Are 8 Bits Insufficient for HDR Test Patterns?

When Portrait Displays acquired Patterns, they announced:

“Patterns™ by Portrait Displays is the first and only macOS test pattern generator to support High Dynamic Range (HDR) on the market. The application uses the latest Metal APIs to support both SDR and HDR color spaces, including (but not limited to) sRGB, BT.709, Adobe RGB (1998), DCI-P3, and HDR in both P3 and BT.2020. It provides users with the tools to validate accuracy throughout the entire system, from the built-in MacBook Pro or iMac Displays to an external device.”

To which Ted Aspiotis claimed that Patterns is somehow not accurate enough, writing, “Patterns’ (from AviCAL) has been working with ColourSpace since August 2020. ColourSpace supports ‘real’ 10/12/16-bit TPG via patchlist or manual RGB sliders (to generate specific RGB triplet). CalMAN supports 8-bit patchlists, and you have 10/12/16 at the output, so it’s not ‘real’ 10/12/16-bit TPG.”

Yet, according to the IDMS (International Committee for Display Metrology), 8 bits is wholly sufficient. In the organization’s Information Display Measurements Standard (IDMS), they write, “HDR10’ signals are encoded with 10 bits per component precision, in order to avoid potential contouring or ‘banding’ artifacts that otherwise can appear in gradients with a lower bit depth encoding. A quantization of 10 bits is rarely needed for metrology test patterns. Therefore, all methods have been developed such that 8-bit precision is sufficient unless otherwise specified.”

Further on, they explain, “It appears obvious to only use of 10 and 12 bits with HDR-related metrology. However, this can cause problems when using video test and pattern sources. While many of those sources will output valid 10- and 12-bit signals, only 8 bits can be addressed by a user e.g., via calibration software or an API for communication with such a device. This might also include the frame buffer used to render test patterns, which is rendered in 8 bits and then converted to 10 or 12 bits before output e.g., to an HDMI output. At first glance, one can argue that even though there is a quantifiable benefit to using 10 or 12 bits for real imagery, a measurement granularity of more than 8 bits is indeed unnecessary for real-world metrological applications that use geometric test pattern.”

Meanwhile, Steve Shaw, CEO of Light Illusion, argues, “I am sorry, but the information provided by the IDMS is incorrect. They have assumed there will be some form of 8-bit conversion in the signal chain, and have adapted their argument to fit that, presenting incorrect information in the process. With modern TPGs, including PGen, SEVEN-G, and even modern graphics cards, there are no such restrictions when partnered with calibration software that can work in variable bit depths – such as ColourSpace. As Ted has pointed out a number of times, Calman is limited to 8-bits. And using 8-bits in a signal path that should be 10-bits, or higher, will introduce calibration issues, as any given 8-bit value will equal any number of 10 or 12-bit values.”

Florian Friedrich is the chair of the HDR Subcommittee of the ICDM and Timo Kunkel, senior color and imaging researcher in the CTO office of Dolby Labs, Inc., did the majority of the work on the HDR chapter of the IDMS.

Update 17.02.2023: When we posed Ted’s response to David Abrams, the ‘originator’ of Patterns™ for Mac, he replied, “The engine in the Patterns application operates at 16-bit precision, or half float. That said, computer systems generally do not output 16-bit precision to the display. As the process goes through the GPU, and out to the display, the signal will be converted based on the system you’re using. For example, it may be 12-bit YCbCr or 10-bit RGB – this is determined by the OS and the external display together. If you would like to enter up to 16-bit values, you may do so in the user interface of Patterns.”

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