Relight, a "digital fill-in flash" filter

This post an update about the work I am doing on shadows/highlights compression and tone mapping. It describes a new filter that works as a sort of "digital fill-in flash", that brightens the dark areas of an image while having a minor effect on the highlights.

Here is a preview of the filter in action (original image of the left, "relight" filter on the right with the corresponding tool dialog opened):

(image source: https://discuss.pixls.us/t/playraw-raw-challenge-backlit/1676)