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HDR: The More Bits, the Better
Apple's new Macs feature 10-bit HEVC decoding for HDR video viewing. To understand why that's important, it's worth a look at an old technology—film cameras

Bright whites, yellow suns, and deep blacks are the goal when it comes to high-dynamic range (HDR) capture and viewing of premium content. After all, as we approach the dog days of summer here in North America, don't we want our captured memories to be equally as brilliant or dark as the real deal?

From pinholes to subpixels, or from 8-bit to 12-bit, the quest for HDR continues to outpace the quest for virtual reality video or even 4K streaming.

In early 2017 we saw several competing HDR options emerge, with one using 10-bit color samples and another using 12-bit color sampling.

To understand why the extra bits matter, we have to harken back to the days when some of us spent parts of the summer in a darkroom, processing 35 mm film rolls or even photosensitive paper through a series of trays filled with various chemicals from household names like Fuji or Kodak.

Images captured on film, either through a pinhole or a more traditional multi-aperture-based camera lens, use the terminology of a “stop.” A stop is essentially a click-point on the camera lens, with each stop designating a different aperture. In film school we were taught to differentiate between f-stops and t-stops, but outside the realm of cinema the use of the term "stop" refers to an f-stop.

The aperture itself determines how deep or shallow the focus will be, which is where we get the terminology for depth of field that's now being tossed about for consumer smartphones. The Apple iPhone 7 "depth effect" feature makes use of two lenses to create a digital version of depth of field.

In addition, the aperture setting had to be balanced with the sensitivity of the 35mm film stock. If too much light reached the film, the image would be washed out (eventually going to all black on the negative, which rendered all white on the printing) but if too little light reached the film, the image would be grainy and dark.

Black-and-white photography, from the masters like Ansel Adams to the more mundane Instamatic 110 cartridge camera I had as a child, is based around the balance of these light and dark areas. While color adds an extra dimension, even good color photographs are a lesson in contrasting darks and lights.

Different film stock had different tolerances or latitudes for these dark and light areas, and the ability to have a wide latitude meant the ability to capture bright whites and dark blacks, rather than just fifty or more shades of grey.

The move to digital capture meant a more muted capture, since most digital sensors had a very limited latitude. That has improved over the past few years, though, with digital sensors beginning to reach a range of latitude that rivals film.

Which brings us back around to HDR and the reason that the bits are so important. The number of bits in the bit-depth for digital capture equates to the number of stops or range of latitude for older film cameras.

An 8-bit capture device offers about 6 stops of latitude, but a 10-bit capture device is capable of almost three times the range of latitude, at almost 18 stops. If more bits are added—going up to the 12-bit offerings now on the marketplace for still image capture and some limited video capture—the range of latitude increases as well.

Unfortunately, just because a camera can capture in the more latitude-friendly 10-bit mode, the image quality is lost when a video is viewed on a playback device that only displays 8 bits of color depth. And that's even assuming that the stream is being delivered in HDR-friendly 10-bit or 12-bit options, because these higher bit-depth options equate to significantly higher data rates.

Movie cineplexes understand this, and focus some of their efforts on eking out much higher dynamic ranges than the typical home HD set. But the homefront is improving, thanks to efforts to introduce various versions of HDR into consumer flat panels.

At the IBC show in Amsterdam in September, expect to see several more HDR choices for both streaming and viewing. In the meantime, enjoy the summertime blacks in the world around you.

[This article appears in the July/August issue of Streaming Media magazine as "Streams of Thought: Summertime Blacks."]

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