While this is an important step towards the creation of broadcast HDR-WCG systems, if HDR-WCG production is to be deployed commercially, it is necessary to use a mix of both conventional standard dynamic range (SDR) and HDR cameras in a single HDR-WCG production.The ability to deliver high dynamic range (HDR) and wide colour gamut (WCG) imagery is crucial to next generation broadcast. It is a key feature of both DVB UHD-1 Phase 2 and the latest ITU-R recommendation: BT.2100.
It is also necessary to derive a high quality conventional ITU-R BT.709 (SDR with gamma non- linearity) programme for regular contribution and transmission. Additionally, it is necessary to cross-map SDR programmes, interstitials and adverts into an HDR-WCG service for transmission.
This paper describes the techniques that have been developed to perform these transforms to meet broadcast production standards in real-time. These techniques are built on the experience gained in the creation of the first fifty HDR theatrical releases, as well as trials with HDR broadcast productions.
Finally, the operational practices to ensure consistency in HDR-WCG production, high quality programme interchange, and a pleasing viewer experience are examined.
Broadcast production today utilises a single colour volume workflow, as majority of footage is captured in one format: SDR (gamma non-linear curve and ITU-R BT.709 (1) colour primaries). With this single colour volume workflow, graphic overlays, adverts, and television programmes are routinely intermixed during production.
In the past year, availability of HDR and WCG televisions has grown substantially as has the demand for HDR-WCG imagery. To meet this demand, the broadcast production workflow must incorporate a multi-format pathway. It is likely that a mix of both SDR and HDR-WCG cameras will be in live production for several years. Additionally, the incorporation of archive SDR material in HDR-WCG production will be required for the foreseeable future.
The colour primaries (red, green, and blue) used in a WCG system are the ITU-R BT.2100 (2) colour primaries. The difference between BT.709 and BT.2100 colour primaries is shown in a chromaticity plot in Figure 1.
As specified in the ITU-R BT.2100 recommendation, HDR will replace the BT.709 gamma curve with an HDR non-linear curve: PQ or HLG. Unlike the traditional BT.709 gamma curve, PQ is an absolute encoding, meaning each code value relates directly to an expected luminance output on a reference display, in a reference environment. In that reference environment, whether a display can produce 500 cd/m2 or 3000 cd/m2, skin tones will remain constant around 26 cd/m2.
The difference between SDR and HDR-WCG reproduction can be better visualised as a colour volume (the colour gamut at all reproducible luminance levels). All colours that a display can replicate are contained within this volume. Figure 2 is a comparison between the BT.709 and the BT.2100 colour volumes. The larger HDR-WCG colour volume not only increases headroom for highlight and shadow detail, it also allows for increased brightness of highly saturated colours.
BT.709 primary blue has a maximum brightness of just 7.3 cd/m2 in a 100 cd/m2 peak white system. Increasing the maximum luminance of the display increases the possible maximum luminance of highly saturated colours, allowing blue to reach 593 cd/m2 in a 10000 cd/m2 peak white system. It should be noted that the HLG system proposed in BT.2100 will not utilize the full BT.2100 colour volume due to the OOTF application on luminance in a display (limited to 103 cd/m2 with a 10000 cd/m2 peak white).
During production, HDR-WCG and SDR imagery will be intermixed, derived, and converted from one source to another. Colour volume mapping is the process used to convert between SDR and HDR-WCG with the goal of preserving hue, saturation, and luminance. ITU-R BT.2100 (2) recommends two colour representations, ICTCP (3) and Y’CBCR, for efficient international programme exchange.
To avoid costly colour conversion in a real-time system, one of these colour representations may be used to perform all operations, including colour volume mapping.Efficient and high quality mapping (including round trip) is essential to successful real-time cross-mapping of SDR and HDR-WCG imagery.
MERGING SDR AND HDR-WCG IMAGERY
In a traditional SDR workflow, all content has the same colour primaries and non-linearity. Eventually an HDR-WCG workflow will have this same property. However, even as the availability of HDR-WCG imagery increases, SDR imagery will remain a common input source for years to come. Therefore, in the transition to an entirely HDR-WCG workflow, a hybrid system must be implemented to merge these two formats into a single container solution from which a high quality SDR programme, that either matches or exceeds current SDR programme practice, can be derived.
Once inside this common container, standard broadcast switchers and other tools will work equally well as with traditional SDR sources. In addition, post-production tools may be employed as necessary (colour correction/LUTS, blends, and graphic overlays).