As I understand it, when I make a change in the tone curve in Adobe Photoshop (or Lightroom) and have selected the RGB Channel option, the following happens: For every pixel in the image, if any of the three color channels has the input value, it will be remapped to the output value. For example: In following image, every pixel has RGB value 0,70,0

enter image description here

If we do the following change in the curve, we haven't change the point that corresponds to 0, but we have changed the point that corresponds to 70 and remapped it to 137. So in every pixel in the image, in red, green and blue channels, if the value is 70, it wll be remapped to 137.

enter image description here

When we have selected only one of the three channels, the change will be applied only to this channel.

Is my understanding of the tone curve correct or something else happens? Are the tones in a colored image actually the 256 values of RGB color system?

  • @SaaruLindestøkke i have read this question, but i don't understand exactly what happens in colored images.
    – Timos Koni
    Apr 16 '20 at 12:58

Notice that the tone curve is set to represent light... it doesn't matter if the image is grey scale or color. In color the 0-255 value of each channel represents the brightness of the tone (i.e. Red, Green, or Blue).

When you make the curve adjustment of 70 to 137 it is not simply changing the number... it is changing the relative value of 70 to a new relative value in each channel based on its' relative perceptual importance to brightness/luminance/light.

Here I placed a magenta rectangle on your image and applied the same curve adjustment; you can see that it is not a simple 70-137 shift (+67)... it only worked that way in your example because there was only one value and it was 70 (the RGB numbers are before/after the curve is applied).

enter image description here

The answer to this question goes some way towards explaining the 0-255 relative brightness issue.

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