I would like to introduce here a technique for darkening the sky in landscape shots, which is based on the use of the channel data itself.
The technique is described using my PhotoFlow editor, but the same steps can be certainly reproduced with GIMP or Krita…
The idea is based on the well-known fact that the red RGB channel is darker than the RGB average in blue areas.
Here is my test image, with the most basic processing:
This is how the Lab L channel (luminosity) looks like:
This is the RED channel:
and this is the B channel:
The next step is to compute the difference between the R and B channels, such that the negative values are represented with RGB values below 50%, and positive values are represented with RGB values above 50%. This can be easily achieved by blending the B channel with the R channel in grain extract mode:
Since the goal is just to darken the blue areas, everything which is above 50% is actually not wanted, because it would lighten the corresponding pixels when applied back to the original image. This can be easily cured by blending a uniform 50% grey layer into the grain extract layer, to obtain this:
The layer is now ready to be blended back with the original image in either overlay or soft light mode. This is the result of a soft light blend:
A contrast layer just before the soft light blend allows to fine-tune the strength of the effect.
The final touch is represented by a copy of the input image which is blended with the darkened one in color blend mode. It allows to restore the original color tonality of the sky, that might get shifted by the overlay/soft light bend.
Here is the final result (a bit exaggerated, to better see where we are going…), and the original RAW file:
_DSC8213.NEF (15.8 MB)
The attached .pfi file contains a deep blue sky layer that can be easily saved as a preset and re-used for any other image.
If anyone is interested in the technique, I’ll be glad to give more details on the different steps and/or the structure of the layers in the .pfi file.
Thanks for looking and for any feedback!