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Color Correcting - the Curves Dialog

Posted on Monday, July 21, 2008:: 1853 Views

The Good news...is that Curves operates much like Levels, it's just housed in a different dialog. Unlike Levels, however, where you can only adjust the black, gray and white points, Curves lets you adjust many more points. You also get a few more options to look at in the Curves dialog, like the color channel curves overlay on the RGB channel, a 45° baseline to use as a reference point, the always important histogram overlay and a handy intersection line that shows you whereabouts on the output graph your new values will be.

The main difference (besides the appearance) between Levels and Curves is the layout of the input and output graphs. In the Curves dialog, the output graph is actually the vertical axis, and as such, reflects the edited pixels whereas the input graph (the horizontal axis) reflects the unedited pixels. The default curve, the straight line that sits at a 45° angle, represents the balance between the input and output graphs. At any point along this default line, the input equals the output. By adding points to the curve and moving them up or down, you change the output values, thus changing the brightness or color.

*Again, for the sake of simplicity, we're going to be looking exclusively at Curves in RGB.

curves diagram
1 Preset Curves Adjustments
2 Select Channel
3 Save / Load Curve Adjustment
4 Edit Points / Draw (Freehand) Curve
5 Auto Curves Adjustment
6 Output Graph (Edited Pixels)
7 Input Graph (Unedited Pixels)
8 Black, Gray, White Point Eyedroppers


curves options
1 Light input / output or Ink input / output
2 Select a smaller or larger grid
3 From left to right: Displays color channel curves in RGB channel, dislpays 45° baseline, histogram overlay, displays line intersecting at point between input and output graphs

Curves Exercise

The easiest way to get your feet wet with curves and see how it works is to dive right in. So let's begin by making a rectangular document, any size, but preferably longer width-wise. I've made a new document that's 300px by 50px at 72 dpi. Now select the gradient tool, making sure that your foreground and background colors are default (if they aren't press D now), and create a black to white, linear gradient from the left to the right. It should look something like this:

black to white gradient

Now add a Curves adjustment layer and open up your histogram palette. Back in Curves: Under Curve Display Options (click the arrow to the left of those words to bring them up) make sure "Show Amount of Light" is selected, and switch to the tighter grid (for the sake keeping yours and mine similar). For now, you can choose to show any of the options at the bottom of the dialog that you want. I usually have all of them checked.

The bottom left corner of the default curve (even though it's a straight line at this point, we're still going to refer to it as a curve) represents our black point. As you select the bottom left point, you'll see that the input and output both say 0. Right now solid black is where it should be. If we move that point up until the output says 50 and the input says 0, we just mapped solid black to 50, which isn't really solid black. As you can see from the gradient, the far left side is no longer solid black, and the histogram shows a large gap on the left.

black at 50 50 black gradient 50 black histogram

What we've just done is the equivalent of moving the black output slider in Levels to 50. We're now saying that any blacks in this image will be no darker than 50. Now grab the white point (the upper right point) and slide it down to 205. The input should still read 255, but the output should now say 205. Notice the white in our gradient (the right side) has gone gray, and the histogram now has a gap on both sides. This adjustment is like moving the white output slider in levels to the left until it reads 205. We're now saying that any whites in this image will be no brighter than 205.

205 white 205 white gradient 205 white histogram

Now press and hold ALT or Option (Mac) so that the Cancel button says Reset, and click reset to reset our curve back to default. Now we'll grab the bottom left point again (the black point), and this time, drag it to the right along the bottom so that the output is 0 and the input is 50. The black in our gradient is now pushed further to the right, taking over some of the darker grays. The histogram now has fewer dark grays and has lost some of it's "skateboard ramp" like appearance. What we've just done is set a new black point, just like sliding the black point input slider in Levels to the right. We're now saying that solid black starts at 50, which is going to compress the gradient image to the right. Notice the output is still 0. That's telling us that our black is still solid black, unlike the value of 50 we had it set to before where it became a dark grey.

new black point new black point gradient new black point histo
As you've probably guessed, it's now time to grab the white point (upper right) and slide it to the left along the top so that the output reads 255 and the input reads 205. Our gradient now shows very little middle gray because the black and white ends have been pushed to the right and left respectively. Our white point adjustment is the same as moving the white point input slider in Levels to the left until it reads 205. Solid white is now 205 which is going to compress our gradient to the left, eliminating some of the intermediate gray values. Since our output is still 255, we know that we still have solid white, it just starts at 205 now. We've eliminated the subtle ramps at each end of the histogram, that represented the dark grays and lights grays just before black and white, by having the black and white points start earlier along the graph.

205 white input new gradient new histogram


 Once again, press and hold ALT or Option (Mac) to change the Cancel button to Reset, and reset the curve. Set a point dead center by clicking once on the curve. The input and output should both be 128. If your values are slightly off, you can manually type 128 into both the output and input fields. Now drag the new mid point, which is going to change the mid tones, up until the input reads 128 and the output reads 180. Again, if you're having difficulty positioning the point on the grid, you can simply enter the desired values in the output and input fields once you're close. Our mid tone value is now brighter at 180 (the output value). As you can see in the gradient, the middle grays have moved to the left, brightening the gradient, but leaving black and white alone.

180 output middle gray gradient mid tone curve

If we were to move the mid point down until we had an input of 128 and an output of 76, the opposite would occur. Our middle grays would move to the right, darkening our gradient. Pretty straightforward stuff so far. In fact, if you were to use Curves by doing just we've done so far, you'd be making the same exact corrections you could make in Levels. Even the black, gray and white eyedroppers work the same. But remember when I said that curves lets you adjust more than just the black, gray and white points? What if we wanted to adjust only the region highlighted in red below?

spot on gradient

This is where Curves can do more than Levels. With the Curves dialog open, the second you move the cursor away from the dialog, it becomes an eyedropper. Click on the spot that you want to isolate, and a point appears on the baseline (the default, unedited curve) along with the input and output values. As you let go the point vanishes, but simply click in the approximate area, and then enter the input and output values that were displayed as you positioned the eyedropper over the specific spot. For my example, I'm going to be affecting the spot with an input and output of 57.

57 57

For the sake of this example, we're going to lighten just this specific region (think about it, this could be a small region of a portrait that you wanted to lighten ever so slightly). If we simply drag the point up so that the output is a higher, brighter value, the rest of the curve moves with it, brightening the image more than we need.

87 57 curve

To tame the unruly curve and isolate the region we want to affect, we're going to add several points to the curve to straighten out the areas we want left alone. First enter 100 for the output, we're going to really brighten up this one spot so we can better see how our adjustment affects only a specific region. Next we'll add eight points to the curve to flatten out the rest. You can try and position them manually or enter the output and input values I used.

8 point curve
 1. - Output: 17 Input: 17
 2. - Output: 29 Input: 29
 3. - Output: 43 Input: 43
 4. - Output: 77 Input: 72
 5. - Output: 89 Input: 89
 6. - Output: 107 Input: 107
 7. - Output: 128 Input: 128
 8. - Output: 179 Input: 179

And here is our gradient now:

spot on gradient

From this simple exercise you can begin to see how you might use Curves to target a specific color in a specific region. For example: Say you want to "warm up" some grass in the background of an outdoor shot. Using the same technique, you would select the Red Channel from the channel select drop down menu, find your target on the curve while hovering over your image with the eyedropper, place your point and drag up slightly, and then keep the rest of the curve in line by adding as many points as necessary. Below is an example of a real life scenario.





For this image, I wanted to eliminate some of the red cast in the greenery in the background. With the eyedropper I targeted the area marked in red, then made the curve you see below. The end result is very subtle, but noticeably cooler. When working in Curves, you'll find that this is often the case. Curves may very well be the "perfectionist's" color correction dialog simple because you can isolate so many different points and adjust until your hearts content, or your fingers cramp up.

red channel curve

Using Curves to Correct Contrast

Curves can also be used to add some contrast to your image. In fact, you can get down right ridiculous with your contrast fine tuning. Those of you who are familiar with Adobe Camera Raw's Tone Curve tab probably use the medium or strong contrast curve in the Point Curve sub tab. You can create the same medium and strong contrast curves in Photoshop's Curves dialog.

Basically, you create sort of a thin, S shape in the RGB channel. Below I created a very simple, medium / strong contrast curve.

med strong contrast

By adding a point or two (or three or however many you desire) to the darker tones and pulling down slightly, adding an anchoring point to the mids (dead center), and a point or two to the lighter values and pushing up, you can achieve subtle to intense shifts in contrast. The more extreme the "S" is, the stronger the contrast. Check out the Strong Contrast Preset inside curves. You can also use it as a starting point and fine tune to your liking. Once you have your curve created that you know you'll use again and again, you can save it as a preset (See numbers 1 and 3 in the Curves diagram up top).

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