Photoshop CS Bible [Electronic resources] نسخه متنی

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 discussed in the preceding pages — 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

Figure 3-1. 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. Figure 3-27 shows an image resized and resampled to 50 percent of its original dimensions. The resampled and original images have identical resolutions, but the resized image has twice the resolution of its companions.

Figure 3-27: An image (top) resized (middle) and resampled (bottom) down to 50 percent. The resized image sports a higher resolution; the resampled one contains fewer pixels.

Resizing an image

To resize an image, use one of the techniques discussed in the "Changing the printing resolution" section near the beginning of this chapter. 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. See Figure 3-2 to refresh your memory.

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 shown in Figure 3-28. As its name implies, the Resample Image check box is the key to resampling.

Figure 3-28: With the Resample Image check box turned on, you can modify the number of pixels in your image.

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. In Figure 3-28, for example, I've changed the first Width value to 669 pixels. The Pixel Dimensions value at the top of the dialog box reflects my change by reading 883.9K (was 1.00M), which shows that the file size has decreased.

To calculate the pixels in the resampled image, Photoshop must use its powers of interpolation, as explained in the "General preferences" section of Chapter 2. 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 generally results in the smoothest effects. When you're enlarging images, the new Bicubic Smoother setting is the way to go — it does its best to reinterpret your image with the smoothest pixel transitions possible. Bicubic Sharper, another new setting, is recommended for preserving the sharpness of an image that you're reducing, or downsampling. Bilinear is faster than any of the other options. 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.

Tip 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 Sharper, 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.

Photeshop Layer styles in Photoshop are dynamic, meaning you never lose the ability to edit their values. Accordingly, the look of a style you've applied to a layer can change when you resample an image. Turn on the Scale Styles check box to avoid unwanted changes in your image's layer styles when you resample. It's important to note, however, that this option is available only if Constrain Proportions is also selected.

Tip 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 want to return to the original size and resolution settings, press the Alt key (or Option key on the Mac) to change the Cancel button to Reset. Then click the Reset button to start from the beginning.

Cropping

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 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. For example, the left side of Figure 3-29 shows an upstanding young lad standing up on a bus seat. This photo clearly captures his rakish charm, but the photo itself appears to have been taken at quite a rake. And did you notice that ghostly face reflected in the bus window? Creepy. Luckily, I can crop around the little fellow's head to delete the extraneous image elements and hone in on his noble visage, as shown in the right of Figure 3-29.

Figure 3-29: Starting with a crooked photograph shot with an Olympus C-3030 (left), I used the crop tool to straighten the image and hone in on the core subject of the piece (right).

Photoshop offers several cutting-edge cropping options — har har — including the capability to crop nonrectangular selections, automatically trim away transparent areas from the borders of an image, correct perspective effects while cropping, and crop and straighten crooked scans. You can read about all these features in the upcoming sections.

Changing the canvas size

One way to crop an image is to choose Image Canvas Size, which displays the Canvas Size dialog box shown in Figure 3-30. The options in this dialog box enable you to scale the imaginary canvas on which the image rests separately from the image itself.

Figure 3-30: Choose Image Canvas Size to crop an image or to add empty space around the perimeter of an image.

Selecting the Relative check box allows you to change the height or width (or both the height and the width) of the canvas by a specific pixel amount. For instance, enter 2 in each option box to change a 100100-pixel image into a 102102-pixel image; enter –2 to crop it down to a 9898-pixel image. Not only can this save you from having to do a little math, but it can really come in handy when batch processing if you want to add the same number of pixels to a batch of different-sized files.

Click inside the Anchor grid to specify the placement of the image on the new canvas. For example, if you want to add space to the bottom of an image, enlarge the canvas size and then click the upper-middle square. If you want to crop away the upper-left corner of an image, create a smaller canvas size and then click the lower-right square. The Anchor grid offers little arrows to show how the canvas will shrink or grow.

To shrink the canvas so that it exactly fits the image, don't waste your time with the Canvas Size dialog box. Using a nifty command, Image Trim, you can automatically clip away empty canvas areas on the outskirts of your image. When you choose the command, the dialog box shown in Figure 3-31 appears. To snip away empty canvas, select the Transparent Pixels radio button. Then specify which edges of the canvas you want to slice off by using the four Trim Away check boxes. Alternatively, you can tell Photoshop to trim the image based on the pixel color in the top-left corner of the image or the bottom-right corner — just click the appropriate Based On radio button. For example, if you have a blue stripe running down the left edge of your image and you select the Top Left Pixel Color radio button, Photoshop clips away the stripe. No trimming occurs unless the entire edge of the image is bounded by the selected color.

Figure 3-31: To quickly snip away transparent areas from the edges of an image, use the Image Trim command.

Using the crop tool

In general, the Canvas Size command is most useful for enlarging the canvas or shaving a few pixels off the edge of an image. If you want to crop away a large portion of an image, using the crop tool is a better choice.

Press C or click the crop icon in the toolbox to activate the tool. To use the tool, you drag to create a rectangular marquee that surrounds the portion of the image you want to retain. But you can control what happens during and after you crop in two important ways:

To help you distinguish the borders of the crop marquee, Photoshop displays a colored, translucent overlay on the area outside the crop box — similar to the way it indicates masked versus unmasked areas when you work in the quick mask mode. Hate the overlay? Deselect the Shield check box in the Options bar. You also can click the neighboring color box to change the overlay color and set the overlay opacity through the Opacity pop-up menu. Note that these controls don't appear in the Options bar until after you create your initial crop marquee.

You have the option of permanently discarding the pixels you crop or simply hiding them from view. Before you drag with the crop tool, click the Delete or Hide radio button in the Options bar to signify your preference. If you choose Hide, you can bring the hidden regions back into view by enlarging the canvas or by using the Image Reveal All command.

If you don't get the crop marquee right the first time, you can move, scale, or rotate it at will. Here's what you do:

Drag inside the crop marquee to move it.

Drag one of the square handles to resize the marquee. You can Shift-drag a handle to scale the marquee proportionally (the same percentage vertically and horizontally). You can also drag the sides of the marquee to resize it.

Drag outside the crop marquee to rotate it, as explained in the next section. This may strike you as weird at first, but it works wonderfully.

Drag the origin point (labeled in Figure 3-32) to change the center of a rotation.

Figure 3-32: When rotating a crop boundary, align the marquee with an obvious axis in your image to determine the proper angle.

Select the Perspective check box in the Options bar, and you can drag corner handles to distort the image. What's the point? Well, the main reason to use this option is to correct convergence problems that occur when shooting images at an angle. For example, let's say you have a billboard in perspective, so that it declines toward the horizon. Using the Perspective check box, you can crop the billboard and remove its perspective so it appears flat.

Although Perspective is a solid feature, it can prove confusing. For starters, you can't preview the results of your changes before applying the crop. Also, in an attempt to conserve important detail, Photoshop tends to stretch the image after applying the perspective. This is actually a good thing — better to stretch an image than trash data — but it requires you to reverse the stretch in a second operation. If this bothers you, you may be better off tackling perspective problems using the Free Transform command, covered in Chapter 12, and do your cropping afterwards.

When the marquee surrounds the exact portion of the image you want to keep, apply the crop by pressing Enter (or Return on the Mac) or double-clicking inside the marquee. You also can click the OK button in the Options bar, which is the giant check mark at the right end of the bar.

If you change your mind about cropping, you can cancel the crop marquee by pressing Escape or clicking the Cancel button, the universal "no" symbol next to the check mark in the Options bar.

Rotating the crop marquee

As I said, you can rotate an image by dragging outside the crop marquee. Straighten-ing a crooked image, however, can be a little tricky. I wish I had a certified check for every time I thought I had the marquee rotated properly, only to find that the image was still crooked after I pressed Enter. If this happens to you, choose Edit Undo (Ctrl+Z or z -Z) and try again. Do not try using the crop tool a second time to rotate the already rotated image. If you do, Photoshop sets about interpolating between already interpolated pixels, resulting in more lost data. Every rotation gets further away from the original image.

Yet another solution is to use the measure tool. Just drag with the tool along the axis you want to make vertical. Choose Image

Cropping an image to match another

You can crop an image so it matches the size and resolution of another image in two ways:

Bring the image you want to crop forward and choose Image Canvas Size. Then, while inside the Canvas Size dialog box, select the name of the image you want to match from the Window menu.

Better yet, use the crop tool in its fixed-size mode. First, bring the image you want to match to the front. Then select the crop tool and click the Front Image button in the Options bar. The Width, Height, and Resolution options automatically update to show the size and resolution of the front image.

Now bring the image you want to crop to the front and drag with the crop tool as normal. Photoshop constrains the crop marquee to the proportions of the targeted image. After you press Enter or Return, Photoshop crops, resamples, and rotates the image as necessary.

Cropping a selection

Another way to crop an image is to create a selection and then choose Image

Cropping and straightening crooked scans

Naturally, you need to begin with a one-layer scanned image consisting of the photos you want to crop out, such as the one on the left side of Figure 3-33. Then choose File Automate Crop and Straighten Photos. As if by magic, Photoshop detects each of the photos in your master scanned image, straightens them according to their borders, copies them to the Clipboard, and pastes them into new image windows, as shown on the right side of Figure 3-33.

Figure 3-33: The new Crop and Straighten Photos command in Photoshop CS takes the pain out of separating and straightening multiple photos in a single scanned image.

If you want to crop out and straighten only certain images from your scan, draw a selection border around the area you want to isolate and carry on as you normally would. If the command repeatedly splits one scanned image into two, select that particular scanned image, hold down the Alt key (Option key on the Mac), and repeat the process. This tells Photoshop that the selected area represents only one scanned image. Keep in mind that the Crop and Straighten Photos command doesn't even need real photos to work its wonders. For example, suppose you have a sheet full of drawings that you want to divide into separate images. You can simply draw a clear border around each drawing and scan in the page. Because Photoshop is now intelligent enough to detect the borders inside of a multi-image scan, you're one leap closer to a collection of mini-masterpieces.