Tyler Pruitt from SpectraCal/Portrait Displays started his presentation by describing a big problem with HDR displays – color volume remapping (which he said includes tone mapping as well). This is needed when the color and/or luminance range of the mastering display and content exceeds that of the presentation display. The bigger “volume” of the mastered content must be mapped down to the lesser capabilities of the display. This includes remapping colors so they “appear” to be similar to the mastered colors as well as tone mapping of the luminance values. This is compounded when content and displays support dynamic metadata where this color volume remapping can happen on a scene-by-scene basis.
The challenge for SpectraCal as a calibration provider is to find a way to measure the accuracy of the remapped colors in such a way that it does not interfere with the remapping algorithms in the display. Plus, color calibration is typically done to a standard, but there is no standard for HDR.
He pointed out an additional problem as well. For one, all HDR panels today still use gamma at the panel level (an issue raised by Intel some time ago – BC4 Intel Makes A Call To Action for UltraHD on PCs) so incoming content with an HDR EOTF (PQ or HLG-based) gets remapped to the native gamma of the panel at the end of the video processing chain. TV calibration controls were developed in the 709/SDR era and did not consider the need for EOTF-to-gamma conversion nor color volume remapping. These functions are done at the beginning of the video processing pipeline whereas the calibration controls are at the end of the pipeline. This can result in some strange behavior with these control knobs.
Because of these issues, Pruitt presented a new method for HDR TV calibration that is independent of HDR color volume remapping behavior, but it will require the cooperation of HDR TV makers to implement. The idea is to calibrate the HDR picture mode(s) with all color volume remapping and EOTF-to-gamma conversion disabled or bypassed. This will allow calibration of the display’s grayscale and gamma response to occur without fighting against these proprietary HDR remapping/conversion algorithms. The other benefit of this approach is that it doesn’t impose a static-calibrated HDR tone map, which has not been standardized.
Pruitt said there are three potential ways to create an HDR bypass mode:
- Use a special set of SMPTE ST 2086 Static Metadata
- Employ a button combination via IR remote control
- Provide a menu option in the television’s OSD menu system
Once the HDR television is in the HDR bypass mode, the display can be calibrated using the normal grayscale, gamma, or white balance controls in the display’s OSD menu system. These controls typically manipulate the last 1D lookup table in the display’s video pipeline.
Measure the Color Volume
The second part of this proposed calibration process for consumer HDR display devices is to measure a “Color Volume Profile.” By measuring the actual color volume performance of the display device, those values can be fed back into the beginning of the video processing pipeline. The display can then use its actual measured performance to more accurately perform the color volume remapping of the HDR content.
The Color Volume Profile will consist of measured values of the following metrics. (These measurements should be made while the display device is still in the HDR bypass mode from the previous step)
- Peak Luminance of White (cd/m²)
- Black Level (cd/m²)
- Red Primary (CIE1931 xy)
- Green Primary (CIE1931 xy)
- Blue Primary (CIE1931 xy)
After the Color Volume Profile has been created, the data needs to be fed back into the display device. Based on feedback from multiple HDR consumer television manufacturers, here are three proposed methods for getting the measured data back into the display device.
- A Color Volume Profile text file, loaded into the television via a USB stick
- Color Volume Profile data entered into the television via OSD menu system
- Color Volume Profile data fed programmatically via a network connection or a serial API
Pruitt said this process is already being implemented for Dolby Vision calibration by LG Electronics with the profile loaded on a USB stick for now, with the other two options being considered for later.
We see this approach as much needed and should be highly valued by the HDR and TV community, so we hope that the TV and other device makers will allow such a by-pass mode. – CC