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Tip #1049: HDR HLG vs PQ on SDR Monitors

Larry Jordan – LarryJordan.com

HLG looks better on SDR than PQ. But PQ looks better on HDR monitors.

Tip #1043 compared the grayscale differences between HDR HLG and SDR. This tip illustrates the differences between watching HLG and PQ on an SDR monitor.

NOTE: The screen shot and the information in this article are taken from a Venera Technologies article.

To display the digital images on the screen, display devices need to convert the pixel values to corresponding light values. This process is usually non-linear and is called EOTF (Electro-Optical Transfer Function).

While SDR uses Rec. 709, HDR defines two additional transfer functions to handle this issue – Perceptual Quantizer (PQ) and Hybrid Log-Gamma (HLG). HDR PQ is an absolute, display-referred signal while HDR HLG is a relative, scene-referred signal. This means that HLG-enabled display devices automatically adapts the light levels based on the content and their own display capabilities while PQ enabled display devices need to implement tone mapping to adapt the light levels.

Under ideal conditions, dynamic PQ based transformation will achieve the best quality results at the cost of compatibility with existing display systems.

As you can see from the screen shot, HLG images look better on SDR monitors than PQ images. However, while PQ based transforms promise to display the best quality results on HDR enabled monitors, in comparison to HLG, PQ requires proper tone mapping by the display device.

EXTRA CREDIT

As you may be able to see in the screenshot, PQ offers more detail in the highlights than HLG.