Understanding the interplay between display limitations, viewer comfort and creative intent is critical for HDR grading. This post examines three pivotal constraints.
Power Budget
In their most accurate picture modes, flagship OLED TVs like the Sony A95L can reach as much as 265 cd/m² full screen brightness, which most OLED TVs can’t begin to approach; so, when grading, it’s good practice to ensure that frame average light levels of static content don’t exceed that figure in order to prevent degradation of the picture.
HDR video content typically has a low APL (<20%). In a study of 41 Warner Bros. titles (over 7 million total frames analyzed, corresponding to approximately 83 hr of content) in the HDR Home format that were mastered on a Dolby Pulsar display (0.005 cd/m2 black level and a peak luminance of 4000 cd/m2), MaxFALL (maximum frame average light level) of 98.6% of the frames was below 170 cd/m2 (the display power budget of the BVM-X300).
Comfortable brightness levels of static images
You might be wondering whether this brightness limitation of some consumer displays isn’t a hindrance when it comes to artistic expression. Unless your goal is to intentionally inflict harm on your audience, it’s probably not a handicap:
In a study by the NHK, images with an average luminance of less than 25% of the peak luminance were considered by participants as comfortable, whereas images with an average luminance that exceeded 25% of peak luminance were judged as too bright by many viewers. In separate tests conducted by the BBC measuring tolerances to brightness jumps in HDR content, having watched video sequences with “peak luminance levels of 1,000 cd/m2 and 4,000 cd/m2, 25% of subjects commented informally that the brightest scenes were uncomfortably bright regardless of any jumps. These scenes had average luminance levels of 268 and 363 cd/m2 on a 1,000 cd/m2 display. Similar comments were not made about the test scenes that had average luminances of 144 and 128 cd/m2 on a 1,000 cd/m2 display.”
Note: “A comfortable overall brightness does not ensure that the content makes good use of the available dynamic range.” ITU-R BT.2408
Graphics White Levels
If graphics white levels are set too high, they will destroy the impact of highlights in the picture, which is why some studios insist on a maximum of 203 cd/m2 unless there is a compelling reason for doing otherwise:
“The video level for the diffuse white, that is the brightest diffuse reflecting part of the scene, is an important aspect of HDR video. The human visual system seems to adapt to this video level and interpret the image accordingly. So, for example, if the image includes graphics, it is important to set that graphics level correctly. If the level is set too high, for example near to the peak brightness of the display, this level is interpreted as the level of a diffuse reflector in the image. Consequently, with the graphic set at this level, there is little impression of speculars or highlights in the image, which looks more like a printed image. However, with the graphic set at an appropriate level, several stops below the peak display brightness, the human visual system interprets higher luminances as highlights and speculars, an effect known as “brilliance”. This leads to the qualitatively greater impact of HDR video and a feeling of being closer to the real world.” – Tim Borer, Display of High Dynamic Range Images Under Varying Viewing Conditions, Research & Development White Paper WHP 369 (British Broadcasting Corporation, December 2019).
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