High dynamic range (HDR) content, such as Dolby Vision, has two major problems. First, how to generate it and second, how to display it. Disney Research in Zürich has a proposed solution to the second issue, at least. Four researchers at Disney, led by Dr. Tunç Aydin, with a fifth colleague from ETH Zürich, published a paper at the recent ACM SIGGRAPH Asia 2014 in Shenzhen, China titled “Temporally Coherent Local Tone Mapping of HDR Video”.
To show the full dynamic range of Dolby Vision content, a display must have 17 – 18 bits of dynamic range. Very few displays have this range, with typical consumer TVs or home theater projectors more typically in the range of 9 – 10 bits. There are other HDR schemes besides Dolby Vision, but they all call for at least 12 bits of dynamic range.
To display HDR content on a non-HDR display, typically tone curves are used to compress the highlights and dark regions. According to the Disney Researchers, “current tone mapping operators either produce disturbing temporal artifacts, or are limited in their local contrast reproduction capability”.
The Disney algorithm is an HDR video tone mapping operator that is said to greatly reduce the input dynamic range, while at the same time preserving scene details without causing significant visual artifacts. The algorithm involves mapping not just a single frame but considers video frames before and after the frame under consideration, as shown in the image. As discussed by the Disney Researchers, the algorithm is suited for use in the post-production process. This would allow, for example, HDR cinema content to be down-converted to normal dynamic range TV content. In principle, however, the algorithm could be embedded in a chip and do real-time HDR to normal dynamic range down-conversion in a TV set.
Disney has posted a video on YouTube that gives examples of the failures of other algorithms, explains how the method works and shows video samples of its algorithm compared to eight prior-art algorithms. Not surprisingly, the Disney algorithm presents the best result.
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I’d previously written on the first issue, acquiring HDR content. The Disney paper specifically mentions the Red Epic Dragon, Sony F55 and F65 and ARRI Alexa XT as being able to natively acquire up to about 14 bits of dynamic range, sufficient for most HDR applications, but ford not reference Dolby Vision. Cameras are on the horizon, however, that can acquire the full dynamic range of Dolby Vision. Digital Cinema projectors using DLP technology can natively show about 12 bits of dynamic range and the projectors developed by Christie and Dolby for Dolby Cinema presumably are able to display more than that. With dynamic backlights, LCD displays can also show HDR content.
While the HDR ecosystem is nowhere near complete yet, this Disney algorithm helps fill in one more missing piece. HDR, no doubt, is coming not only to the cinema but to home TV as well. – Matthew Brennesholtz
