The Local Contrast tool adds local contrast to an image by applying an unsharp mask with a large blur radius. It it an easy way to give an image a little more 'punch'. In effect, the image is blended with a blurred version of itself, amplifying the local tones (highlights get lighter, shadows get darker), thus creating more contrast. The contrast can be tuned by several parameters explained below.
This tool was first implemented by G'MIC and then ported to RawTherapee. Its effect is applied in L*a*b* space and only on the lightness channel. Its position in the processing pipeline is after the Shadows / Highlight tool and before all other tools that operate in L*a*b* space.
Determines the extent of the local contrast (i.e. the radius of the blurring). Higher values give a smoother, but stronger contrast. Lower values give a more localized, less pronounced contrast.
Determines the overall strength of the effect. Higher values amplify the differences between the original image and the blurred image, thereby increasing contrast.
The "Darkness Level" parameter modifies only those areas of the image that were darkened with respect to the original. Higher values amplify the change (making darker parts even darker), lower values diminish the change. N.B. A value of 0 means the local contrast is only modified by making the image lighter.
The "Lightness Level" parameter works similarly, but only on areas that were lightened.
Note that setting both the "Darkness Level" and "Lightness Level" to 0 effectively disables the tool.