When HDR content is graded by a colourist, using a professional monitor that is capable of 100% of DCI-P3 and either 1,000 or 4,000 nits of peak brightness, this information is included as metadata which can be expressed in one of two ways – as either static or dynamic metadata. In the case of static metadata there are two key numbers, the black level (usually 0 or 0.05 nits) and the peak luminance (usually 1,000 or 4,000 nits), which applies to the entire film. However, when it comes to dynamic metadata, the black level and peak brightness can be changed on a scene by scene and even frame by frame basis.
That is why there has been a lot of publicity surrounding dynamic metadata because, the less capable an HDR display, the harder it is to reproduce a good HDR image using static metadata. The reason that tone mapping HDR content using static metadata is much harder, is that the TV must take the maximum colour volume that applies to that material and fit it into the TV’s own colour volume for the entire length of the content. Although some scenes will have very high peak luminance highlights, the majority won't and so the tone mapping ends up over-compressing this much larger number of less demanding scenes. The result is that often HDR content can appear rather dim overall because the tone mapping is transposing less bright scenes based on a peak brightness number that only applies to a small number of actual bright scenes. The tone mapping is essentially over-compensating for the peak luminance number included in the static metadata.
Whilst there are industry standards when it comes to the colour gamut (DCI-P3 and Rec. 2020) and PQ EOTF (SMPTE ST2084) that are used for HDR content, the same is not true for tone mapping. The reality is that each manufacturer is free to tone map the HDR content on their HDR TVs in any way that they feel is appropriate. This means that some manufacturers will tone map precisely against the PQ EOTF up to the limit of the display's capabilities. As a result the content creator's artistic intention has been retained but it also means that some scenes can appear overly dark and peak highlights can appear clipped (lose detail). The alternative approach is to deviate from the PQ EOTF and perhaps boost the mid-range of the curve in order to lighten the majority of scenes. Whilst this can result in an image that is less dark overall and avoid clipping, it's also unfaithful to the original intentions of the content creators.
These problems associated with static metadata are the main reason for the recent move towards dynamic metadata, either using Dolby Vision or HDR10+. The benefit of dynamic metadata is that it can be adjusted from scene to scene, thus reflecting the fact that certain scenes have a much lower dynamic range, one that the TV in question is perfectly capable of displaying. This means that more of the content is being displayed on a one-to-one basis, as in the image above, and only a few scenes require actual tone mapping. The result will be a superior HDR experience, especially with less capable TVs.
Ultimately tone mapping is a necessary and important solution to the problem of modern video content far exceeding the display capabilities of the current generation of TVs. It will be a long time before consumer TVs can fully display all HDR material exactly as it was graded by the content creators but in the meantime tone mapping provides the best way of ensuring that an HDR TV can display this content to the best of its abilities