This tutorial is not so much a tutorial as an explanation on how the RGB curves could be used in luminosity mode.
Let’s start with an image:
Now lets have a look at the blue and red channels of this image:
It is clearly visible, that in the blue channel there is a strong contrast between the blue sky and the red bell tower, whereas in the red channel the tower is almost as bright as the sky and contrast is quite strong within the tower.
Following these observations we can first use the blue curve to reduce the overall contrast between the sky and the tower and the red channel to add contrast to the structures of the tower. In this example I did this using a blue curve, which pulled up the dark tones and pushed down the brighter ones (basically a flattened contrast curve, an inverse S curve), and finally using a red curve, which would lift the brighter parts and lower the dark tones (a contrast curve, an S curve).
Here you can see the result after applying only the blue curve:
And finally the result after applying the blue and the red curve:
We didn’t use the green curve in this example but the same logic applies when using the green curve.
Hope you enjoyed this little demonstration of how to use the RGB curves in luminosity modelogic works for all the channels.