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BBC Hybrid Log-Gamma points to screen-agnostic HDR

At IBC, the BBC presented an award-winning whitepaper which showed how existing screens can show High Dynamic Range content.

The technology can be built into an HEVC Main 10 signal that would also provide a signal to non-HDR displays – providing a screen-independent implementation of the technology reliant on HEVC.

It all got very technical in the presentation, so we’ll try to outline the main thrust of the technology here, but at the core of the matter, the BBC wants to change the way that content is treated right from the moment of capture, in order to create video that can be carried along the same pipes to screens and interpreted according to the ability of the screen.

Presented and written by Andrew Cotton and Tim Borer, the BBC R&D findings present a system which promises to greatly simplify the creation of HDR video, as long as the panels are capable of processing 10-bit video via HEVC – meaning that this is a technology that will go hand-in-hand with the adoption of UHD, as the standard requires 10-bit video. While there might be issues with some 4K panels, which won’t support the higher frame rates or color depth of the full UHD standard, it seems unlikely that Hybrid Log-Gamma will be commonly found on screens with only 1080p resolutions.

10-bit color depth is around four times the size of 8-bit (1,024 shades per primary color vs. 256), which allows for an image to avoid the stepping artifacts and banding that can occur around the subject, which is particularly obvious in elements of any frame which are out of focus. Using the much greater color depth, those artifacts can be avoided.

On stage, Cotton noted that dynamic range is measured in terms of contrast (stops), rather than brightness. Printed images have a contrast of around 100:1, or about 7 stops, while the human eye is able to perceive about 14 stops of contrast. According to the paper, 8-bit video manages around 6 stops, while 10-bit achieves around 10 stops – still not achieving the full capability of the human eye.

So with a shift in the capture of the image (the opto-electric transfer function, from light at the source to digital data in the camera and workflow), to utilize the increased color depth and contrast data, a much better picture quality can be achieved via 10-bit video. This shift would also negate the need for additional metadata in the stream.

In addition, the Hybrid Log-Gamma implementation allows color grading at 600 nits, with an upper limit of 4,000 nits. Currently, the industry standard is still stuck at 100 nits, mostly due to the legacy of showing content in dark cinemas rather than brightly lit living rooms or outside screens – which would necessitate a higher baseline. With most current TVs capable of achieving upwards of 400 nits, just regrading the content to match today’s screen technology would have a profound impact on quality.

With the higher grading, the display brightness wouldn’t be such a determining factor on the quality of the image. The higher baseline brightness in the image would allow for more subtlety in the picture, without needing to use very high brightness displays and trying to find a way to make their power consumption comply with stringent EU regulations.

“We are delighted that IBC has recognized our paper on a display-independent HDR television system,” said Andrew Cotton and Tim Borer. “We believe that the technical solution we have presented provides the best universal approach for HDR, allowing it to rapidly become part of mainstream UHD offerings internationally.”

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