好文分享，有關HDR的亮度

chhanthony

http://www.lightillusion.com/uhdtv.htmlHDR - WHAT IT REALLY MEANS FOR PICTURE LEVELS
The following is taken directly from the ST2084 (PQ EOTF) specification.
This EOTF (ST2084) is intended to enable the creation of video images with an increased luminance range; not for creation of video images with overall higher luminance levels. For consistency of presentation across devices with different output brightness, average picture levels in content would likely remain similar to current luminance levels; i.e. mid-range scene exposures would produce currently expected luminance levels appropriate to video or cinema.

The PQ based ST2084 HDR specification defines reference white (normal diffuse white) as being 100 nits, which is exactly the same as for SDR (Standard Dynamic Range) displays, as specified for mastering (grading) grade-1 applications. With PQ HDR above 100 nits is for spectral highlight detail only. This shows that the Average Picture Level (APL) of a PQ HDR display will not be significantly different to a SDR display.
Regardless of the maximum peak brightness the display can generate the diffuse white will always be around 100 nits.
The BBC's HLG HDR standard uses 75% of the signal (input) range as nominal diffuse white, which is obviously a 'variable, as the HLG standard is a 'relative' standard, not 'absolute' as with PQ. This is consistent with have any standard SDR TV and running it at higher peak luma levels for daytime viewing, as is the case in most home lounges.
The nominal nits value for diffuse white will vary with the peak brightness of the display, with a 1000 nit display having diffuse white around 175 nits, and a 4000 nits display around 600 nits, depending system gamma (see later for info on HLG system gamma).
So the reality is that HDR should just ADD to the existing brightness range of SDR displays, meaning that more detail can be seen in the brighter areas of the image, where existing SDR images simply clip, or at least roll-off, the image detail.
The following histogram is a simplified view of the difference between a SDR (Standard Dynamic Range) image, and its PQ HDR equivalent.
Note that the APL (Average Picture Level) remains approximately consistent between the SDR and ST2084 HDR images, with just the contrast range and specular highlight levels increasing.



While this is a PQ based graph, the same is true of HLG based HDR.
If this approach to HDR is understood, and images created based on using the additional brightness range for specular highlights, the true potential of HDR will be realised.