Mapping of SDR into HDR

This group was brought to my attention today. Nice discussion going on.
I've a few points and comments re the discussion of mapping/converting SDR into PQ HDR.

Fredrik Hubinette wrote:
"SDR should be mapped into HDR as when played in a reference environment,
the composited result should then be adapted to the viewing environment."
I agree.

First, backing up a bit, there is the issue as to how PQ should be displayed, and the comments (that I find nonsensical) that there is some fundamental difference between how PQ (and its "absolute" mapping of code values to pixel brightness) and SDR work in practice.

Consider SDR: the EOTF was (belatedly) documented in BT.1886<http://www.itu..int/rec/R-REC-BT.1886-0-201103-I/en>, with reference brightness documented (also belatedly) in BT.2035<http://www.itu.int/rec/R-REC-BT.2035-0-201307-I/en>. Two documents that basically say use gamma 2.4 with 100% at 100 nits.. The Scope of BT.2035 states:

"This Recommendation prescribes a method allowing HDTV producers or broadcasters to establish a reference viewing condition for evaluation of HDTV program material or completed programmes that can provide repeatable results from one facility to another when viewing the same material. This includes the display device and the surrounding environment." (my emphasis).

BT.2100<http://www.itu.int/rec/R-REC-BT.2100-1-201706-I/en> defines a reference viewing environment, as well as reference PQ EOTF. The intention is that they go together. There is no intention expressed that the reference PQ EOTF should be used in a non-reference viewing environment.

So what happens in practice with SDR? People watch SDR video in brighter environments than specified in BT.2035, and they use brighter displays than specified in BT.2035.

So what should happen in practice with PQ HDR? People will watch in brighter environments than specified in BT.2100, and they will use brighter pixels than specified in BT.2100. To my mind, there is no fundamental difference in philosophy or practice between SDR and PQ HDR. The only difference is that with SDR the curve and the scaling of the curve were specified in separate documents. We have learned how to write better specifications; with BT.2100 we tried to define a complete HDR TV system in a single document.

In the limited amount of PQ content to date, we have seen mid-tone brightness similar to that of SDR. See: https://www.dolby.com/us/en/technologies/dolby-vision/operational-guidelines-for-pq.pdf. A version of this study is included in the new Report ITU-R BT.2408<http://www.itu.int/pub/R-REP-BT.2408-2017> (see Annex 1).

If full-scale 100 nit SDR white is mapped into PQ at 300 nits, what will happen? (Hint: on a PQ reference display it will look like a living room SDR display.) Typically the 100 nit (on an SDR reference display) SDR white will display at say 300 nits on a consumer display intended for use in a bright room. If an HDR display is set in a mode for best results in a bright room, it may likewise display a 100 nit PQ pixel at 300 nits. For an SDRx3 => PQ mapped image, the SDR white would then display at 900 nits on the living room PQ display, making SDR whites come out 3x brighter than HDR content.. More important than white level would be mid-tone levels, especially the level of faces. We certainly don't want SDR faces displaying 3x brighter than HDR faces. So far, we've seen faces in PQ at levels close to those for SDR (using reference monitors for both). It is possible that over time, with more experience and familiarity with HDR images, HDR levels may creep up. It is also apparent that broadcast content often employs higher levels than movie or dramatic content.

There is an approved (but as yet unpublished) update to Report BT.2390-2<http://www.itu.int/pub/R-REP-BT.2390-2-2017> that has an added section on SDR->HDR mapping. An excerpt is below (note that this language was very carefully crafted).

"10.1.1  Display referred mapping of SDR into PQ
The following procedure may be followed to achieve consistent mid-tone luminance levels when mapping standard dynamic range content into PQ.
Standard dynamic range ITU-R BT.2020 content should be mapped to PQ by applying the ITU-R BT.1886 display EOTF and then applying the PQ EOTF-1. For unity mapping the peak signal of standard dynamic range content should be set to 100 cd/m2 or 51% PQ.
Unity mapping does not change the display of the SDR content (it will display on the PQ HDR reference monitor the same as it displayed on the reference SDR monitor). Thus, no OOTF adjustment of the SDR display light signal is necessary.
If the SDR content is being inserted into HDR programming, and there is desire to more closely match the brightness of the HDR content, and that brightness is known, scaling can be done to bring up the brightness of the mapped SDR content. Scaling should be performed with care lest scaled SDR content, in particular skin tones, becomes brighter than in the HDR content."

Hope the above helps with understanding.
Best regards,

Craig Todd
Sr. VP and CTO
Dolby Laboratories, Inc.
1275 Market St.
San Francisco, CA 94103, USA
T  415-558-0221  M 415 672-0221
www.dolby.com<http://www.dolby.com/> |ct@dolby.com