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The ins and outs of High Dynamic Range (HDR) in cinema
Posted on Thursday, February 16, 2017

HDR in cinema

 

High Dynamic Range is a heavily debated topic in the technology community: not only linked to cinema, but also to television and content creation.


In previous articles, we have tackled the (non)sense of some of the extremely high contrast ratio numbers that are claimed as well as the different angles involved when developing and building a high(er) dynamic range projector.

 In this article, we’ll zoom out a bit and won’t focus on the projection aspect. We look at the wider eco-system instead: what does it take and who’s involved to get HDR-content on my screen?

UHD Premium

To start our story, we need to step away from cinema and go back in time to the activities of the ‘UHD Alliance’ between 2010 and 2015. This industry consortium works on Ultra HD (3840x2160, consumer version of ‘4K’) and has founding members from entertainment, electronics, and technology fields. Apart from working on standardization and interoperability (focused on TV and BluRay); they identified ‘UHD Premium’ in January 2016 – note that’s it’s not a spec, but a label by the consortium.

The goal was “to bring a new, differentiated entertainment experience that delivers a premium expression of creative intent using next-generation audio?visual technologies”. For the first time, the group went beyond resolution only: color gamut, contrast ratio, bit depth etc. were all included in the key properties. This is what makes the ‘UHD Premium’ definition unique.

UHD Premium defines HDR as: 

  • SMPTE ST2084 EOTF (see below for more explanation) 
  • A combination of peak brightness and black level either:

 

More than 1,000 nits peak brightness and less than 0.05 nits black level (aimed at LCD TV’s)

OR

More than 540 nits peak brightness and less than 0.0005 nits black level (aimed at OLED TV’s)

 

 This is closest we ever got to a specification for HDR in the movie and TV industry. SMPTE defines it without quantified metrics: “A High Dynamic Range System (HDR System) is specified and designed for capturing, processing, and reproducing a scene, conveying the full range of perceptible shadow and highlight detail, with sufficient precision and acceptable artifacts, including sufficient separation of diffuse white and specular highlights.”

This UHD Premium definition put a stick in the sand for content creators – often those same creators that make the movies that are screened in cinemas. It’s only logical that they wanted to leverage this across their content (workflow). Understanding the UHD Premium definition will help you understand what’s currently under discussion and what might be coming into cinema.

SMPTE ST2084 EOTF

EOTF stands for ‘Electro Optical Transfer Function’; which basically means “how do you translate bits into nits?” or “how do you translate electrons into photons?”. The content is handed over to cinemas in the form of a DCP: the digital cinema package is a digital file – lots of bits – that needs to be loaded into the server, then sent to the projector and finally projected on screen – lots of nits. In order to guarantee the creative intent of the director, a standardized translation step is needed. Since the director isn’t present in the booth for every screening, he cannot preserve his creative intent live and on the fly (“I want to make the actor’s suit more reddish”). You have to trust in the way he stored his intent in the bit values of the DCP (“pixels that show actor’s suit = (4021, 212, 54)”). This is exactly what the EOTF does: it tells the projection system how to represent a certain input value.

Historically, this EOTF curve in digital cinema was referred to as the ‘gamma curve’: the shape could be described by a power law, where the exponent was named after the Greek letter gamma with a typical value of 2.6 for cinema, and between 2.2 and 2.4 for television.

One of the risks of the introduction of higher dynamic range is visible banding or contouring in the image. If you try to describe more information (higher dynamic range) with the old description (gamma curve, 12bit) the quality could be insufficient and quantization steps could become visible.

Picture courtesy of Fringe Focus, source: https://fringefocus.com/wpcontent/uploads/2010/02/slide12.jpg 


Hence the introduction of a new EOTF that’s tuned to the need of HDR: the so-called SMPTE ST2084 (it has been standardized by SMPTE) or PQ curve (Perceptual Quantizer) in common speech. The good thing – from a cinema screening viewpoint – is that the projector actually doesn’t care what’s behind the EOTF. Its processing modules just do the math and don’t ask questions. All Barco cinema projectors can have the PQ curve active as EOTF.

Picture courtesy of Intel, source: https://image.slidesharecdn.com/siggraphscotthdrv100d-160824214933/95/highdynamic-range-hdr-demystified-39-638.jpg?cb=1472075547