This is the sixth installment of a six-part HDR “survival guide.” Over the course of this series, I hope to impart enough wisdom to help you navigate your first HDR project successfully. Each day I’ll talk about a different aspect of HDR, leaving the highly technical stuff for the end. You can find part 5 here.
Thanks much to Canon USA, who responded to my questions about shooting HDR by sponsoring this series.
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There are several competing standards for HDR distribution. Each has consequences for the cinematographer and their work.
The Dolby Vision standard utilizes the PQ curve (see appendix) for image encoding. It also specifies 12-bit encoding with a peak brightness of 10,000 nits as defined in the ST2084 specification, although Dolby currently recommends a target peak white value of 4,000 nits.
Dolby Vision content is currently mastered within the P3 color gamut, although it is capable of reproducing imagery in any gamut from Rec 709 to Rec 2020 depending on the capabilities of the display. Rec 2020 color is far beyond what modern displays can produce but leaves room for future growth.
Dolby’s key strength is that it provides dynamic metadata that instructs a proprietary decoder chip, built into a television, how to best adjust imagery to fit within the constraints of a consumer display on a frame-by-frame or shot-by-shot basis. If a program was mastered on a monitor that exceeds the specs of the television on which it is being viewed, either in dynamic range or color gamut, then the dynamic metadata that travels with the program tells the decoder chip how to expand or contract each shot’s color gamut and dynamic range to fit within that display’s abilities.
The HDR10 standard, which is championed by a number of television manufacturers who don’t want to license technology from Dolby, also uses the PQ curve. It aims for 10-bit encoding, encompasses the same Rec 2020 color gamut, and has been adopted as the Blu-ray DVD encoding standard. Most online streaming services offer both it and Dolby Vision as options. (HDR10 can be implemented in HDR TVs as a software upgrade, whereas Dolby Vision TVs require a built-in chip.)
Where Dolby Vision’s dynamic metadata aids in adjusting color gamut and peak brightness to match a television’s capabilities on a shot-by-shot basis, HDR10 incorporates only one instruction that applies to the program overall. There is some discussion about adopting a shot-by-shot metadata scheme similar to Dolby’s, but this has not been finalized.
The biggest difference is that there is no specification for what happens if a program’s peak white exceeds the capabilities of a consumer TV. It is up to each manufacturer to develop a roll-off scheme—likely some sort of highlight-only gamma curve—to compress highlights in a pleasing way that will retain some of the artistic integrity of the image.
This is of some concern to the discerning cinematographer.
The third impending standard is HLG. It is backwards compatible across a wide range of TVs as it employs a gamma curve that becomes progressively flatter as brightness increases, much like a log curve. A consumer TV will reproduce brightness levels as high up the curve as it can and roll off the rest.
In theory, the same content is viewable on both an old SDR TV and a new 1,000 nit HDR TV, although with dramatically different results.
HLG’s ever-flattening brightness curve doesn’t allow for the same size color gamut as other formats, so highlight saturation is reduced compared to Dolby Vision and HDR10.
Currently, HLG is only being considered for broadcast television, where legacy sets will be an issue for years to come.
THINGS TO REMEMBER
- Dolby currently offers the best scheme for adapting imagery to HDR televisions.
- Dolby and HDR10 are both currently available as video streaming options.
- HDR10 is the de facto Blu-ray standard.
- HLG is meant to be an over-the-air broadcast standard only.
- You have no control over which of these technologies will preserve or distort your creative vision.
Wrapping it all Up
There are a lot of unanswered questions about HDR origination and broadcast, but there are certain on-set practices that should make the transition easier. Key among these is to use an HDR monitor to train your eye to recognize when technical issues will arise regarding highlights, shadows and camera movement. There aren’t a lot of cheap or bright on-set monitors available right now, but at least one—the Canon DP-V2410—seems to be bright enough, dark enough, light enough and affordable enough to be a good on-set reference monitor.
Beyond that, it’s important to remember that bit depth matters, especially when the camera’s entire dynamic range will be reproduced on a display with little or no tonal compression. When broadcast standards call for 10-bit and 12-bit deliverables, it pays to shoot at a higher bit depth to leave room for grading in post, while pushing your tonal scale farther from the Barten Ramp boundary. 10 bit RGB is the bare minimum and won’t leave you much room in post. 12-bit is better, and 16-bit is best.
Grading will be a new experience. One colorist told me that there weren’t any limits as to what he could do, given well-exposed material at a high bit depth. This is exciting news for any cinematographer who is included in the grading process. Sadly, at least in short form work, this is not always the case.
As display dynamic range, color depth and resolution increase, our margin for error on set decreases. At the same time, HDR gives us a license to push imagery to perceptual realms never before possible in either film or video. The dynamic range of a projected film print can’t compete with HDR. Like film, it will take some time to learn to evaluate a scene’s visual impact strictly by eye and light meter. Fortunately, such training can happen in real time thanks to the availability of set-friendly HDR monitors.
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The author wishes to thank the following for their assistance in the creation of this article.
David Hoon Doko
Shane Mario Ruggieri
Disclosure: I was paid by Canon USA to research and write this article.