Liz Pieri of Dolby Labs gave a presentation explaining why in HDR displays, one should really use the ICtCp color space instead of the Lab space. The reason is that using the DeltaE2000 metric for calculating color accuracy, which is based on CIELab, produces more errors especially in darker colors.
Pieri explained that CIELab was developed for the paint and fabric industry and is based on paint chips. It is only really a valid model of human vision for these materials in a limited luminance range. While it was adapted for SDR displays and worked fairly well, the wider range of luminance and colors in HDR displays has revealed the flaws of this model.
Dolby has developed a different color model called ICtCp. This color model has been approved as an encoding space by the ITU, but Pieri wants others to now consider it as a valid human visual model as well.
She then began to describe some experiments and analysis Dolby has done to compare color accuracy – via a JND metric, for L*a*b*(as MacAdam data) vs. ICtCp. The chart below shows that the mean square error (MSE) for ICtCp is better than DelatE2000 over the limited luminance range of the MacAdam dataset.
Pieri also looked at some data that 3M gathered a couple of years ago that focused on the wide colors near the BT.2020 primaries (the study was done to look at how to establish a tolerance to establish a 2020-compliant display). The data shows that in the green, DeltaE2000 and ICtCp have similar performance, but ICtCp outperforms in the yellow and red.
Finally, Pieri described data taken with a Dolby Laser cinema projector at 0.1, 25 and 1000 cd/m². Subjects were shown a 4-segment test patch with one patch having a slight chromatic shift. This was done over a variety of colors and the three luminance levels, shifting the color slightly in three directions at each test point. As shown in the data, the two performed similarly for the midtones and highlights, but ICtCp was better for the dark tones.
The chart summarizes the finding with recommendation to use ICtCp for HDR/WCG images as DeltaE2000 is inconsistent. – CC
