SSIM-Motivated Two-Pass VBR Coding for HEVC

We propose a structural similarity (SSIM)-motivated two-pass variable bit rate control algorithm for High Efficiency Video Coding. Given a bit rate budget, the available bits are optimally allocated at group of pictures (GoP), frame, and coding unit (CU) levels by hierarchically constructing a perceptually uniform space with an SSIM-inspired divisive normalization mechanism. The Lagrange multiplier <inline-formula> <tex-math notation="LaTeX">$\lambda$ </tex-math></inline-formula>, which controls the tradeoff between perceptual distortion and bit rate, is adopted as the GoP level complexity measure. To derive <inline-formula> <tex-math notation="LaTeX">$\lambda$ </tex-math></inline-formula>, Laplacian distribution-based rate and perceptual distortion models are established after the first pass encoding, and the target bits are dynamically allocated by maintaining a uniform Lagrange multiplier level for each GoP through <inline-formula> <tex-math notation="LaTeX">$\lambda$ </tex-math></inline-formula> equalization. Within each GoP, rate control is further performed at frame and CU levels based on SSIM-inspired divisive normalization, aiming to transform the prediction residuals into a perceptually uniform space. Experiments show that the proposed scheme achieves high accuracy rate control and superior rate-SSIM performance, which is further verified by subjective visual testing.