Posted on October 5, 2020 By Phil Jones
I have seen a lot of displays that look great on a test pattern but less appealing when viewing real world HDR content. Lately, calibrators, reviewers, and salespeople have been pointing at test patterns and scenes and stating “it is bright and there is no clipping” to try to prove that one projector’s dynamic HDR tone mapping algorithm is better than its competitor.
Sometimes chasing HDR highlights results in an SDR-like picture. In an attempt to show all the HDR brightness information on a low brightness display, it downconverts HDR to just SDR. While all the brightness information is there, the image looks flat.
Video cameras like video displays have a limited amount of dynamic range. HDR footage is shot in a Log format which preserves most the shadow detail and highlights in a scene within a camera’s limited dynamic range capture capability.
Cameras can shoot video in several different Log modes, which are different gamma curves designed to capture as much of the brightness information as possible using the camera’s sensor. This prevents loss of detail in overblown highlights and shadowy areas. While all the detail is present, the image looks completely washed out because that is the only way you could squeeze all the brightness information into a limited space. While all the shadow and highlight info is visible, it doesn’t look like HDR.
The content recorded in a Log curve is color graded to restore its compressed brightness range during post-production. As the colorist adjusts the brightness curves, colors start to look vibrant, blacks look deeper, and highlights are brighter.
During the HDR production, bright highlight details are not a colorist or director’s primary focus. Most movies and TV shows are color graded on mastering monitors like the Sony BVM-X300, which has a maximum brightness of 1000 nits. The colorist may occasionally check to verify that the peak highlights exceeding 1000 nits are still visible; their main concern is the scene’s overall look.
For several years, I had a Sony BVM-X300 sitting on my desk. This reference display is widely considered one of the best mastering displays in the world. I even had the opportunity to grade some raw S-Log footage for HDR demos on flat panels at tradeshows. No one ever walked up to that BVM-X300 and said the HDR did not look great because of a single clipped cloud in the sky. I know that all the highlight detail is still there because I can see it in my scope. So if the finished content is played it back in the future on a higher brightness display, that highlight detail will be visible.
Even if the colorist has an extremely bright HDR mastering display, when doing a “trim pass” they may choose to clip a few highlights in the scene to make the image look more dynamic. If most content creators are not hyper-focused on a few clipped highlights, why should you?
When you compress a large brightness range into a narrow brightness range to record it on a camera, the video looks flat. Why do people believe that if all that brightness information is compressed into the limited brightness range of a projector that the result will not be a flat-looking picture as well? If you bend the tone mapping curve too much to preserve highlights, you might end up converting HDR imagery into SDR.
When comparing two projectors with comparable contrast capability, neither projector will be able to be brighter while simultaneously showing more extreme highlight detail, producing more vibrant colors, and delivering deep blacks. Tone mapping (whether it is dynamic or not) requires the projector to compromise something (highlights, brightness, color, or black level). For a projector to produce an “HDR like” image, the best option might be to clip an occasional bright highlight.
Sony’s goal is to give the viewer an experience close to that of an HDR flat panel even if the projector must clip some highlights for a few seconds in a scene. After a processor like the X1 analyzes an HDR scene, it must make a hard decision about what information to display.
Most of the onscreen objects in a 4,000nit movie have a brightness of less than 100 or 200nits, so the video processor might choose to reproduce the brightness of most a scene as accurately as possible and clip a few highlights to produce a better HDR picture. I would call that intelligent dynamic tone mapping.
During my review of the VW915ES, I typically left the HDR Dynamic Contrast setting on LOW with excellent results regardless of the content mastered brightness. While the HDR Dynamic Contrast usually makes the right decision, sometimes you may want to finetune the look of an HDR movie or scene. This can be done with the press of a button – three levels in the Dynamic HDR Enhancer let you quickly tweak the look (how much you bend the brightness curve).
Whether a highlight is clipped is not the best way to judge a good HDR image and it also does not determine if the projector/TV is actually dynamic tone mapping. When watching HDR content, sometimes brighter highlights will still be clipped, but Sony believes this is necessary to keep most of the image on screen as close to the director’s intent as possible.
Different projector manufacturers utilize different HDR tone mapping algorithms to try to deliver the best picture possible. There is no right or wrong way to dynamic tone map because it is always a compromise, so just choose the HDR look you prefer. Since no projector today can accurately reproduce the entire HDR signal, isn’t that what really matters?
The most significant upgrade to the VPL-VW915ES is that its video processing is now powered by a version of Sony’s powerful X1 processor that has been optimized specially for projector applications. This processor is what gives the projector the ability to do scene by scene HDR tone mapping
As mentioned in the VW915ES review, that unit delivered the best image quality I have ever seen in my test lab room. I can confidently attest that the VW915ES produced the best looking, most eye-popping HDR picture I have ever seen on my screen in my test lab earning it one of our Hot Product Awards.
Click below to check out the VPL-VW915ES review.
In the future, as the brightness and contrast of home theater projectors increase, the clipped highlight information can be restored without making the HDR image look flat. For example, the X-1 processor in a 10,000 lumen Sony VPL-GTZ380 will clip fewer bright highlight details than an X1 processor found in the 2,000 lumen VPL-VW915ES. In fact, the GTZ-380 is bright enough to reproduce HDR mastered at 1,000 nits on a 100” screen with zero tone mapping.
BTW, if you are lucky enough to be able to afford an insanely bright projector like the GTZ380 for your home theater, you don’t have to worry about the HDR image being too bright. Since each pixel in HDR content is assigned a specific brightness when accurately reproduced that pixel will have the same brightness on a 10,000 nit projector or a 2,000 nit projector.
In addition to a rated brightness of 10,000 lumens, 100% DCI-P3 color coverage, and a 16;000:1 native contrast the flagship VPL-GTZ380 is also equipped with an even bigger brain. The X1 Ultimate is probably the most powerful video processor ever utilized in a home theater projector. Originally designed for Sony’s state of the art Master Series LCD and OLED TV, the X1 Ultimate adds two additional processing features.
First is Object-based HDR Remaster which analyzes and adjusts the colors and contrast of individual objects on the screen. So while some projectors can adjust contrast frame by frame, it done along a single contrast curve, the X1 Ultimate adjust each object in each frame individually using multiple contrast curves per frame. The Dynamic HDR Enhancer and Object based HDR Remaster features work together to maintain even more of the highlight and shadow detail found in HDR content.
The second feature called Dual Database Processing ensures even better 4K upscaling. Two powerful image improvement databases work together, dynamically improving images in real time. 4K X-Reality PRO™ upscales images to near 4K clarity while a second database cleans the picture and reduces on-screen noise.
To learn more about Sony’s flagship home theater projector, check our product introduction article.
Even when HDR content is mastered for professional cinema, the colorist and director still make hard decisions on what brightness information to keep, what to compress, and what to throw away. Since consumer HDR content is mastered for a much brighter flat panel, similar choices must be made by the projector’s video processor.
By combining high-quality components with powerful processing, projector manufacturers like Sony continue to improve the HD, 4K, and HDR image quality of their home theater projectors.
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