Graeme Nattress was kind enough to answer some questions we had about highlight roll-off in RED’s Output Transform LUT.
According to the Red.com website, ‘very soft’ highlight roll-off compresses highlights the least, while ‘hard’ compresses highlights the most. But ‘very soft’ makes HDR footage resemble SDR and ‘hard’ allows highlights to sparkle. So, what exactly does compression mean in this instance, and is it a bad thing?
Graeme Nattress: Highlight roll-off is all about how you map a high dynamic range source image down to an image suitable for a lower dynamic range display, ideally preserving as much of the image as possible.
So yes, the goal is to make HDR look like SDR, i.e. fitting all the HDR range back down into the smaller SDR range. That can (perhaps will) make the image a bit “flat” because there’s (a) an awful lot of DR to bend down into range, and (b) the output transforms are “general” in that they will always “work” on pretty much any footage, but if your footage either doesn’t employ the full range, or has bright things you don’t really want to see detail in or keep really bright, they could be less than optimum.
It’s “compression” in the global sense, in that across an image, any brightness no matter where on the image will be similarly gently curved into visible range. Contrast that with dynamic HDR tone-mapping where the goal is similar – to map an HDR image into SDR, but it will work on a spatial basis so a bright area will get a very different compression curve to a dark or mid-range region. So no, it’s not a bad thing, but it’s a creative choice, and one that can vary based on your footage and how you shot it. The output transforms are “general” solutions to this problem, and as you note, the sub-optimal nature will manifest as highlights not “popping” at max brightness.
If you wanted to experiment with highlight roll-off, you could use the “none” transform and draw curves on a grading node ahead of the output transform. This can be tricky to do as they’re finicky to get right, hence the easy access to the choice of basic shapes as part of the output transform. If you want to play with output transforms and the various building blocks that make them up, and get to see the shapes of the curves used.
If you have a LUT tool like Lattice, you can open them and visualize to sorta see what is going on. If you view them like this, you can see the SDR highlight roll-offs don’t look all that different, which is why it’s hard to make them yourself with a curve, but much easier for me to do it in code where I can finely tweak by the numbers….
But on their website, doesn’t RED warn against setting highlight roll-off to ‘none’?
Graeme Nattress: ‘None’ is advised against as it is unforgiving, and can produce some very ugly images, but it’s useful for those that wish to build their own output transforms or rolloff curves…. hence the warning…. But if you can use it and grade the footage carefully, it can work….
Which setting do colorists who grade shows for Netflix and HDR theatrical presentation use?
Graeme Nattress: Netflix uses ACES for grading. It’s similar to IPP2 in that there’s a log image you grade and an output transform you apply that targets the display. Like IPP2 they have SDR and HDR options you can play with.
ACES is applying highlight roll-off as well… would it be doing something similar to what medium contrast/soft highlight roll-off is doing in IPP2, by chance?
Graeme Nattress: For the SDR modes it’s similar. For HDR, you need a very different curve as you can preserve much more image data before the roll-off needs to be engaged.
Don’t colorists also use highlight roll-off to emulate the film look, or is it just for mapping HDR to lower dynamic range displays?
Graeme Nattress: For building a film look, indeed there’s some roll-off involved. As film images get brighter, they start to get less bright than they should the brighter they get, and then the “print” curve is going to gently curve in the shadows to add contrast and roll off the highlights.
Are most colorists who are doing an IPP2 HDR workflow using medium contrast/soft highlight roll-off?
Graeme Nattress: For HDR they’ll be using SMPTE 2084, and probably with no roll-off at all, rather they’ll grade to bring in any errant highlights that go too bright, or they’ll let the grading software handle it somehow.
As so often happens, we were more confused after speaking with Graeme than before. How is setting highlight roll-off to ‘hard’ applying more compression than ‘very soft’? Doesn’t setting ‘User Nits Limit’ to 1,000 ensure that highlights don’t exceed the capabilities of the reference monitor? Is the fact that ACES RRT is applying an even more aggressive highlight roll-off than IPP2 one of the reasons why so many HDR shows on Netflix look like SDR? Six years ago, a contributor to ACES Central wrote, “I’ve found with digital capture and color grading of the project that I am often lighting to very high contrast ratios. Much higher than I would do when we captured on film with a photo/chemical output. I find that these images do not fit into the RRT. And there is no way for ACES to adapt to this. So, as it is, ACES is forcing my lighting style to match the RRT. This is a step backwards towards “the old days of film” in a way.” It would appear that the same holds true today.
Thoughts on Highlight Roll-Off
Highlight roll-off is a characteristic of photochemical film that many believe is as important as dynamic range. HDR has no highlight roll-off whatsoever. If we want to emulate the film look, rolling off highlights is a necessary evil.
The perils of applying excessive highlight roll-off:
- Erodes contours, destroying the volume of shapes and surfaces
- Tosses out texture and detail
- The image becomes flat
- Lessens the impact of specular highlights that are requisite for HDR
- Impacts colors, making them less saturated.
- Image resembles SDR
The advantages of applying an appropriate amount of highlight roll-off:
- Flattering to faces, more forgiving of imperfections
- Decreases perceived sharpness that persons of discernment find objectionable
- Fewer hotspots, minimizing the number of power windows required
- Lower contrast reduces the intrusion of judder that is aggravated by today’s high contrast, high luminance displays
- It’s a key component to achieving the analogue film look
Setting a speed limit is the #1 factor to consider prior to grading.
Strategy: Prior to grading, one of the very first considerations should be to determine a speed limit for the project. Setting a speed limit shouldn’t be an arbitrary decision that’s imposed even before looking at the footage. When asked whether he had any tips for those undertaking Dolby Vision HDR grading for the very first time, Siggy Firstl, senior colorist at Company 3, gave the following advice:
“Understand the look [that] the filmmakers are wanting and establish early on the sort of bright[ness] levels, how far you want to push the highlights… That’s probably […] the number one thing to establish before you even start doing the creative color… That will change the look of the show and also, if you say for instance go too bright in establishing that light and then you get into coloring, it can put a lot of extra work on you if you’re having to sort of force the highlights down the whole time.”
Budgeting for highlights
At the cinema, it’s not atypical for highlights to be nearly 3X brighter than diffuse white:
“In traditional imaging, the range allocated to these highlights [specular highlights and emissives] was fairly low and the majority of the image range was allocated to the diffuse reflective regions of objects. For example, in hardcopy print the highlights would be 1.1x higher luminance than the diffuse white maximum. In traditional video, the highlights were generally set to be no higher than 1.25x the diffuse white. Of the various display applications, cinema allocated the highest range to the highlights, up to 2.7x the diffuse white.”
Highlight Roll-Off
Deciding how much highlight roll off to apply is a balancing act between preserving artistic intent and maintaining detail and texture. Choosing the amount of highlight roll-off that works best for the project, then tailoring it to each scene using tools like contrast/pivot, custom curves and power windows is a good strategy. Setting highlight roll-off to ‘none’ is not recommended. The ‘User Nits Limit’ should be identical to the project luminance level:
“Some professional HDR monitors will hard clip when their brightness range is exceeded and produce artifacts. If this is the case, then you can use the “User Nits Limit” to set a limit to the encoded HDR image. The Highlight Rolloff control will be in effect and alter how the full HDR range up to 10,000 nits is mapped into the specified brightness range.” red.com
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