1.
Introduction
Three-dimensional video (3DV) has attracted people's attention as an emerging new media. Owing to limited transmission bandwidth, efficient compression of 3DV, which consists of texture video coding and depth map coding, is a key enabling factor for its applications. In Ref. 1, research on texture video compression has been carried out by adopting the interview correlations between different viewpoints besides temporal and spatial correlations. Also, many works have focused on depth map coding recently.2, 3
In Ref. 2, the motion estimation process was skipped by directly using the motion vectors (MVs) of texture videos, thus reducing the coding cost for depth maps. While the method in Ref. 3 generated three candidate modes and MVs for depth map coding using the motion information of texture videos. Inspired by these previous works, the motion information of texture videos was utilized in this letter for improving the depth map coding efficiency.
In H.264/AVC, to improve the video coding efficiency of interprediction, the median of the MVs of neighboring coded blocks was applied to predict the MV for the current coding block. Thus, the accuracy of predicted motion vector (PMV) highly affects the coding efficiency of motion-compensated prediction.4 Among the existing proposed methods for improving the efficiency of the predictive coding of MV, the competition-based PMV selection method proposed by Laroche5 has been adopted to the Key Technology Area (KTA) due to its significant bit rate reduction for texture videos. Motivated by the MV-competition method,5 this letter proposed a texture video-assisted motion vector predictor for depth map coding.
2.
Motion Correlation Analyses Between Texture Videos and Depth Maps
2.1.
Motion Similarity Analysis between Texture Videos and Depth Maps
For the object belonging to the same macroblock, the MVs of texture videos and depth maps should be the same in a sense of physics. However, the texture video MVs cannot represent the physical displacement values of the object due to texture complexity. Thus, the MVs between texture videos and depth maps may be coincident when reflecting the displacement of the same object. While in other cases, since the pixels in the coding block belong to different objects, or even if they are located in the same object, due to the texture complexity, the MVs between texture videos and depth maps are not the same, even not similar, as shown in the following experiment.
In the experiment, texture videos and depth maps were encoded with H.264/AVC using four quantization parameters (QPs), i.e., 27, 32, 37, 42. The MVs of texture videos and depth maps for each 4×4 block were extracted and analyzed. An MV similar evaluation criterion is introduced in Eq. 1
Eq. 1
[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{eqnarray} E_{mv} (i,j,k) = \left\{ {\begin{array}{*{20}c} {1,} & {{\rm }\left\| {{\bf MV}_t (i,j,k) - {\bf MV}_d (i,j,k)} \right\| < T} \\[3pt] {0,} & {{\rm otherwise}} \\ \end{array},} \right.\nonumber\\ \end{eqnarray}\end{document}Eq. 2
[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} P_{td} = \frac{{16 \times \sum\nolimits_{k = 1}^{N_f } {\sum\nolimits_{i = 1}^{{H / 4}} {\sum\nolimits_{j = 1}^{{W / 4}} {E_{mv} (i,j,k)} } } }}{{H \times W \times N_f }}, \end{equation}\end{document}Table 1
Percentage of similar MVs between texture videos and depth maps.
| Ptd (%) | ||||
|---|---|---|---|---|
| Sequences | QP = 27 | QP = 32 | QP = 37 | QP = 42 |
| Breakdancers view 02 | 59.02 | 67.80 | 76.36 | 84.60 |
| Breakdancers view 04 | 59.42 | 68.91 | 77.39 | 85.71 |
| Ballet view 02 | 75.64 | 80.84 | 86.70 | 90.12 |
| Ballet view 04 | 77.01 | 82.42 | 87.40 | 89.81 |
| Bookarrival view 08 | 77.47 | 81.39 | 83.89 | 86.35 |
| Bookarrival view 10 | 75.17 | 79.39 | 83.48 | 86.51 |
As seen from Table 1, the value of Ptd becomes larger with the increase of QP value for a certain sequence. It is because there are much more skipped macroblocks (MBs) and 16×16 modes in depth map coding than texture video coding,3 while for texture video coding, the number of such MBs and modes increases for higher QP, thus resulting in the increased percentage of similar MVs between texture videos and depth maps. To summarize, when coding depth maps, the MVs of several blocks in texture videos can be used as PMVs for associated blocks in depth maps; while for other blocks with relatively large MV differences, the texture video MVs should not be used.
2.2.
Accuracy Analysis of Employing Texture Video MV as PMV for Depth Map
We compared the accuracy of using the median PMV and texture video MV as PMV for depth map, and the results are described in Table 2.
Table 2
Comparison results of motion vector difference.
| Ptm(%) | ||||
|---|---|---|---|---|
| Sequences | QP = 27 | QP = 32 | QP = 37 | QP = 42 |
| Breakdancers view 02 | 15.55 | 25.93 | 34.73 | 46.19 |
| Breakdancers view 04 | 18.93 | 28.68 | 42.26 | 53.50 |
| Ballet view 02 | 62.92 | 75.67 | 86.89 | 92.35 |
| Ballet view 04 | 63.28 | 79.51 | 87.86 | 92.63 |
| Bookarrival view 08 | 77.82 | 84.51 | 87.98 | 92.10 |
| Bookarrival view 10 | 72.03 | 77.83 | 85.05 | 90.54 |
In the experiment, the MV for the depth map block is denoted as MVd, and the median PMV and PMV based on texture video MV are PMVm and PMVt, respectively. ∥MVDm∥ represents the vector difference between MVd and PMVm, and ∥MVDt∥ represents the vector difference between MVd and PMVt. In Table 2, Ptm indicates the percentage of the blocks with ∥MVDt∥−∥MVDm∥≤0 in all the blocks of the picture. From Table 2, the value of Ptm increases with the increase of QP value for the certain sequence, which is mainly attributable to the occurrence of more skipped MBs and 16×16 modes in texture video coding for higher QPs. Thus, we can draw the conclusion that for some blocks of depth maps, it is more accurate to select the MV of corresponding texture video as PMV for depth map than the median PMV.
3.
Texture Video-Assisted Motion Vector Predictor for Depth Map Coding
Motivated by the analyses in Sec. 2, a texture video-assisted motion vector predictor for depth map coding was proposed. In the proposed algorithm, both the median PMV and texture video MVs are adopted to predict MVs for depth maps.
In order to obtain the optimal PMV between the median PMV and the proposed PMV for depth maps, the rate-distortion optimization criterion5 is employed to select the PMV with the minimum rate-distortion (R-D) cost. The Lagrangian cost function is given by
Eq. 3
[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} J({\bf S},{\bf I}|\lambda) = D({\bf S},{\bf I}) + \lambda R({\bf S},{\bf I}),\end{equation}\end{document}Eq. 4
[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} {\bf PMV}^* = \arg \min \{ J[{\bf S},{\bf I}({\bf PMV}_i |_{i \in \left\{ {t,m} \right\}})|\lambda]\}.\end{equation}\end{document}4.
Experimental Results and Analyses
Experiments were carried out using three 3DV sequences as shown in Table 3. In Table 3, the left view and right view were used to synthesize the virtual view in the position of the central view. Experiments were performed on the KTA test platform version 2.6r1 (KTA2.6r1). Context-based adaptive binary Arithmetic coding and four QPs (27, 32, 37, 42) were used.
Table 3
Sequences parameters.
| Sequences | Resolution | Left view | Central view | Right view |
|---|---|---|---|---|
| Breakdancers | 1024×768 | view 02 | view 03 | view 04 |
| Ballet | 1024×768 | view 02 | view 03 | view 04 |
| Bookarrival | 1024×768 | view 10 | view 09 | view 08 |
The performance comparison among the proposed method, H.264/AVC, and the MV-competition (temporal PMV + median PMV) method5 is tabulated in Table 4. In Table 4, the Bjontegaard delta (BD) bit rate6 denotes the percentage of average bit rate savings at the same coding quality; while the BD peak signal-to-noise ration (PSNR)6 indicates average increase of PSNR at the same coding bit rate. As seen from Table 4, the achieved average BD bit rate is −3.68% compared with H.264/AVC. The percentages of selecting texture video MV as optimal PMV among skip and 16×16 MBs, as well as other block types in the depth map coding, are provided in Table 4. For depth map coding, the block types other than skip and 16×16 MBs mostly occur in the object edge with discontinuous depth values, the MVs among neighboring blocks have relatively large differences. Thus, for these block types, the texture video MV would be more accurate than the median PMV and can be selected as the optimal PMV for depth maps. Thereby, the coding bits for MV difference in depth maps are saved. Moreover, from Table 4, it can be seen that the proposed method can give better coding results than the MV-competition method.
Table 4
Coding efficiency of different test sequences.
| H.264/AVC | MV- competition | Proposed | Percentage of Texture MV(%) | Compared with MV-competition | Compared with H.264/AVC | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sequences | PSNR (dB) | Bit rate (kbit/s) | PSNR (dB) | Bit rate (kbit/s) | PSNR (dB) | Bit rate (kbit/s) | Skip & 16×16 MBs | Other block types | BD PSNR (dB) | BD bit rate (%) | BD PSNR (dB) | BD bit rate (%) |
| Breakdancers view 02 | 47.74 | 707.32 | 47.72 | 705.24 | 47.74 | 702.22 | 0.84 | 27.34 | 0.05 | −0.91 | 0.15 | −2.78 |
| 43.99 | 372.83 | 43.95 | 365.7 | 43.96 | 364.63 | 1.85 | 29.88 | |||||
| 40.64 | 189.9 | 40.63 | 184.52 | 40.63 | 181.02 | 2.57 | 32.38 | |||||
| 37.23 | 93.68 | 37.29 | 90.84 | 37.26 | 90.44 | 2.81 | 33.86 | |||||
| Breakdancers view 04 | 47.68 | 705.43 | 47.67 | 700.03 | 47.64 | 699.25 | 0.88 | 27.66 | 0.03 | −0.74 | 0.13 | −2.56 |
| 43.91 | 364.97 | 43.89 | 360.93 | 43.92 | 361.02 | 1.75 | 30.18 | |||||
| 40.57 | 180.89 | 40.61 | 177.07 | 40.60 | 175.20 | 1.91 | 32.49 | |||||
| 37.1 | 86.45 | 37.1 | 83.43 | 37.15 | 82.60 | 2.06 | 33.95 | |||||
| Ballet view 02 | 47.55 | 605.08 | 47.57 | 602.25 | 47.58 | 601.60 | 1.02 | 29.10 | 0.10 | −1.50 | 0.24 | −3.94 |
| 43.98 | 349.71 | 43.95 | 343.69 | 43.89 | 341.66 | 1.95 | 34.52 | |||||
| 39.94 | 184 | 39.97 | 179.68 | 39.95 | 172.90 | 2.06 | 40.43 | |||||
| 35.73 | 80.62 | 35.81 | 77.35 | 35.72 | 74.92 | 2.85 | 44.19 | |||||
| Ballet view 04 | 47.9 | 561.69 | 47.93 | 551.83 | 47.92 | 551.53 | 1.05 | 30.85 | 0.05 | −1.09 | 0.20 | −3.52 |
| 44.23 | 337.75 | 44.24 | 330.55 | 44.22 | 328.68 | 1.44 | 36.70 | |||||
| 40.16 | 178.2 | 40.17 | 173.85 | 40.15 | 172.37 | 1.75 | 42.03 | |||||
| 36.17 | 78.53 | 36.23 | 77.64 | 36.17 | 71.56 | 2.06 | 44.04 | |||||
| Bookarrival view 08 | 45.89 | 336.48 | 45.85 | 329.25 | 45.87 | 327.02 | 1.14 | 33.72 | 0.06 | −1.34 | 0.24 | −4.89 |
| 42.68 | 177.13 | 42.7 | 170.2 | 42.69 | 169.27 | 1.75 | 37.60 | |||||
| 39.77 | 95.05 | 39.76 | 91.62 | 39.72 | 88.36 | 1.87 | 39.91 | |||||
| 36.69 | 52.49 | 36.74 | 50.75 | 36.65 | 49.67 | 2.74 | 41.39 | |||||
| Bookarrival view 10 | 46.1 | 461.51 | 46.13 | 458.36 | 46.11 | 457.77 | 1.81 | 30.81 | 0.06 | −1.30 | 0.21 | −4.39 |
| 42.87 | 238.03 | 42.93 | 237.07 | 42.89 | 230.65 | 2.01 | 36.33 | |||||
| 39.85 | 122.18 | 39.84 | 114.83 | 39.81 | 113.38 | 2.95 | 39.44 | |||||
| 36.6 | 62.18 | 36.6 | 61.54 | 36.58 | 59.28 | 3.05 | 41.48 | |||||
| Average | 0.06 | −1.15 | 0.19 | −3.68 | ||||||||
To further verify the effectiveness of the proposed method, the qualities of the synthesized virtual views were also compared as shown in Table 5. As seen from Table 5, the average BD PSNR of synthesized virtual views using the proposed method was improved up to 0.13 dB compared with the median PMV utilized in H.264/AVC.
Table 5
PSNR comparison of the synthesized virtual view.
| Sequences | BD PSNR (dB) | Average (dB) |
|---|---|---|
| Breakdancers virtual view 03 | 0.08 | |
| Ballet virtual view 03 | 0.11 | 0.13 |
| Bookarrival virtual view 09 | 0.20 |
5.
Conclusions
Based on the analyses of motion correlations between texture videos and depth maps, the texture video MV can be used as a candidate PMV for depth map coding in the proposed method. Experimental results have verified that the proposed method can achieve higher coding efficiency than the median PMV utilized in H.264/AVC.
