No Metrics to Describe HDR WCG Displays

As today’s consumer displays can attain brightness levels up to fifteen times higher than legacy SDR screens, the traditional 2D color gamut diagram no longer suffices to characterize the behavior of displays, necessitating a third axis to describe luminance. Nevertheless, although color volume plays a far more important role in color reproduction in HDR than color gamut, with few exceptions, manufacturers and calibrators persist in measuring displays in terms of color gamut. We reached out to HDR evangelist Vincent Teoh of HDTVTest to find out why:

“There is currently no universally accepted colour volume metric that correlates with real-world performance. The site to which you’re referring is probably Rtings who measure and publish colour volume figures, so let’s analyse the normalised DCI-P3 ITP coverage of a few OLED TVs they’ve measured:

Sony A8H (2020 OLED): 88.1%

Sony A9F (2018 flagship OLED): 87.8%

LG A1 (2021 entry-level OLED): 87.2%

LG B9 (2019 entry-level OLED): 86.6%

LG G1 (2021 flagship OLED): 86.3%

Sony A90J (2021 flagship OLED): 78.5%

As you can see, among these OLED TVs, the Sony A90J had the lowest measured colour volume using Rting’s methodology, yet is clearly the one that’s able to saturate brighter colours the most (and which we praised), because of the post-processing of the BRAVIA XR engine on the A90J which analyses real-world content, not test pattern. There is no correlation between a high colour volume and the eventual perceived HDR impact using current methodology. Furthermore: 1) Current colour volume metrics are derived from CIELAB which is based on a single state of adaptation, so it’s not necessarily well suited to HDR. 2) Current metrics only describe the outer boundaries of the colour volume, without revealing any information about the distortion within (i.e. accuracy is not accounted for) While distilling the complex subject of HDR colours down to a single number is appealing for both reviewers and consumers, our experience and testing led us to conclude that colour volume (measured using current methodologies) in no way accurately represents real-world viewing, and can even be downright misleading, and so we haven’t dedicated any resources to measuring this metric which is more hype than substance”.

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