Resampling And Cropping

After you bring up an image whether you created it from scratch or opened an existing image stored in one of the five billion formats its size and resolution are established. Neither size nor resolution is set in stone, however. Photoshop provides two methods for changing the number of pixels in an image: resampling and cropping.

Resizing Versus Resampling

Typically, when folks talk about resizing an image, they mean enlarging or reducing it without changing the number of pixels in the image. By contrast, resampling an image means scaling it so the image contains a larger or smaller number of pixels.

With resizing, an inverse relationship exists between size and resolution size increases when resolution decreases, and vice versa. But resampling affects either size or resolution independently.

Resizing an Image

To resize an image, use one of the techniques I've discussed. To recap briefly, the best method is to choose Image>Image Size, turn off the Resample Image check box, and enter a value into the Resolution option box.

Resampling an Image

You also use Image>Image Size to resample an image. The difference is that you leave the Resample Image check box turned on.

As its name implies, the Resample Image check box is the key to resampling. When Resample Image is selected, the Resolution value is independent of both sets of Width and Height values. (The only difference between the two sets of options is that the top options work in pixels and the bottom options work in relative units of measure such as percent and inches.)

You can increase the number of pixels in an image by increasing any of the five values in the dialog box; you can decrease the number of pixels by decreasing any value. Photoshop stretches or shrinks the image according to the new size specifications.

At all times, you can see the new number of pixels Photoshop will assign to the image, as well as the increased or decreased file size. To calculate the pixels in the resampled image, Photoshop must use its powers of interpolation. The interpolation setting defaults to the one chosen in the Preferences dialog box.

But you can also change the setting right inside the Image Size dialog box. Simply select the desired method from the Resample Image pop-up menu. Bicubic results in the smoothest effects. Bilinear is faster. And Nearest Neighbor turns off interpolation so Photoshop merely throws away the pixels it doesn’t need or duplicates pixels to resample up.

Here are a few more random items you should know about resampling with the Image Size dialog box:

  • This may sound odd, but you generally want to avoid adding pixels. When you resample up, you’re asking Photoshop to make up details from thin air, and the program isn’t that smart. Simply put, an enlarged image almost never looks better than the original; it merely takes up more disk space and prints slower.

  • Resampling down, on the other hand, is a useful technique. It enables you to smooth away photo grain, halftone patterns, and other scanning artifacts.

One of the most tried-and-true rules is to scan at the maximum resolution permitted by your scanner and then resample the scan down to, say, 72 or 46 percent (with the interpolation set to Bicubic, naturally).

By selecting a round value other than 50 percent, you force Photoshop to jumble the pixels into a regular, homogenous soup. You’re left with fewer pixels, but these remaining pixels are better. And you have the added benefit that the image takes up less space on disk.

  • To make an image tall and thin or short and fat, you must first turn off the Constrain Proportions check box. This enables you to edit the two Width values entirely independently of the two Height values.

  • You can resample an image to match precisely the size and resolution of any other open image. While the Image Size dialog box is open, choose the name of the image you want to match from the Window menu.

  • If you need help resampling an image to the proper size for a print job, choose Help>Resize Image to bring up the Resize Image Wizard. The dialog box walks you through the process of resampling step by step.

It’s really for rank beginners, but you might find it helpful when you want to turn the old brain off and set Photoshop to autopilot. (Note that Adobe uses the word “resize” simply because it’s friendlier than “resample.” Whatever it’s called, this command does indeed resample.)

If you ever get confused inside the Image Size dialog box and you want to return to the original size and resolution settings, press the Alt key to change the Cancel button to Reset. Then click the Reset button to start from the beginning.

Photoshop remembers the setting of the Resample Image check box and uses this same setting the next time you open the Image Size dialog box. This can trip you up if you record an action for the Actions palette. Suppose that you create an action to resize images, turning Resample Image off. If you later resample an image turning on Resample Image the check box stays selected when you close the dialog box. The next time you run the action, you end up resampling instead of resizing. Always check the status of the check box before you apply the Image Size command or run any actions containing the command.

Cropping Image

Another way to change the number of pixels in an image is to crop it, which means to clip away pixels around the edges of an image without harming the remaining pixels. (The one exception occurs when you rotate a cropped image or use the new perspective crop feature, in which case Photoshop has to interpolate pixels to account for the rotation.)

Cropping enables you to focus on an element in your image. Version 6 offers several new, cutting-edge cropping options—har har—including the capability to crop nonrectangular selections, automatically trim away transparent areas from the borders of an image, and correct perspective effects while cropping.