Curves input and output

[#1]

RE: Photoshop curves. I don't understand why adjusting the combined RGB channel's output numbers aren't reflected in the individual RGB channels. For example, if I set the combined RGB channel's white point input from 255 to an output of 245 the separate red green and blue channel output numbers aren't affected (the output remains 255 instead of 245). Conversly the same is true if I set the whitepoint output for each of the three separate red green and blue channels to 245 from an input of 255 the combined RGB channel's output numbers remain at 255. Shouldn't the combined RGB channel reflect the changes made in each of the three separate RGB channels (ie. an output of 245?

[#2]

RE: Photoshop curves. I don't understand why adjusting the combined RGB channel's output numbers aren't reflected in the individual RGB channels. For example, if I set the combined RGB channel's white point input from 255 to an output of 245 the separate red green and blue channel output numbers aren't affected (the output remains 255 instead of 245). Conversly the same is true if I set the whitepoint output for each of the three separate red green and blue channels to 245 from an input of 255 the combined RGB channel's output numbers remain at 255. Shouldn't the combined RGB channel reflect the changes made in each of the three separate RGB channels (ie. an output of 245?

[#3]

That's the way that Photoshop Curves work. Photoshop treats the composite RGB channel separately from the individual R, G, and B channels. As you observed, if you change the RGB composite curve the individual R, G, and B channel curves do not change. However, if you choose an individual R, G, or B channel and manipulate the curve, you will see the change reflected for that individual channel. 

[#4]

As to WHY it works that way, well, it makes sense to me. If a change to the RGB composite channel was reflected in the individual channels, it would be confusing. I wouldn't be able to tell whether I changed an individual channel curve. If I did change an individual channel curve, Photoshop would have to add that change to the change made in the RGB channel -- that's a lot of math, and it would make it difficult for a user to know exactly how much an individual channel was changed.

[#5]

Indeed Barbara I understand what you're saying but it still seems odd to me because the composite channel seems to only affect lightness (also contrast) which presumably also would affect the individual channels. Perhaps changing the composite channel to read "lightness" instead of "RGB" would help clarify. It's not really a big deal because I use the info window to check my numbers anyway. I do wish however that at least we could have the option of displaying the input/output levels for white and black points from the composite channel in the individual RGB channels.

[#6]

I see what you mean. However, the way each individual user thinks about and uses the settings often does not correspond to the mindset of the programmers at Adobe. Personally, I like the way the channels panel displays things, but that's probably because after many years I am simply used to it. 

It sounds like you are a sophisticated Photoshop user so you have probably noticed that when the composite RGB channel is active in the Channels panel, and when sampling with the black point, white point and gray point eyedroppers, the changes are reflected in the individual channels the way you prefer.

[#7]

Thanks Barbara, Yes, I see that behavior while viewing in the Channels panel. I find that a little cumbersome so, being a creature of habit, I'll probably just continue using the info panel when things get a little confusing. Thanks for your help. Now I know that others have realized the same behavior when using curves.