SSIM-Motivated Two-Pass VBR Coding for HEVC

Abstract

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 λ, which controls the tradeoff between perceptual distortion and bit rate, is adopted as the GoP level complexity measure. To derive λ, 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 λ 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.