I have just committed a new implementation of the local contrast adjustment tool.
The new tool is based on the same incremental guided filter that is also used for the shadows/highlights tool, and was inspired by the discussions and examples from this thread.
As the name suggests, the tool allows to control the amount of local contrast in an image. It extracts the high-frequency parts using an edge-preserving guided blur, and then adds back the same high frequencies, either enhanced or suppressed.
The effect of the filter is reduced when the blurred image approaches black or white. This filter is therefore mostly acting on the mid-tones.
Here is an example of the filter in action, with default parameters. It is particularly effective in enhancing the texture of the fur and wings of the butterfly, without generating disturbing halos.
Amount=100, radius=64, threshold=20:
The tool is available in the latest packages.