Although the human eye is capable of detecting thousands of different hues and tones in a single glance, we often end up seeing exactly what we expect to see. For example, take three pieces of white paper collected from different sources (a white napkin, a sheet of copier paper, and a catalog page, for example). Individually, each sample looks as though it's neutral white. It's not until you place them side by side that you see a color cast--subtle tints of yellow, blue, or magenta.
Such color casts commonly create problems in scanned photographs. An image may appear neutral on screen (and perhaps even in a printed proof), but when you place it next to other images or on a bright white sheet of paper, you discover a slight tint.
Clearly, some images should have a color cast (for example, a photo taken at the beach just before sunset), but many others should not--and you must remove the tints manually. Using Adobe Photoshop, I'll show you how to strip away unwanted color casts simply by neutralizing the black, white, and gray pixels in your image.
DAVID BLATNER is a coauthor of Real World Photoshop 5 and the author of The QuarkXPress 4 Book (both Peachpit Press, 1998) and other books from Peachpit Press. He can be found at http://www.moo.com.
1. Find the Highlights At first glance, the bird's feathers in this photograph appear to be white, but closer inspection reveals a blue or cyan cast. By neutralizing the image's white and black pixels, you can correct the entire image and return the eagle to its true shade.
To locate the image's highlight and shadow pixels, use Photoshop's Threshold command, which converts the photograph to a high-contrast black-and-white image. From the Image menu, open the Adjust submenu and select Threshold.
Find the image's highlights by moving the Threshold slider (A) to the right end of the histogram. This reveals the location of the lightest areas: at the eagle's crest and below the eye (B).
Move the slider in the opposite direction to find the image's darkest areas--in this case, the feather tips in the lower left corner.
Click on Cancel to leave the dialog box and return to your color image.
2. Create Adjustment Layers Once you've identified the highlights and shadows, use an adjustment layer to correct the image. These layers let you apply a tonal or color adjustment (such as Curves or Hue/Saturation) as a layer rather than changing the actual image data.
Adjustment layers offer several advantages over applying changes directly to your document. First, you can always go back and change layers without adversely affecting the image itself. Adjustment layers also give you an easy way to judge results--just turn the layer on and off.
Create a new adjustment layer by command-clicking on the New Layer button at the bottom of the Layers palette (A).
In the New Adjustment Layer dialog box, specify what type of layer you want to use (B). In this case, I've chosen Levels. (If you prefer, you can also use Curves here; you would follow the same steps). Click on OK.
3. Adjust Your Highlights Next, you need to change your image's highlight and shadow pixels--which currently have a color cast--to the correct neutral colors. To do this, you'll use the black and white eyedropper tools in the Levels dialog box.
Tip: When you create the adjustment layer, Photoshop automatically opens a Levels dialog box. Later, if you want to change your settings, simply double-click on the adjustment layer in the Layers palette to reopen it.
In the Levels dialog box, double-click on the white eyedropper tool (A) to display the Color Picker dialog box.
The Color Picker dialog box lets you specify the correct neutral color for your highlights. To retain some ink in the lightest parts of your image, don't use solid white for the highlight. Here I've set brightness to 92 percent, which results in an 8-percent-neutral color (B).
By using the Brightness setting, you avoid the hassle of figuring out proper RGB and CMYK values for neutral colors. You might want to select a darker neutral, depending on the needs of your output device. Click on OK.
4. Choose Your Pixel You're now ready to snap the image's highlights to the new, correct eyedropper color. Take care, though; picking a pixel outside the highlight area can drastically degrade image quality. If that happens accidentally, just choose a different, more appropriate pixel.
If you don't see the Info palette, open it from the Windows menu. If the palette doesn't already display RGB values, open Palette Options from the window's pull-down menu (A) and change one of the Modes to RGB.
With the adjustment layer's white eyedropper tool selected, press command-plus (+) to zoom in on the area of the image containing the highlights you found in step 1.
Using the Info palette's numbers as a guide (B), search the area for the pixel that best represents a neutral white--it should have approximately equal R, G, and B values.
When you find the right pixel, click on it with the white eyedropper. Photoshop then forces that pixel to match the color you set in the Color Picker. Once you select a pixel, the Info palette displays its values before and after the effect (separated by a slash). If you don't like the change, use the first set of numbers to choose a different pixel.
5. Set Your Shadows After you've set the highlight, it's time to adjust the shadows. Repeat steps 3 and 4 using the black eyedropper tool. Sometimes it's difficult to find a true neutral black, so you may wish to target a slightly off-neutral color.
With the adjustment layer's Level dialog box still open, double-click on the black eyedropper tool to reopen the Color Picker dialog box.
Set the black eyedropper tool to a neutral, near-black color. (In this case, I didn't want a completely black shadow because the feathers are actually brownish, so I added a tiny amount of red (A).) Then click on OK.
With the black eyedropper tool selected, click on a pixel in the darkest area of your image. (Thanks to step 1, I know the darkest pixels appear in the lower left corner of the image.) The pixels will then snap to the neutral--or off-neutral--color you selected.
You can see from the finished image that the eagle has finally lost its blue cast.