Oct 15, 2018

Tonemapping: Why do Recordings and TV images look different when using 4K60 Pro HDR passthrough?

High Dynamic Range (HDR), together with Wide Color Gamut (WCG) can display more vibrant, vivid and realistic colors, thanks to the much higher contrast ratios of televisions, and the expanded pallet of colors modern TVs are capable of reproducing.  

A problem arises when HDR content needs to be converted to SDR (Standard Dynamic Range). There are no industry standards for how to convert HDR content into SDRHDR and SDR operate within different boundaries when it comes to color, brightness, and color space.

Converting HDR to SDR is known as Tonemapping. Various algorithms have been created to tackle the problem of converting HDR to SDR and keeping the SDR image as close to the HDR image as possible.  

The question arises of how to convert an image that has a much wider range of color and brightness, to a much more constrained image.  

What would happen if you simply clipped any brightness and color values from the HDR image that SDR cannot display? The image would then end up very flat and look desaturated.

How about keeping detail in very bright areas (for instance, looking at the sun)? The overall image brightness can be decreased, but then detail in dark areas would be lost.

Like above, when trying to conserve detail in a dark image, the total brightness of the image would need to be raised, giving the look of grayish blacks, and potentially blowing out detail in brighter elements in the scene.  

4K Capture Utility employs a commonly used tonemapping algorithm which tries to keep the image as close as possible to the original.