One of the most interesting papers at SMPTE 2017 was delivered by Tyler Pruitt, Technical Liaison for SpectraCal, maker of the CalMan display-calibration software. He proposes a new methodology for HDR calibration, including a measured color-volume profile for each display.
The paper begins by pointing out that calibrating HD SDR displays is relatively easy. There’s one standard for peak brightness, color gamut/volume, and other factors to which mastering monitors and consumer displays must conform in order to reproduce the video as the content creator intended. The goal of calibration is to adjust a display so its performance is as close to these standards as possible.
With HDR, however, there is no such standard. Mastering and consumer displays can have various peak-brightness levels and color volumes, and these values are generally higher in mastering displays than they are in consumer displays. As a result, a consumer display must adjust the incoming HDR signal so these elements fit within its particular capabilities.
This process is called tone mapping or color-volume mapping. Unfortunately, there is no standard for how this should be accomplished, so each manufacturer must come up with its own way of doing it. In addition, the display’s behavior will be different with a signal that was mastered on a 1000-nit monitor compared with a signal that was mastered on a 4000-nit monitor.
As Tyler states in his paper, “The fundamental concept of color calibration for displays is that you are calibrating to a known standard. However, the non-standardization of HDR color volume remapping provides a unique challenge for proper color calibration of the HDR picture mode(s) in consumer HDR televisions. Without a standard to calibrate to or validate against, there is no objective way to determine if an HDR display is properly calibrated.”
Another problem is that the display panels within all current LCD and OLED HDR TVs are still based on the gamma EOTF (electro-optical transfer function), which determines how brightness code values in the signal are converted to light emanating from the panel. So, an incoming HDR signal that uses the PQ or HLG EOTF must be mapped to the panel’s native gamma, which is typically 2.2 or 2.4.
In most consumer displays, tone mapping is performed before the signal reaches the calibration controls. This makes HDR calibration especially difficult, since each display’s tone-mapping algorithm behaves differently and can easily affect the calibration.
As illustrated here, most HDR TVs accept a signal via HDMI and convert the YCbCr encoding to RGB. Next, the TV performs its tone mapping—called color-volume remapping in this diagram—before the signal reaches the calibration controls. After all that, the signal reaches the panel itself.
In response to all these considerations, Tyler proposes a new methodology for HDR calibration. Specifically, he recommends that TV manufacturers include a setting that allows calibrators to disable the display’s tone mapping and EOTF mapping. This “HDR bypass mode” would let a calibrator adjust the display’s grayscale and gamma response without worrying about how its HDR remapping algorithms are affecting the calibration.
Another part of Tyler’s proposal is a measured Color Volume Profile. The calibrator would measure a display’s actual color performance, and those values would be fed into the display. As a result, the display would be able to perform color-volume mapping more accurately according to its native capabilities.
The Color Volume Profile would include peak luminance and black level in nits as well as the coordinates of the red, green, and blue primaries. These values would be fed back into the display via USB memory stick, entered into the display’s menu system, or over a network.
SpectraCal has developed a similar system with LG for Dolby Vision, and it hopes to do the same thing for HDR10 and other HDR formats with the rest of the industry. I strongly recommend that all display makers heed this proposal and work toward a standard that brings the order and stability of SDR calibration to the HDR world.