Curves on Color Channels...

When fudging about with the curves in the individual color channels, is that saturation, or what?

Say you take the green channel and pull it straight down from the middle. What exactly is that doing? Saturation, hue? Something else?

Wow, great question! Isn't it funny how you can use something every day, and instinctively know exactly how it works, yet not be able to explain it!

Taken in isolation, a green channel midtone adjustment is both a saturation and lightness adjustment. After all, the range of each channel is from 0 (black) to 255 (completely saturated colour). So to move the middle of the green slider up would be to move it towards spectral green (is that the right word?), therefore lightening and saturating; to move it down would be to move it towards black, ie darkening and desaturating.

However, you can't actually look at it that way, because a curves adjustment on any channel adjusts all colours which contain that channel's colour. In your example, to pull green down does all sorts of things: darkens greens, saturates magentas, pinkens neutrals and all other colours to varying degrees.

So no, a curves channel adjustment isn't doing a Hue or Saturation adjustment, which can be performed on individual colours, or on all colour equally, and doesn't affect neutrals. Curves affects all colours, but not equally, and definitely affects neutrals.

Post deleted!

I cannot even express what I want to say!

Obviously you change color.
If you add contrast to a channel, you'll add saturation for that color..
If you pull up the center of the green channel, you'll add green to the mid tones, without affecting the highlights and shadows much. You'll also alter green saturation: It'll get higher in the shadows, lower in the highlights.

At least, that's what I'd think

It also matters what blending mode the adjustment layer is set to obviously...

Saturation measures the amount of gray in a color. It is distinct from basic hue. It is distinct from lightness (how light or dark a color appears).

Contrast is different. You can desaturate an image and any difference in lightness remain. That's one of the techniques for converting a color photo to monochrome.

Contrast makes lighter pixels lighter and darker pixels darker. Middle tones remain unchanges.

What happens when you adjust a single channel with Channels? It depends on the relationship between the channels before and after. You will get a change in hue, if you move the channels further/closer. But, if you make them equal at some point, you will be introducing gray and reducing saturation. With regard to saturation and lightness, they can go all over the place.

You can see for yourself what happens. Add three or four sample points to a photo. Set each on the Info palette to HSB. Then make your Curves maneuvers and watch what happens.

Cheers,

Mitch

Damo77 wrote in post #6730559
Wow, great question! Isn't it funny how you can use something every day, and instinctively know exactly how it works, yet not be able to explain it!

Taken in isolation, a green channel midtone adjustment is both a saturation and lightness adjustment. After all, the range of each channel is from 0 (black) to 255 (completely saturated colour). So to move the middle of the green slider up would be to move it towards spectral green (is that the right word?), therefore lightening and saturating; to move it down would be to move it towards black, ie darkening and desaturating.

However, you can't actually look at it that way, because a curves adjustment on any channel adjusts all colours which contain that channel's colour. In your example, to pull green down does all sorts of things: darkens greens, saturates magentas, pinkens neutrals and all other colours to varying degrees.

So no, a curves channel adjustment isn't doing a Hue or Saturation adjustment, which can be performed on individual colours, or on all colour equally, and doesn't affect neutrals. Curves affects all colours, but not equally, and definitely affects neutrals.

Thanks amigo. This so far is closest in getting at what I wanted to know.

The reason I'm asking is I'm working on emulating some of my PS actions as presets in Lightroom, and unfortunately Lightroom does not have the simple curves for each color channel, but they have sliders for all kinds of things including saturation, contrast, luminance, calibration, etc.

I'm simply trying to figure out what sliders within LR I need to use and in which combination I need to use them in order to do the same thing as moving the curve around in PS.

In other words, it goes a little deeper than just playing with the curves and observing what happens...I already do that and have actions created based upon that experience.

First. LR has Curves.

Second, with respect, Damo77 is mistaken. Pulling a single channel up does not necessarily lighten. Nor does pulling it down necessarily darken. Nor does pulling a single channel up necessarily increase saturation. Nor does pulling it down necessarily decrease saturation.

As someone already noted, the curve is applied to every pixel. Unless every pixel starts out the same value, a Curves adjustment, even to a single channel, will affect different pixels differently. How it affects an individual pixel depends on where it starts and where it ends.

Saturation and brightness are separate. RGB does not isolate them, which is why it is difficult to predict what will happen when you adjust a single channel.

You can see for yourself faster than you can compose a reply. Drop three or four sample points on a photo. Pick a saturated color, pick a midtone gray, pick a highlight, pick a shadow. Set them to show HSB. Then watch the S (saturation) value and you make various Curves maneuvers. You will see that Damien is well-intentioned in his advice but he is mistaken.

There is no simple answer to your question, precisely because RGB does not separate hue, saturation, and brightness.

Cheers,

Mitch

Of course, but It's an RGB tone curve. It doesn't have the full functionality as the Photoshop curve tool. That's why I started the thread - to figure out what the workaround would be.

Not everything in PS has a workaround in LR.

I have tools that do make adjustments to individual channels with Curves. My TLR Color Compensating Filters action set is one example. There is no simple formula for converting a Curves adjustment in Photoshop to a simple combination of pulling of sliders in LR.

I understand what you want to do. I'd like to do it, too. What you propose to do, will not work.

Consider the saturation slider. That is going to increase/decrease the saturation of every pixel by the same amount (until an individual pixel gets pushed to 100% with an increase or 0% wth a decrease). Curves, however, is designed not to affect every pixel identically. It is designed to affect some pixels more than others. The same maneuver -- pulling one point along the curve for one channel -- will have many different effects.

Let's say you pull the Red curve so 128 becomes 135. Saturation for some pixels can increase. Some will remain unchanged. Some can even decrease. Increasing Red in isolation does not necessarily increase saturation. For pixels that were say 128,0,0 -- yes. They are now 135,0,0. Their saturation increases. But, what about pixels that were 128,135,135? The same adjustment to the Red channel made them neutral gray. Their saturation just fell to 0%. They are 135,135,135.

Cheers,

Mitch

Now if I could have only put into words what Mitch has said I would not have needed to delete my post.

It would have not been nearly as well said or as clear to understand. Nicley put Mitch!

gmitchel850 wrote in post #6734894
Not everything in PS has a workaround in LR.

I have tools that do make adjustments to individual channels with Curves. My TLR Color Compensating Filters action set is one example. There is no simple formula for converting a Curves adjustment in Photoshop to a simple combination of pulling of sliders in LR.

I understand what you want to do. I'd like to do it, too. What you propose to do, will not work.

Consider the saturation slider. That is going to increase/decrease the saturation of every pixel by the same amount (until an individual pixel gets pushed to 100% with an increase or 0% wth a decrease). Curves, however, is designed not to affect every pixel identically. It is designed to affect some pixels more than others. The same maneuver -- pulling one point along the curve for one channel -- will have many different effects.

Let's say you pull the Red curve so 128 becomes 135. Saturation for some pixels can increase. Some will remain unchanged. Some can even decrease. Increasing Red in isolation does not necessarily increase saturation. For pixels that were say 128,0,0 -- yes. They are now 135,0,0. Their saturation increases. But, what about pixels that were 128,135,135? The same adjustment to the Red channel made them neutral gray. Their saturation just fell to 0%. They are 135,135,135.

Cheers,

Mitch

That's pretty much what I was asking...

What curves do, and if it's possible to duplicate it in LR. I'm willing to accept "no, it's not possible" as an answer. Thanks Mitch.