[Tutorial] RGB curves in luminosity mode



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.


@McCap Another great tutorial! It appears to be applicable not just to RawTherapee but processing in general. Perhaps consider changing the category and tags to reflect that.


Ok, deleted the rawprocessing tag, thanks.

(Carmelo Dr Raw) #4

Nice tip, thanks a lot!

As @afre mentioned, this technicque can be applied in several image processors, like GIMP and PhotoFlow (DT as no RGB curves, AFAIK). Here is how the “RGB curves in luminosity mode” can be used in PhF:

As a side note, one can obtain very interesting results by applying a whole channel onto the original data in luminosity mode. For example, applying the RED channel in luminosity mode usually results in darker skyes, as if one had used a red filter when taking the picture…

(Gabriele ) #5

Hi ,
where I can find it in GIMP 2.8/2.9 ?
I tried but I am no able :roll_eyes:


(Carmelo Dr Raw) #6

@dafrasaga - Open the image, duplicate the background layer, set the blend mode to Lightness (LCH), and finally apply Colors -> Curves to this new layer. Here is how I edited the red channel:

Hope this helps!

GIMP 2.9 layer modes
(Morgan Hardwood) #7


(Olaf Berthelsen) #8

Hi’ McCap

Another interesting post…
How did you create the black and white image of the red and blue channel using RT?
Did you use the channel mixer and the black and white tool or the black and white tool alone?


Just the black and white tool. There you specifically select color channels.
And thanks. :slight_smile:

(nosle) #10

Just in case someone is not aware. You can use the little coloured rectangles to show black and white versions of select RGB channels.

Here the red channel is displayed in bw.

(Morgan Hardwood) #11

Not completely up to date yet, but:

(Olaf Berthelsen) #12

Hi’ McCap

So you enable the Black-and-White tool, select Method: Channel Mixer, Preset: Absolute RGB, Color Filter: None and adjust the sliders until the numbers under the Preset-line are R:100% G:0% and B:0% to select the red channel?

All the different possibilities in the Black-and-White tool could need some further explanations and examples (tutorials)…:o)!


I hope it came across, that the B&W channel mixer is absolutely irrellevant for this tutorial. You can see the R, G and B channels of your image by simply clicking “R”, “G” or “B” on your keyboard or use the tools shown by @nosle.
I only look at the channels to see where contrast is and what to do with it.

The B&W pictures I show, I made only to visualize what I was explaining. I created them using the steps you tell but selected the according color filters. I don’t even know what Preset I used as it wasn’t important.

(Olaf Berthelsen) #14

Hi’ McCap

Thank you. I got you all wrong. Because there is both a separate channel mixer tool and a channel mixer section of the black and white tool.

It’s really smart that you can select the channels by just typing R, G og B on the keyboard, I was not aware of that.