For the higher coding performance than the previous video coding standards, High-E ciency Video Coding (HEVC) adopts an intra prediction method with 35 modes, which requires heavy computational complexity. Intending to reduce this complexity, we analyzed the role of modes and proposed a scheme that contains two rough mode decision (RMD) processes with a customized set of modes to be tested in the rst stage. The second stage of the RMD is calculated for a maximum of 4 modes. As compared to the default encoding scheme in HEVC test model HM-16.20, experimental results show that the proposed method reduces encoding time up to 22.74% with negligible loss of coding e ciency.
In recent years, there has been a growing interest in services related to the
transmission and storage of high and ultrahigh de nition videos. The video
coding standard H.264/Advanced Video Coding (AVC) [
Mainly due to the new coding tools and the exible data structures, HEVC
provides a signi cant improvement in compression e ciency compared to its
predecessors H.264, especially when operating on high-resolution video content [
HEVC contains several elements improving the e ciency of intra
prediction over earlier solutions. HEVC design supports a total of 35 intra prediction
modes, including Planar, DC and 33 angular modes, as presented in Figure 1,
which contribute to representing di erent texture and object edge direction more
precisely [
JSAT D = SAT D + pred
(1) where SAT D represents the absolute sum of Hadamard transformed residual signal for a PU. pred is a Lagrange multiplier, and Rpred represents the number of bits for the prediction mode. The number N is varied depending on the PU size. The N is set to f8, 8, 3, 3, 3g for 4 4, 8 8, 16 16, 32 32, and 64 64 PU, respectively.
In the second step, three most probable modes (MPM), which are derived
from the intra modes of the left and top neighboring PUs [
where SSE represents the sum of the squared errors between the original CU and the reconstructed CU. Rtotal is the total number of bits used for encoding with this mode.
In this way, the RDO process has to check only a maximum of 11 modes instead of all 35, and so the computational load can be reduced. However, the complexity is still high, since, in the RMD step all the 35 modes need to perform the cost calculation, the number of modes for RDO is still large. In this paper, a fast intra mode decision is proposed to further reduce the complexity of HEVC intra coding while maintaining the RD performance. 2
Theoretically, 35 intra modes play the same role and their probability of choice is equal. However, the results of analyzing some videos of various categories have given a di erent perspective. To perform the statistical analyses of frequently chosen modes, HEVC reference software HM-16.20 was used to encode a set of video sequences of di erent classes and resolutions. Statistical results for test sequences in class B and the sequence "PeopleOnStreet" are shown in tables 1 and 2 with the four most frequent modes are represented in bold. Figure 2 shows diagrams re ecting the probability (P ) of the choice of speci c prediction modes when encoding test video sequences.
Statistics show that Planar and DC prediction modes are most likely for all video sequences. However, the probability of being the best choice for the vertical and horizontal modes (Angular10 and Angular26) is much greater than the other angular modes. Fig. 2. Mode selection probability.
There has been signi cant work to speed up the intra mode decision process. Based on the above analysis, this paper proposes a scheme for reducing intra modes in RMD and hence increasing the encoding speed, the descriptions of which will be described below. 3
Instead of all 35, we prefer to test the most probability modes. For this purpose, a set of modes was created, including DC, Planar and Angular modes (2 + 4i) where 0 i 8. As a result, only 11 modes are tested using the RMD process to nd modes with the lowest cost. Let's call the 2 best modes FM ( rst mode) and SM (second mode). After that, a exible step (RMD2) was added after the rst. The input data for RMD2 depends on what the FM and SM are. The general scheme is shown in gure 3 with the mode selection algorithm for the second step is presented as follows: - For PU 16 16, 32 32 and 64 64: check if FM is not Planar, DC or vertical mode (0, 1, 26) then perform the second step of calculating RMD2 for 4 adjacent FM modes: F M 2, F M 1, F M + 1, F M + 2, after that update the list of candidates. Otherwise, RMD2 is skipped.
- For PU 8 8 and 4 4: if FM and SM are Planar or DC modes, the encoder will add MPM modes (if not already included in the candidate list) and perform the RD cost calculation step. In other cases, the second step RMD2 will check some other modes F M 2, F M 1, F M + 1, F M + 2 (if FM is an angular mode) or SM 2, SM 1, SM + 1, SM + 2 (if FM is DC or Planar).
According to the proposed scheme, in the second stage of calculating cost RMD2, we can calculate a maximum of 4 more modes surrounding FM and SM. The minimum and the maximum number of checked modes is 11 and 15, respectively, which reduces computational load. After that, the RDO process will be performed with MPM modes added to the candidate list. 4
The proposed scheme has been implemented on top of the HEVC reference
software HM-16.20. A set of standard video sequences in ve classes covering a
wide range of resolutions and use cases (see Table 3) was tested using the All
Intra-Main con guration and four values of the quantization parameter QP 22,
27, 32 and 37 as speci ed by [
THM 16:20 (3) where THM 16:20 denotes the time consuming of the default HM-16.20 and Tprop represents the time consumed by the proposed algorithm.
Di erent sequences are obtained di erent results, due to di erent detail and complexity. The RD curves of the proposed algorithm and the HM for some sequences are shown in Figures 4 and 5. It can be seen that the proposed algorithm achieves almost the same PSNR on di erent bitrates. By interfering with the mode selection for the RMD process, the number of modes to be tested has been signi cantly reduced. It can be argued that the proposed scheme requires less computational load, while the coding performance remains almost at the same level compared to the original HEVC encoder. It may be recommended to be combined with an algorithm that optimizes the CU separation process.