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Photoshop® CS4 Bible by Dan Moughamian, Simon Abrams, Stacy Cates

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9.3. The Curves Command

The Curves command is one of the most powerful features in Photoshop. It can be used to make an image look lighter, darker, or more detailed. Color balance can be precisely adjusted and undesirable color casts can be removed. Composite color channels and individual color channels can be targeted, as well as any segment of an image's tonal range.

A key strength of curves is that many points along the tonal range of an image can be adjusted (up to 14), whereas Levels is limited to three points of adjustment. Finding the part of the curve that corresponds to the part of an image you want to target is very convenient when using curves.

With the Curves dialog box open, you can move your pointer over the image in the image window and hold down the mouse button on the part of the image you want to target. You can also hold down the mouse button while you drag your pointer over an area of the image. Either of these actions cause a point to temporarily display in the corresponding areas along the curve while you drag. If you want to place a point on the curve by clicking directly in the part of the image you want to target, simply press Ctrl/ while you click in the image.

Both Curves and Levels can display 0–255 numeric values (black to white). This scale is also used to describe color levels in RGB color mode, a commonly used color mode for images displayed on-screen. ...

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