HDR video exposure normalisation behavior across platforms

This post is a work in progress.

Website with PQ and HLG HDR video test charts

https://andrewkeyanzhe.github.io/100_203_nits_PQ_HLG_patterns/4_compare.html

Behaviour of Chrome on Windows

These settings are used:

• HDR is enabled in Windows settings
• The Windows OS SDR brightness is set to 100 nits (5% on the slider)

The following screenshot will show the correct nits when viewed on a device with SDR brightness set to 100 nits

• on Windows, set SDR brightness to 5%
• on MacBooks, set the brightness setting to in between 7 and 8 clicks from the bottom (out of 16)

• We can see that for Chrome on Windows, PQ HDR videos are displayed without normalising to OS SDR brightness in nits, so video pixels’ nit values sent to the display matches that encoded in the video
• However, HLG HDR videos are normalised to OS SDR brightness in nits, using a factor of OS SDR brightness in nits/203. So the 100 nit HLG pattern is shown at around 49 nits. ⚠️ This does not match the pixel nits as encoded in the HLG video, when one assumes the 1000 nit HLG transfer function (commonly used by reference displays and DaVinci Resolve)

Comparison of PQ and HLG video test pattern rendering in Chrome on Windows