In this work, we study the suitability of high dynamic range, wide color gamut (HDR/WCG) objective quality metrics to assess the perceived deterioration of compressed images encoded using the Hybrid Log-Gamma (HLG) method, which is the standard for HDR television.
Several image quality metrics have been developed to deal specifically with HDR content, although in previous work we showed that the best results (i.e., better matches to the opinion of human expert observers) are obtained by an HDR metric that consists simply in applying a given standard dynamic range metric, called visual information fidelity (VIF), directly to HLG-encoded images.
However, all these HDR metrics ignore the chroma components for their calculations, e.g., they just consider the luminance channel. For this reason, in the current work, we conduct subjective evaluation experiment in a professional setting using compressed HDR/WCG images encoded with HLG and analyze the ability of the best HDR metric to detect perceivable distortions in the chroma components, as well as the suitability of popular color metrics (including ∆ITPR, which supports parameters for HLG) to correlate with the opinion scores. Our first contribution is to show that there is a need to consider the chroma components in HDR metrics, as there are color distortions that subjects perceive but that the best HDR metric fails to detect. Our second contribution is the surprising result that VIF, which utilizes only the luminance channel, correlates much better with the subjective evaluation scores than the metrics that do consider the color components.
Technical Depth of Presentation
What Attendees will Benefit Most from this Presentation
Researches, Engineers, and Technologists who are related to HDR/WCG image compression for content distribution and archive purposes
Take-Aways from this Presentation
1. Objective quality metrics specifically developed for HDR images are not suitable for HLG-based HDR image coding; a combination of an HDR-to-SDR tone mapping curve and an SDR metric is appropriate
2. To evaluate HDR/WCG compressed images precisely, a metric which uses both luminance and chroma components for the calculation should be required
3. Which metric is suitable for HDR/WCG image coding using the HLG method? We found a defect in the latest color difference metric.