2019 LG TVs – Custom Tone Mapping Explained
By Chris Heinonen on
A new feature on some 2019 LG TVs, including the updated OLEDs, is the ability to define a custom tone mapping curve using CalMAN. As this is the first TV to offer this option, I’m going to explain the basics of tone mapping, talk about how the process works on the new LG models, and then the potential benefits of having this capability.
Tone Mapping Overview
To understand the benefits of this new feature, you have to understand the basics of tone mapping. HDR content allows you to have much brighter highlights than before, and many of those highlights are brighter than a TV can display. HDR content has a peak light level, which the TV uses to determine how to display those highlights. The most common peak highlight levels (MaxCLL or Maximum Content Light Level) are typically 1000, 4000, and 10,000 nits. An OLED can typically display up to 700-750 nits, while the best LCDs can display around 2000 nits when calibrated. But based on the HDR data, there are highlights even brighter than this, and that’s where tone mapping comes in.
The tone mapping in a display adjusts how highlights too bright for the TV are shown. The simplest way is to clip highlights. With this method on an OLED TV, it displays everything up to 700 nits, and then everything above that is displayed as if it was 700 nits. This gives you the maximum possible light output but it also loses you the most detail as those bright objects, like clouds or snow, are now just white with no texture or details visible.
For most tone mapping, you instead display everything at the correct values up to a certain point, perhaps 150 or 300 nits, and then reduce the brightness of objects above that. In this case, the clouds you see won’t be as bright, but they’ll still be brighter than anything else on the screen. At the same time, you’ll maintain the details in them so they actually look like clouds and not just a white blob in the sky.
When I review a TV, I often watch some early scenes on the 4K disc of Pan that contains some very bright highlights. Depending on how the TV handles the tone mapping I might see details in the sky or they might disappear, and a bubble might have a rainbow of color on it or it might be solid white. There is also a scene in Batman vs. Superman where Ben Affleck has a white shirt that can either be completely white when a TV chooses to clip or have wrinkles and texture if it is tone mapping the highlight down to what the TV can display.
What is important is that neither of these options is wrong, but just different approaches to how to display content. Until now, how a TV chose to tone map was pretty much decided at the factory and you had to live with it, but with the new 2019 LG models, you are given some control over the process.
Defining a Tone Map
Before now, tone mapping in LG OLED displays was based on a peak light output of 700 nits and a set of tone curves. If your OLED was dimmer or brighter, the tone curves were the same and it slightly impacted what you saw. Now you are given the choice for some of the settings.
First, you’re able to set the peak light output of your own OLED display. Brighter or dimmer, this option lets you get the most accurate tone curve for your display since it knows when to start rolling off to preserve highlights. Second, you can set up the roll-off point for three different MaxCLL levels. Since the most common levels for content are 1000, 4000, and 10,000 nits that is what is selected in the menus here. Now for each of those you can define your own roll-off point. With 1000 nits using 70% is common and that’s what is selected here. For 4000 nits they have selected 60% and for 10,000 nits they’ve selected 50%.
Once you upload these to the LG TV, which only takes a single click, you can then use CalMAN to verify their accuracy and adjust the 1D grayscale LUT if necessary. Then you have your own custom tone maps for the most common content, so what does that give you?
Benefits and Decisions
The main option you get here is control. Say you have an OLED that does 750 nits and you want to watch 1000 nit content with as little clipping as possible. Simple set the 1000 nit mastering peak to roll off at 100% and now it will display content exactly as it should be up to 750 nits and then clip above that. It will give you peak brightness, but you’re also going to wind up with lots of highlights that might look bad since they’ll have no detail.
Maybe with 10,000 nits content, you want to have it start tone mapping earlier, at 30%, so the dynamic range of the highlights is larger. There isn’t much 10,000 nit content out there now but there is lots of 4000 nit content. You can now test and see what tone mapping levels look best to you, and it doesn’t take long at all if you have access to CalMAN.
Content also often has very different light levels than what the MaxCLL says. It might be mastered with a peak light level of 4000 nits, but maybe that’s a single highlight in the film and everything else is dark? Perhaps you prefer to see more of the midrange details than the bright details, or maybe you easily notice when the image clips and can’t stand it. Now you’ll have the ability to adjust it through trial and error to determine what you like best. I’m also sure there will be a long thread at AVS Forums in the future, where everyone compares their settings and tries to decide what they think is best, even down to single movies.
My initial thought was to create a custom map for 4,000 nit content that tracks perfectly up past 80% or so, keeping everything as bright as possible with the loss of some range in highlights. Apparently, this is what many professionals have also thought, but when you do that you lose so much highlight details that it looks like clipping when they appear. It will be very interesting to test this out myself with real-world content and see what looks good, what looks worse, and where the sweet spot is.
These custom options also don’t work with LGs dynamic tone mapping. If you have that enabled then these will be bypassed as the TV will evaluate it on a frame by frame basis to determine what the tone curve should be.
Other LG Features
Another new feature this year is HDR that automatically adjusts the tone curve based on ambient light. A problem with HDR has been that with fixed light levels, dark content can look great in a home theater but any light washes it out. When I watch the hilltop scene in the final Harry Potter, I do it in the dark because if there is any stray light it washes out almost all the detail.
Using the ambient light sensor on the TV, the HDR tone curve adjusts itself to raise the level of dark content when the room is bright. The brighter it is, the most it adjusts it. You’d never want this for calibrating, but for watching HDR in the living room it might not be bad. It’s something I’ll need to test and see how it works in practice, but I’ll be interested to see if they can figure it out. Dolby has their own version of this technology when the LG utilizes when watching Dolby Vision content with this feature enabled.
There is also an integrated test pattern generator inside of the LG OLED models. If you’re calibrating with CalMAN, it can create its own test patterns so you don’t need to connect anything over HDMI and it can do it with full 10-bit triplets. You’ll still need software and a meter, but it makes the process that much easier and more reliable.
Finally, you have the option now to disable the white boost with HDR content. This greatly reduces the peak luminance down to only 400-450 nits but preserves saturation where you can see highlights sometimes start to be washed out when it gets very bright. This behavior is noticeable when I compared a WRGB OLED (as all consumer models are) to a professional RGB OLED as the highlights on the consumer model started to turn white while the professional retained the color. In a dark environment where 400-450 nits can be enough, this option might offer improved colors with HDR highlights, but it is another feature we will want to test to see how it looks in use.