Subjective assessment of the impact of a content adaptive optimiser for compressing 4K HDR content with AV1

TL;DR

This study evaluates how a content-adaptive optimizer affects subjective quality in 4K HDR AV1 video compression, revealing insights into perceptual metrics and the influence of film grain, with implications for HDR streaming quality assessment.

Contribution

It provides the first comprehensive subjective assessment of HDR content optimization with AV1, comparing expert and non-expert viewers and analyzing the correlation with objective metrics.

Findings

Some perceptual metrics correlate well with subjective scores.

Expert and non-expert viewers yield similar subjective assessments.

Film grain significantly impacts perceived quality under compression.

Abstract

Since 2015 video dimensionality has expanded to higher spatial and temporal resolutions and a wider colour gamut. This High Dynamic Range (HDR) content has gained traction in the consumer space as it delivers an enhanced quality of experience. At the same time, the complexity of codecs is growing. This has driven the development of tools for content-adaptive optimisation that achieve optimal rate-distortion performance for HDR video at 4K resolution. While improvements of just a few percentage points in BD-Rate (1-5\%) are significant for the streaming media industry, the impact on subjective quality has been less studied especially for HDR/AV1. In this paper, we conduct a subjective quality assessment (42 subjects) of 4K HDR content with a per-clip optimisation strategy. We correlate these subjective scores with existing popular objective metrics used in standard development and show that some perceptual metrics correlate surprisingly well even though they are not tuned for HDR. We find that the DSQCS protocol is too insensitive to categorically compare the methods but the data allows us to make recommendations about the use of experts vs non-experts in HDR studies, and explain the subjective impact of film grain in HDR content under compression.