Shadows/Highlights
Introduction
Use this tool to brighten the shadows or darken the highlights of an image.
This tool has received a new engine in RawTherapee 5.5, it now uses an edge-aware fast guided filter to prevent halos and operates by default in RGB space to preserve saturation.
Usage
Use it in moderation to preserve a natural look. If the photographed scene has a high dynamic range (very deep shadows and very bright highlights) then use the Dynamic Range Compression tool to compress the dynamic range to a more manageable level, and then optionally use this Shadows/Highlights tool on top of that.
Interface
Color Space
Adjusting shadows and highlights in the RGB space preserves image saturation which usually looks more natural than working in L*a*b* space which tends to desaturate affected areas. However, in some cases working in RGB space may oversaturate the shadows, in which case you should switch to L*a*b* space.
Shadows
Allows you to brighten the darkest parts of the image
Highlights
Allows you to darken the brightest parts of the image.
Tonal Width
Shadows/Highlights Tonal Width allows you to control how bright an area must be for it to be affected by the highlights slider, and how dark an area must be for it to be affected by the shadows slider. Though the underlying math is a bit more complicated, think of a histogram - the highlights tonal width specifies the range of tones from the white end of the histogram which the highlights slider will affect, and the shadows tonal width specifies the range of tones from the black end of the histogram which the shadows slider will affect. The higher the tonal width value, the more tones are affected.
Radius
The value of the Radius slider influences the effective area of the Shadows and Highlights sliders.