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Friday, June 7, 2019

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:

The relight filter is based on the recently introduced new version of the shadows/highlights tool. This tool is very powerful but has quite a lot of parameters, and is therefore rather difficult to use.

One of the typical uses of the shadows/highlights tool is the recovery of back-lit images, where a rather dark subject is framed in front of a bright background. Such type of images often lack information in the mid-tones, that can be therefore compressed in order to provide room for the shadows. Usually such images are taken by exposing for the highlights, therefore it is often desirable to leave the highlights untouched or only slightly compressed.

The shadows/highlights tool is well adapted to process this kind of back-lit images. Nice results can be obtained by keeping the highlights compression to zero, and only adjusting the shadows compression, as shown here:

With this kind of settings, the effect of the filter in the shadows regions is very close to a positive exposure adjustment, and hence colors are very well preserved. Here is a comparison with a simple exposure compensation on the left of the image:

The output of the shadows/highlights tool often has a tendency to be on the soft side, and can profit from some mid-tones contrast increase. This can be achieved using the tone mapping tool, configured in a way that does not compress the highlights but introduces a S-shaped tone adjustment. Here is a before/after comparison:

Wrapping all the above together, I have created a new "relight" tool that combines the shadows compression and the mid-tones contrast adjustment in one single interface, with just three parameters:
  1. the strength of the fill-in effect (equivalent to the shadows compression strength)
  2. the range of tonal values that get brightened (equivalent to the shadows compression strength)
  3. the amount of mid-tones contrast

The new relight tool is also accessible from a tool shortcut that looks like a lightning, just below the curves icon.


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