Colourist Bryan McMahan Talks on HDR Video and Creating Better Pixels

High dynamic range videowas one of topics that interested vendors and visitors most at the IBC2015 exhibition – how to create, display and broadcast it, and its effect on viewers’ reception of content.Modern VideoFilmpost production participated in a 4K/HDR panel discussion in August 2015, shortly before IBC, titled ‘The Future of Post: 4K and HDR’, sponsored by Sony and KeyCode Media and moderated by Larry Jordan.

The panel included Modern Video Film's award winning senior digital colouristBryan McMahan, known most recently for ‘Knight of Cups’ and ‘The End of the Tour’, as well as Michael Cioni, president of Light Iron post studio, and Michael Whipple, executive director of post-production technologies at Sony Pictures Entertainment.

Bryan discussed his experiences grading HDR video, which he has done using both Dolby Vision and Samsung SUHD, two of the major professional systems for HDR display. Bryan also recently completed the 4K remastering of the feature film ‘Braveheart’, which included an HDR version as one of its deliverables.

Capturing HDR

Based on these experiences, Bryan now feels that HDR creates a new visual palette for DPs and colourists to work with.Digital Media Worldhad a chance to ask Bryan some questions regarding the colourist’s point of view on this topic, including how HDR recording might affect the relationship of colourists with DPs. Both will have access to more options for fine-tuning looks as digital cameras become more sophisticated and capable of capturing a wider colour gamut or range of colours, and a higher dynamic range - the ratio between the maximum and minimum measurable light intensities within which a sensor can record detail.

Traditionally, when using standard dynamic range cameras, the aim is to present a range of luminance similar to what we see in everyday life through our own eyes, which continuously adjust to the broad changes in dynamic range that occur naturally in the environment. Your brain continuously interprets this information so that you can see in a wide range of light conditions. Until recently, most digital camera sensors haven’t been able do this very effectively, and HDR images were produced by capturing and merging in post two or more different, narrower-range exposures of the same subject.

However, producing images this way is a completely different task to making use of a modern digital sensor’s high dynamic range. The sensor’s dynamic range depends on the size and number of photosites it contains, located at each pixel, and how quickly they reach saturation. The dynamic range of a digital camera is usually represented by a comparison of maximum light intensity measurable at pixel saturation to minimum light intensity measurable – yielding the contrast ratio expressed as f-stops, which describe the total light range in powers of 2. An example is a contrast ratio of 1024:1, which would be listed as a dynamic range of 10 f-stops because 210 = 1024.

Contrast ratios, and therefore dynamic ranges, are increasing in new cameras. The RED Raven, introduced at IBC, claims a range of over 16.5 f-stops. Regardinggrading softwareto work with this kind of imagery, Bryan said, “As cameras become more sophisticated so will colour correction systems, but this has always been the case. Because HDR display requires access to the most range the camera can give us, the software in the colour correctors needs to give us the ability to use all of those bits.”

Role of the Colourist

In some ways, Bryan expects HDR capture to extend the role of the colourist further into production-side activities, similar to the effect that creating and using on-set LUTs had on their role. “I think the situation will be very similar, but the one difference is that the monitors used on set most likely won't have the same range in luminance or in bit depth as the HDR mastering monitors or HDR projectors have. I believe this fact will create a closer relationship and greater trust between the cinematographer in the field and the colourist he or she will be working with on the final result.”

An especially interesting comment Bryan made during the panel discussion was that once he was used to working in the HDR format, it takes him the same or even less time to grade an HDR master as an SDR video. He explained this a little further.

“The information that a camera captures, whether it's a digital camera or especially a film camera, is much greater than the displays or projectors that are currently on the market,” he said. “Home monitors, whether standard definition or high definition, are in the Rec 709 colour space, which can’t reproduce all of the colour that has been recorded. Therefore, colour must be manipulated to work within the limitations of the display.

Bending Colour to Fit

“However, because HDR displays are closer to P3 colour space, which is a larger colour gamut, I don't have bend the colour to fit. The same goes for the luminance of the image. With HDR I don't need to spend as much time worrying about clipping skies, hot spots on buildings, windows, cars, faces and so on. Also we finally have good blacks in the picture that are rich but let you see into them. I don't have to crush the blacks to get good contrast.  

“P3 is also the colour space for digital cinema so I don't need to convert colour space from the DI process to the home version, but even with that large colour gamut, the luminance is still limited compared to what the HDR can produce. Having this luminance to work with means I spend less time trying to fit a large peg into a small hole.

“Currently, I think success in grading HDR footage depends on both a person’s expertise as a colourist, and the software he or she is using. The colour correction systems are just coming up to speed with this format and, for that matter, so are the colourists. Like any new format it takes a little time to get used to working on it, and time to gain the knowledge of how things convert to different displays.

“But as always, the most important point is the communication between the cinematographer, the director and storyteller as to how their story should be seen. Just because my car will go 200 mph doesn't mean I have to drive it that way all the time.”

New Starting Point

Bryan is uncertain about how much specialized training most colourists will need to feel competent at working with and taking advantage of HDR content. It takes him a few days to get used to the HDR image, and after that a few days to go back and re-do what he started. He said, “My base setting or starting point will most likely change and evolve as I become more familiar with the HDR workflow. So far, there are still very few home displays on the market and as others start to appear we will need to figure out what the conversion process is to all of these other displays.”

Regarding monitoring, the luminance output of available HDR capable monitors, which are still very expensive, varies considerably and depends on whether the displays are LED or OLED. “As the manufacturers improve their products, specifications become a moving target. This is going to be a big challenge and means that collaboration between the on-set crew and the post production team producing the final product is imperative,” Bryan noted.

At this point in HDR’s development, Bryan anticipates that the format will have a greater impact in the home than on feature film production. “There is no doubt that HDR projection is a far superior theatrical experience and that the industry will be moving in that direction, but theatre chains will have to justify the expense of new cinema projection systems,” he said, “While to date, manufacture of home HDR projection systems is limited, I’m pretty confident that by the end of the year multiple options on displays for homes will be available - the major streaming companies that stream TV shows and movies to households today want shows that are in HDR now!”

Visual Change

Bryan feels distributors are facing a similar visual change as they did when they went from standard definition to high-definition. Viewers subjectively see more detail in a HDR image, even if the number of pixels is the same, and once they have seen HDR imagery, it will hard to turn back to SDR programming. When describing their Dolby Vision system for displays, Dolby Laboratories maintains that while 4K televisions have more pixels and the new UHD TV standards include high frame rates, neither of these two standards makes each pixel better able to represent the full range of brightness people see in reality. This kind of improvement comes from more dynamic range and wider colour gamut.

“But for viewing, it's really not that big a change on the equipment side apart from the display,” Bryan reflected. “As far as production goes, camera crews will need more storage because a 16-bit file is preferred and the workflow may be 12-bit rather than 10-bit, but the cameras are the same. HDR does not define resolution so what you are currently working in, other than your display, should not have to change drastically.”  www.mvf.com