Analysis of the depth-shift distortion as an estimator for view synthesis distortion

• Analysis of the use of depth-shift distortion for low complexity 3D Video encoders.
• Depth-shift distortion can substitute synthesis distortion at frame or MB-row coding levels.
• SAE is preferable to SSE to aggregate the depth-shift distortion of a depth region.

Aiming for 3D Video encoders with reduced computational complexity, we analyze the performance of depth-shift distortion in depth-image based rendering algorithms, incurred when coding depth maps in 3D Video, as an estimator of the distortion of synthesized views. We propose several distortion models that capture (i) the geometric distortion caused by the depth coding error, (ii) the pixel-mapping precision in view synthesis and (iii) the method to aggregate depth-shift distortion caused by the coding error in a depth block. Our analysis starts with the evaluation of the correlation between the depth-shift distortion values obtained with these models, and the actual distortion on synthesized views, with the aim of identifying the most accurate one. The correlation results show that one of the models can be used as a reasonable estimator of the synthesis distortion in low complexity depth encoders. These results also show that the Sum of Absolute Error (SAE) captures better the distortion on a depth block than the Sum of Squared Error (SSE).The correlation analysis is performed at three levels: frame, MB-row and MB. Results show that correlation values are consistently high at the frame level and for most MB-row positions, while lower values are achieved at the MB level and for specific rows at the MB-row level. Finally, to assess the results obtained by the correlation analysis, the different depth-shift distortion models are employed in two algorithms of the rate-distortion optimization (RDO) cycle of the depth encoder: (i) Quantization Parameter (QP) selection and (ii) mode decision. We evaluate the QP selection algorithm at three levels: frame, MB-row and MB, and the mode decision at the MB level. At the frame level, results show that the use of depth-shift distortion is equivalent to synthesis distortion, with the advantage of a lower computational complexity. At sub-frame levels, the results are consistent with the comparative correlation results, giving guidelines for the use of an efficient depth-shift distortion model on low complexity depth encoders.