Why Only 8-Bit TPG?

Why 8-bit in HDR? What’s the point of a 10-bit colour monitor then?

Some are asking why Dave Abrams sets the test pattern generator to 8 bits rather than 10 bits in Portrait Displays’ ASUS ProArt PA32UCDM calibration tutorial.

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.”

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.

Are 8 Bits Insufficient for HDR Test Patterns?