How does the RGB tone curve affect pixel values in Photoshop or Lightroom?
Asked 4/16/2020
2 views
2 answers
0
When using the Tone Curve in Photoshop or Lightroom with the composite RGB curve selected, does it remap each channel value independently? For example, if a pixel is RGB(0,70,0) and I move the curve so input 70 maps to output 137, will any channel value of 70 become 137 in every pixel? And if I adjust only the red, green, or blue channel curve, is the change applied only to that channel? More generally, in a color image, are the 'tones' just the 0–255 values of the RGB channels?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
1
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).
The answer to this question goes some way towards explaining the 0-255 relative brightness issue.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
6y ago
0
Generated from our catalog & community — verify before relying on it.
Your understanding is only partly correct.
With an individual channel curve selected, the adjustment applies only to that channel. So changing the green curve remaps green channel values while leaving red and blue unchanged.
With the composite RGB curve, the curve represents overall light/brightness, not simply “replace every 70 with 137 in all channels.” In a simple case like a pixel value of RGB(0,70,0), that 70 may end up at 137, but the RGB curve is better thought of as changing tonal/luminance response across the image rather than doing a flat numeric substitution everywhere.
In color images, each channel does contain numeric values from 0–255 (in 8-bit images), and those values represent the intensity of red, green, and blue for each pixel. But “tone” in editing usually refers to perceived brightness/lightness, which is related to the channel values rather than being a separate set of 256 color tones.
So:
- composite RGB curve: adjusts overall tonal/light response
- red/green/blue curve: adjusts only that channel
- 0–255 values are channel intensities, not the whole story of perceived tone
Recommended products
UniqueBot
AI6y ago
Your Answer
Related Questions
How do the RGB and individual channel curves interact in a single Photoshop Curves adjustment?
Why does Photoshop’s RGB picker still use 0–255 in a 16-bit image?
What does Photoshop’s RGB histogram show, and how is it different from luminosity?
How can I correct a blue haze in darktable, and can I adjust RGB channels separately?
Why do Photoshop Curves and Lightroom Tone Curve affect RGB values differently?