I was about to link to an old article on Secrets about HDR but figured that might be too much of a deep dive. Luckily Chris Heinonen just posted a nice little summary of HDR tone mapping on his site. https://referencehometheater.com/201...ing-explained/
The following is quoted from Chris Heinonen in his article linked above. In the quote he is referencing a new customizable tone map feature on the new generation of LG OLEDs. While this is not specifically talking about projectors it’s still a good read and the concepts are similar.
“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.”