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Don't just adjust curves understand them!

Added on Friday 23rd of April 2010 05:10 am EST
 

Application:

Adobe Photoshop CS/CS2/CS3/CS4

Operating Systems:

Macintosh, Microsoft Windows

Curves are an integral tool for advanced image editing. But while many folks use the Curves command all the time, they still don’t quite understand why they do what they do.

To understand curves, we’ll:

Access the Curves dialog box.

Differentiate between input and output values.

Explain how channel selection affects your curve adjustments.

Expose hidden secrets and shortcuts so you can become a curves power user.

Photoshop is chock full of image editing tools, one of the most sophisticated being the Curves command. And while you can float by with a limited knowledge of the tool, you’ll make more precise image adjustments when the Curves dialog box isn’t Greek to you. So let’s dive in and uncover why curves do what they do.

Access the Curves dialog box

We’re big fans of non-destructive image editing. (See the article, “Don’t destroy those pixels! Try these five non-destructive Photoshop techniques” in this issue for more on salvaging your image pixels.) So while we’d normally apply a curve via an adjustment layer, the point of this article is to explain how the curve actually changes your image.

Because the CS4 Adjustments panel looks a lot different from the Photoshop Curves dialog box—even though it works in the same way—for our example we’ll use the Curves dialog box. To access this dialog box, open any RGB image and then select Image > Adjustments > Curves. You can also press [command]M ([Ctrl]M in Windows) to access the dialog box. The Curves dialog box appears, as shown in Figure A. While there are some interface differences between versions, our focus is on the actual curve box, which is the same in all versions.

A

Read the curve

The Curves dialog box graphs the values of your image’s pixels. Every image channel is essentially a grayscale image. When combined they provide your image its color. But every pixel in every channel has an intensity value between 0 and 255, 0 representing black and 255 representing white. The points along the curve represent the full tonal range of the selected color channels from dark to light, as shown in Figure B. In RGB mode, the graph represents brightness values from 0 to 255. In CMYK, it shows ink density values from 0% to 100%.

B

The horizontal axis graphs the original intensity values, also known as input levels. The input levels are your pixel’s current values upon opening the dialog box (before any edits). The vertical axis graphs the output levels, or the values your pixels will take on when you click OK. When you open the Curves dialog box, the input and output levels are identical, so the default graph is a diagonal line.

As you modify the line by adding and moving control points, the line’s shape changes to correspond to the new values. Then, when you click OK, Photoshop remaps any pixel values that change because of your curve. So if you click on the curve and see an input value of 150 and an output value of 156, that means that every pixel in the selected channel(s) that started with a value of 150, Photoshop will remap to a value of 156, hence lightening the pixels.

The gradient bars to the left and below the graph indicate the location of the highlights and shadows in the graph, as represented in Figure B. Our example shows the shadow tones in the lower-left corner and the highlights in the upper-right corner, which is the default for RGB images. If you prefer, you can flip the display by selecting the Pigment/Ink % option button in CS3/CS4, or by clicking on the double arrow icon in the lower gradient bar in CS/CS2.

 

Tip: Input and Output values are valid only for your current session in the Curves dialog box. Photoshop doesn’t apply any changes to your file until you click OK. But once you do so, the dialog box closes and ends your session. If you return to the Curves dialog box later, you’ll see the default diagonal line again, unless you apply the curve ...