Gamut Rings Greatly Overemphasize Light Areas of Color Volume

“Arbitrarily placing the HDR peak intensity white at L=100 grossly distorts the Lightness curve, and would not be a good Lightness predictor.”

Chris Lilley, W3C, 2020

Charles Poynton and David A. LeHoty argue that the color volume metrics used to calculate gamut rings are flawed, greatly overestimating actual coverage, allowing manufacturers to make exaggerated claims for their displays:

“We have presented various luminance/lightness metrics. If a color area metric is based upon any of the L*, sRGB, or BT.1886 quantities – for example, the a* and b* chroma components of CIE LAB used to represent area directly, or the a* and b* components used to estimate area used in a gamut ring calculation [4, 10] – then the overestimation by a factor of two in the lightness dimension is squared, resulting in an overestimation of perceptual effect by a factor of four. If a color volume metric is based upon any of these quantities – for example, cubic delta‐E (∆E3) – then the overestimation by a factor of two in the lightness dimension is cubed, resulting in an overestimation by a factor of eight. These metrics greatly overemphasize the light areas of the color area or volume, compared to metric such as PQ that has a better perceptual foundation. A LAB‐based metric with a white reference chosen a factor of five below HDR peak white might estimate a space as having 64 million colors, where a PQ‐based metric might more realistically estimate 8 million.” – Charles Poynton, “Luminance, Brightness, and Lightness Metrics for HDR” in Color and Imaging Conference, 2022

A proposal has been made to use CIE LAB to quantify HDR gamut. We argue that CIE L* is only appropriate for applications having contrast range not exceeding 100:1, so CIELAB is not appropriate for HDR. In practice, L* cannot accurately represent lightness that significantly exceeds diffuse white – that is, L* cannot reasonably represent specular reflections and direct light sources. In brief: L* is inappropriate for HDR. We suggest using metrics based upon ST 2084/BT.2100 PQ and its associated color encoding, ICTCP.

Charles Poynton

“In the new world of HDR, L* is an inadequate measure of lightness. L* covers only about a 100:1 ratio of luminance. Because L* is effectively limited to diffuse white at the top end, it cannot represent direct light sources and specular highlights usefully—and both are important aspects of HDR imagery. Concerning color gamut, the display community historically has estimated color extent as the area of the triangle formed by display primaries in CIE (x, y) chromaticity space. We explained how the ICTCP colorspace, as utilized by BT.2124, is more useful than L*, LAB, and (x, y) to estimate HDR image data’s perceptual parameters. We can expect consumer manufacturers to overstate color volume measurements just as they have overstated contrast ratios in the past.” – Comparing Displays Using Luma Contours and Color Volumes, David A. LeHoty, Charles Poynton

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