Enhance local contrast.
This is achieved using either a local laplacian filter (the default mode) or an unnormalized bilateral filter. Both modes work exclusively on the L channel from Lab. The local laplacian filter has been designed to be very robust against unwanted halo effects and gradient reversals along edges.
- Choice of local laplacian filter or bilateral grid. The following sections define the controls available for each of these modes.
- Adjust the coarseness of the details to be adjusted.
- How strongly the algorithm distinguishes between brightness levels. Increasing this parameter results in a more contrasty look.
- Add or remove detail. Higher values will increase local contrast.
To understand the parameters of the local laplacian filter, one can think of it as applying a curve to the image, similar to the following graph:
This curve will be applied to the image in a way that works locally and avoids halo artifacts.
The local laplacian mode also supports shadow lifting and highlight compression, similar to the shadows and highlights module.
- Add or remove detail. Higher values will increase local contrast. This will insert an S shaped element in the center of the curve, to increase or decrease local contrast.
- This affects one end of the S shaped contrast curve, effectively increasing or compressing contrast in the highlights. A low value will pull the highlights down.
- Similar to the highlights parameter, this affects the other end of the S shaped contrast curve, and will increase or decrease contrast in the shadows. A higher value gives more contrast in the shadows. A lower value lifts the shadows and can effectively simulate a fill light. Note that this is done via local manipulation of the image. However, this means that a completely dark image cannot be brightened in this way – only dark objects in front of bright objects are affected.
- midtone range
- This controls the extent of the S shaped part of the contrast curve. A larger value will make the S wider, and thus classify more values to be midtone range and fewer values to belong to the highlights and shadows part. In more high dynamic range settings it can be useful to reduce this value to achieve stronger range compression by lowering the contrast in the highlights and the shadows. Note however that for really strong HDR scenarios this may work best in combination with a base curve which pre-compresses the range, perhaps with an approximately logarithmic base curve. The exposure fusion feature in the base curve module may lead to more pleasing results at times, but is also more prone to producing halo effects.
This setting can cause banding artifacts in the image if pushed to extreme values. This is due to the way darktable computes the fast approximation of the local laplacian filter.