How do the RGB and individual channel curves interact in a single Photoshop Curves adjustment?
Asked 1/6/2011
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2 answers
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In Photoshop Curves, I understand a single curve on its own. For example, if an individual Red channel curve maps input 100 to output 50, then pixels with a red value of 100 become 50.
What confuses me is how the composite RGB curve interacts with an individual channel curve when both are on the same Curves adjustment layer.
For example, if I set the composite RGB curve so 100 maps to 50, and then set the Red channel so 100 maps to 125, I don’t get 125 for red values that started at 100. Similarly, other combinations give unexpected results.
What is the mathematical relationship between the composite RGB curve and the Red/Green/Blue channel curves in the same Curves layer?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
1
Hokay, Turns out I was over-thinking the problem (no way!). The answer is values from Curve1 go through Curve2. So on the RGB100 -> RGB50 a R100 value becomes R50, then in the R100 -> R125 curve the R50 value becomes R73.
Voila.
Originally by user1819. Source · Licensed CC BY-SA 4.0
user1819
15y ago
0
Generated from our catalog & community — verify before relying on it.
In a single Photoshop Curves adjustment, the composite RGB curve and the individual channel curves act sequentially, not independently.
The practical way to think about it is:
- The pixel value is first remapped by the composite RGB curve.
- That result is then passed through the selected channel curve (Red, Green, or Blue).
So if RGB 100 → 50, a red value of 100 first becomes 50. Then the Red curve is applied to that new value. If your Red curve maps 50 to about 73, the final result is R73.
That’s why you don’t get the result by simply adding or averaging the two curve settings at input 100. The second curve is working on the output of the first one.
In short: the curves are effectively chained together, with the output from one becoming the input to the next.
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AI15y ago
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