The HDR module optimises local dynamic range allocation for smaller details (e.g. on a more local level) than the Contrast module; the HDR module works primarily medium-to-small features in the image.
The HDR module complements the Sharpen module and is generally a more flexible and powerful alternative that generally achieves artifact-free results. Examples of use cases are bright galaxy cores where small detail is still recoverable in the highlights.
The HDR module does not exacerbate noise grain like simpler dynamic range algorithms, factoring in noise propagation into the size of the final detail enhancement. As such, it is meant after your non-linear dataset has been stretched, for example using the Development or AutoDev modules.
As with most modules in StarTools, the HDR module comes with a number of presets;
Going beyond the presets, more detailed adjustments can be made, starting with the 'Detail Size Range' parameter. This parameter is highly influential on the end result. It governs the range of detail sizes HDR should concentrate on, in order to bring out the most detail. Keeping this value small will see small detail accentuated. However, using larger values will see both small and large structural detail modified. Using larger values will progressively dig out larger scale structures and can be quite effective in highlighting these.
A selection of different algorithms to bring out detail exists. These are chosen through the 'Algorithm' parameter;
In order to throttle how much the shadows and highlights respond to the enhancements, a brightness mask is used, the power of which is controlled by the 'Dark/Bright Response' parameter.
Human nor machine will be able to discern detail objectively or with certainty.
If you just loaded an image, all pixels in the whole image will be set in the mask, so every pixel will be processed by default.
The 'Intricacy' parameter controls how much smaller scale detail should prevail over larger scale detail.
Nevertheless the result can look quite pleasing when simply browsing past the image in a Facebook feed.
There is no one perfect solution, but rather a range of approximations to the "perfect" solution.
You can convert everything you see to a format you find convenient. Give it a try!