  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.
