HDR: Part 25 - Creative Technology - Displaying Contrast

Several things are widely held to be true about HDR. Subjectively, it looks nice; most people instinctively like it. Also, it’s often described as being about higher brightness, and while that’s rather missing the point, it’s still very common to specify displays solely on the basis of their peak light output. More power is good, right?

Well, cooler heads know that the “dynamic range” part of the term is closer to contrast than brightness, but whichever way we spin it, people who’ve been dealing with it for a while have been raising concerns that may make some of the more ambitious HDR standards look a little overambitious, even to the point of being hard to watch. Given that this has been an objection also levelled at stereoscopic 3D, which has struggled to establish itself several times amid accusations of eyestrain, anyone interested in furthering HDR – which is many people – would do well to at least engage the argument.

It’s perhaps a flaw of the film and television production sector that it doesn’t pay quite enough attention to the realities of cinema exhibition and home viewing, which may differ quite significantly from the standardized, idealized environments experienced by film and TV finishing people. Peter Putman is an SMPTE member with more than four decades of experience in video and display technology who has given talks at the Hollywood Professional Association’s tech retreats for years. His recent talk, part of the 2021 online retreat, presented some observations about display brightness that might raise eyebrows among people who have fond memories of Dolby’s four-thousand-nit Pulsar display. That display regularly wowed trade show attendees with beautiful material featuring the light of the setting sun gleaming from the polished wings of a vintage aircraft. Pretty as it was, though, there have long been suspicions that what looks good on a trade show floor might not actually be desirable for either theatrical or home distribution.

Natural Contrast

Putman’s narrative begins with some all-important real-world research. While preparing his HPA presentation, he says, “we had this massive snowstorm. I went outside and took some spot meter readings. The sun was over a hundred thousand nits as you'd expect, and the snow was 16,000 to 20,000. The darkest corner of my garage was 320 nits. The actual contrast ratio was fifty to one. I keep coming up with that number.” Scenes of sunlit snow are sometimes hard to look at with the unprotected eye, but with such a high contrast natural scene equating – excluding the sun – to only six or so stops of dynamic range, Putman’s interest was piqued.

“At the same time,” Putman continues, “I was testing some LEDs in a very, very big and completely darkened theater, seating several thousand. People were saying ‘this thing is too bright, it hurts my eyes.’” These tests involved an off-the-shelf LED video display with a 3mm dot pitch that’s far from the cutting edge in 2021, and capable of three or four hundred nits. It’s a similar technology to that seen in every major outdoor advertising venue in the world, from the Las Vegas strip to London’s Piccadilly Circus. LED video walls in general represent the most powerful display technology currently available, capable of enough brightness and contrast to overcome midday sunlight.

Spectacular LED Colorimetry

With their staggering horsepower, inky blacks and spectacular colorimetry, LED displays might seem a shoo-in for cinematic exhibition. Traditional projection technology has traditionally aimed for fairly modest brightness – typically 48 nits, equal to fourteen foot-lamberts, or about half what TV standards require for normal reference displays. “For the average viewing in a cinema,” Putman says, “we struggled for years to get to the sixteen foot-lamberts that the SMPTE standards want. And [at one presentation] Dolby were saying that the maximum should be thirty foot-lamberts [102 nits] – maybe a bit more. That’s for specular highlights.” As such, even movie theaters advertised as capable of high dynamic range have sometimes involved about as much dynamic range as the last several decades of home TVs.

Later in 2019, a demonstration of Samsung’s Onyx display demonstrated the wisdom of Dolby’s caution. Putman describes the test material: “Arri had a test reel of HDR clips, heads against a white background, and some clips from Avengers which were more specular. So we were evaluating this footage, and people were complaining it was too bright. I asked the engineer how bright it was – he said ‘it’s not much, it’s two hundred nits max, and we’ve gone as high as three hundred nits.’ The whole room was lit up. People next to me, the chairs – everything. To achieve the same luminance levels on a projection screen that size would require at least sixty thousand lumens… I'm not even aware anyone makes a sixty thousand lumen projector.”

Here, Putman touches on a key subject: the viewing environment. Standards do recognize, if somewhat vaguely, the importance of the brightness and color of the room surrounding a display, and effectively all domestic displays are much brighter than the hundred-or-so nits of the standard, because the average lounge does not exist in the state of Stygian gloom typical of a grading suite. There is something to the idea that this makes a mockery of having a director of photography step into a blacked-out tent from a sunlit exterior location and immediately assess the image on a monitor, no matter how carefully calibrated it is; it also means TVs may legitimately need more power than grading reference displays.

Optimal Viewing Conditions

The issue for exhibition, though, is that cinemas generally are blacked out almost completely – irritating emergency-exit signs notwithstanding – and as such the standards required of mastering displays, which may exceed 1500 nits, may not be appropriate. “The LED manufacturers are only starting to clue into this,” Putman says. “They're starting to learn about it. They'll say ‘I can deliver a wall that'll do 1600 nits’. They're thinking ‘Hey, let's make it bright.’ Well, if the movie theater was fully lit, if it was outside, that might work, but if we’re darkening the room you don’t need that much.” The future for drive-in movie theatres, it seems is literally bright, but more conventional viewing environments may need to be a little more cautious.

If anything, we have simply reconfirmed that the issue is not brightness – it’s contrast, albeit in two different ways. For an image to be legitimately high dynamic range, that image must have high contrast. The between the image and the surrounding environment, however, must be limited to keep the viewer comfortable. This is also why projected HDR is difficult; even if the cinema auditorium is painted flat black, the clothing and people in it are not, and achieving the minuscule black levels required for HDR contrast levels can be impossible. Increasing brightness merely increases the brightness of the reflected light pollution.

This interacts with the capabilities of various common display technologies in ways that can be complex. OLED displays have very high contrast because they can achieve very low black levels, but often aren’t that bright; very bright OLEDs suffer short life, especially of the blue-emitting elements. TFT-LCD displays are effortlessly brighter than OLEDs, but their black level, and thus contrast, is poorer.

Improve Contrast Ratio

Even so, either can produce enough contrast for reasonable HDR, as Putman continues. “I have a fifty-five inch LG OLED in my home theater. This is an OLED running at 130 nits, for average bright content and diffuse white, but even with those luminance levels, there’s enough light output to illuminate the room.” Many people would not instinctively consider that a true HDR display, but Putman refers again to contrast ratio. “I did the Portrait Displays calibration and the display has a contrast ratio easily exceeding 1,000:1. However, the ambient contrast ratio between the brightest image on the screen and brightest reflective surface in my darkened family room is just 62:1. The actual display contrast is much higher and capable of high dynamic range. After calibration, small area white is 700 nits. That’s easily 20 stops higher than ‘just above black.’”  The mathematics are reasonably instinctive: “If your display is a hundred nits, if you want fifty-to-one contrast ratio, your blacks need to be two nits… an OLED can do that easily, a quantum-dot LCD can do that.”

And, based on the two-hundred-nit Samsung LED video wall tests, Putman’s non-HDR, domestic OLED, in a dark room, might be capable of uncomfortable brightness given sustained bright picture content. “Am I not getting an HDR experience?” Putman asks. “A few years ago at the HPA Tech Retreat, Gary Demos and Joe Kane were promoting the idea of a mezzanine-level HDR specification, since just about every TV, LCD and OLED, can easily achieve it. Can we then show HDR on it? We can, on the OLEDs, but what's the viewing environment?”

Content, as ever, is king. The numbers we’ve discussed are maximums, not averages, and colorists are free to choose how brightly to render a scene, give or take systems like Dolby Vision which might choose to alter things to suit various displays. Arri’s heads against white are, perhaps, a harsh test for excessive brightness. “Just because we have all this horsepower doesn't mean we have to use all this horsepower,” Putman warns. “What's a comfortable level for them to be watching at? What's a desirable contrast ratio where you preserve those surges of high dynamic range?”

Maintaining Immersive Viewing

That parallel with stereoscopic 3D is unavoidable. It, like HDR, required (and to the extent it’s still done, requires) a light touch to keep the audience comfortable for the duration of a two-hour feature. That’s something which has occasionally created friction between producers who might breeze through a room, viewing the image only briefly, and push for a more immediately-spectacular result.

A healthier approach, Putman goes on, might be modeled on sound mixing. “If you have a hundred-watt amplifier, you're not listening to audio at a hundred watts all the time. The average level you’re listening to, assuming a symphony that has peaks and valleys, is something like three to five watts of audio. You have that extra headroom for cymbals or an orchestra hit. If you put an 800Hz tone into a hundred-watt amplifier and run it full bore, that's painfully loud in a living room.” The visual equivalent might be the difference between the brightness of a diffuse white surface and the specular highlight on a piece of chrome in the sun: the HDR display makes the highlight look realistic without dazzling the audience.

“Some companies were perhaps a little overly enthusiastic about very high peak luminance levels with HDR reference monitors, without taking the viewing environment into consideration,” Putman says. “People are running around saying ‘we want four thousand nits!’. We don't need that. Four thousand nits is an LED running in Wembley Stadium, outdoors. If the sky is at fifteen or twenty thousand nits, you've got to be able to see the display. But no, a four thousand nit display never belongs in a home. A two thousand nit doesn't belong in the home. When we attended the demo of the Samsung Wall at their executive demo center at Irvine, I asked about the peak brightness and they said 1600 nits. I said ‘I can't watch this with the room lights off! I need sunglasses, it's too bright!’”.

Realistic Experience

In the end, Putman is certainly not dismissive of the potential for improvement in theatrical exhibition, which, as we’ve seen, has long struggled to match the brightness even of home TVs. Demanding HDR makes things even more difficult. “To do in the theater what people are seeing at home with systems like HDR10+ and Dolby Vision, well, you can't use conventional projectors. You have to use an emissive [that is, LED] display. The projector guys can claim they can do it, but emissive imaging is just not the same as reflective.” A white screen, in the end, is still a white object, and at least some light will bounce back from the room and illuminate it, increasing the black level and reducing contrast below that critical 50:1 number. That, in the end, is the key: not brightness, but contrast.

“For the bulk of material,” Putman concludes, “we're going to be using a fraction of available illuminance on LED displays. If we think about this intelligently, we can give people an HDR experience that feels realistic, but you're not really running a display all that hard to do that. It's more about the contrast ratio than the sheer nits. For cinematic and immersive viewing, we already have way more horsepower than we will ever need.”

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