (ASIC) Huang [9] k 20 fps k Huang [9] k 14 fps k Sheng [10] k 28 fps 64x32 + 2x96x32 = k Shih [11] k 19 fps 160x = k Liu [12] k 49 fps 96x32 + 2Nx k Chao [13] k 2048xl fps 144x32 + 2x16x32 = k Shih [14] k 50 fps N 20.9k Tobajas [15] k 77 fps k Parlak [16] FPGA (0.13 um) k 352x fps k Lai [17] k 58 fps k Chung [18] k 63 fps k

7 720 IEEE Transactions on Consumer Electronics, Vol. 59, No. 3, August 2013 Since the block sizes, conditions used to determine whether an edge will be filtered or not, conditions used to determine the strength of the filtering that will be applied to an edge, and the amount of computations performed in filtering operations are different, the amount of computations performed by HEVC DBF hardware and H.264 DBF hardware will be different for the same video frames. In HEVC DBF algorithm, 53% of the operations are performed in the decision part, and because of the data dependencies most of these operations are performed sequentially. However, this is not the case for H.264 DBF algorithm. Since the pixels in neighboring edges can be filtered in parallel in HEVC DBF algorithm, HEVC DBF hardware can use large number of parallel datapaths. However, this is not the case for H.264 DBF hardware. Because, the pixels in the neighboring edges of 4x4 blocks overlap in H.264 DBF algorithm. V. CONCLUSION Since HEVC DBF algorithm has high computational complexity, in this paper, the first HEVC DBF hardware in the literature is proposed. Two parallel datapaths are used in the hardware in order to increase its performance. The proposed hardware is implemented in Verilog HDL. The Verilog RTL code is verified to work correctly on an FPGA board. The proposed HEVC DBF hardware can code 30 full HD () video frames per second. Therefore, it can be used in consumer electronics products that require a real-time HEVC encoder or decoder. REFERENCES [1] B. Bross, W.J. Han, J.R. Ohm, G.J. Sullivan, T. Wiegand, High Efficiency Video Coding (HEVC) Text Specification Draft 6, JCTVC- H1003, Nov [2] G. Correa, P. Assuncao, L. Agostini, L. A. Silva Cruz, Complexity Control of High Efficiency Video Encoders for Power Constrained Devices, IEEE Trans. on Consumer Electronics, vol.57, no. 4, Nov [3] E. Kalali, Y. Adibelli, I. Hamzaoglu, A High Performance and Low Energy Intra Prediction Hardware for High Efficiency Video Coding, Int. Conf. on Field Programmable Logic and Applications, Aug [4] P. List, A. Joch, J. Lainema, G. Bjøntegaard, M. Karczewicz, Adaptive Deblocking Filter, IEEE Trans. on CAS for Video Technology, vol. 13, July [5] Y. Adibelli, M. Parlak, I. Hamzaoglu, Energy Reduction Techniques for H.264 Deblocking Filter Hardware, IEEE Trans. on Consumer Electronics, vol. 57, no. 3, Aug [6] A. Norkin et al., HEVC Deblocking Filter, IEEE Trans. on CAS for Video Technology, vol. 22, no. 12, Dec [7] J. Vanne, M. Viitanen, T. D. Hamalainen, A. Hallapuro, Comparative Rate-Distortion-Complexity Analysis of HEVC and AVC Video Codecs, IEEE Trans. on CAS for Video Technology, vol. 22, no. 12, Dec [8] E. Ozcan, Y. Adibelli, I. Hamzaoglu, A High Performance Deblocking Filter Hardware for High Efficiency Video Coding, Int. Conference on Field Programmable Logic and Applications, Sept [9] Y.W. Huang, T.W. Chen, B.Y. Hsieh, T.C. Wang, T.H. Chang, L.G. Chen, Architecture design for deblocking filter in H.264/JVT/AVC, IEEE Int. Conf. on Multimedia and Expo., July [10] B. Sheng, W. Gao, D. Wu, An Implemented Architecture of Deblocking Filter for H.264/AVC, IEEE Int. Conf. on Image Processing, Oct [11] S. Y. Shih, C. R. Chang, Y. L. Lin, An AMBA- compliant deblocking filter IP for H.264/AVC, IEEE Int. Symp. on CAS, May [12] T. M. Liu, W. P. Lee, T. A. Lin, C. Y. Lee, A memoryefficient deblocking filter for H.264/AVC video coding, IEEE Int. Symp. on CAS, May [13] Y C. Chao, J. K. Lin, J. F. Yang, B. D. Liu, A high throughput and data reuse architecture for H.264/AVC deblocking filter, IEEE Asia South Pacific Conf. on CAS, Dec [14] S. Y. Shih, C. R. Chang, Y. L. Lin, A near optimal deblocking filter for H.264 advanced video coding, IEEE Asia South Pacific DAC, Jan [15] F. Tobajas, G. M. Callico, P.A. Perez, V. de Armas, R. Sarmiento, An Efficient Double-Filter Hardware Architecture for H.264/AVC Deblocking Filtering, IEEE Trans. on Consumer Electronics, vol. 54, no. 1, Feb [16] M. Parlak, I. Hamzaoglu, Low Power H.264 Deblocking Filter Hardware Implementations, IEEE Trans. on Consumer Electronics, vol. 54, no. 2, May [17] Y.-K. Lai et al., A Memory Interleaving and Interlacing Architecture for Deblocking Filter in H.264/AVC, IEEE Trans. on Consumer Electronics, vol. 56, no. 4, Nov [18] H.-C. Chung, Z.-Y. Chen, P.-C. Chang; Low Power Architecture Design and Hardware Implementations of Deblocking Filter in H.264/AVC, IEEE Trans. on Consumer Electronics, vol. 57, no. 2, May BIOGRAPHIES Erdem Ozcan received B.S. degree in Electronics Engineering from Sabanci University, Istanbul, Turkey in He is currently pursuing M.S. degree in Electronics Engineering at Sabanci University, Istanbul, Turkey. His research interests include digital hardware design for digital video processing and coding, and low power digital hardware design. Yusuf Adibelli received B.S. and M.S. degrees in Electronics Engineering from Fatih University, Istanbul, Turkey in 2005 and 2007 respectively. He received Ph.D. degree in Electronics Engineering at Sabanci University, Istanbul, Turkey in He is currently working as a Researcher at the Scientific and Technological Research Council of Turkey (TUBITAK). His research interests include digital hardware design for digital video processing and coding, and low power digital hardware design. Ilker Hamzaoglu (M 00-SM'12) received B.S. and M.S. degrees in Computer Engineering from Bogazici University, Istanbul, Turkey in 1991 and 1993 respectively. He received Ph.D. degree in Computer Science from University of Illinois at Urbana- Champaign, IL, USA in He worked as a Senior and Principle Staff Engineer at Multimedia Architecture Lab, Motorola Inc. in Schaumburg, IL, USA between August 1999 and August He is currently an Associate Professor at Sabanci University, Istanbul, Turkey where he is working as a Faculty Member since September His research interests include SoC ASIC and FPGA design for digital video processing and coding, low power digital SoC design, digital SoC verification and testing.