1 A painter paints a picture. He paints it at whatever size he thinks is appropriate. (Or, perhaps, on the only piece of canvas he can afford on that particular day.)
2 A patron likes the artwork, but the painting is too large for the frame that works best with the dining room table. Yeah, patrons can be like that, can’t they?
3 The patron asks the artist to make the painting fit the frame.
4 The artist decides between cropping and resampling. He can grab a pair of scissors and cut off some of the painting (cropping) or painstakingly re-create the painting from scratch at a smaller size. Thankfully, Photoshop does the “repainting” for you, using Image Size with its resampling algorithms.
5 The artist charges the patron for the extra work. (Don’t forget this final, crucial step!)
Cropping cuts away part of the image to meet a target size. Resampling retains the entire image, but shrinks or enlarges it to meet the target size.
Picking an image resolution
After you have the concept of resampling under your belt, how do you know what size you should be resampling to? How many pixels do you need? Here are your general guidelines:
Photos for your inkjet printer: Inkjet printers are stochastic printing devices: That is, they use a series of droplets to replicate each pixel in your image, as shown in Figure 2-12. In theory, the optimal image resolution is one-third of the printer’s rated resolution. However, most printers don’t need an image resolution higher than 300 ppi. (For fine art prints from my high-end Epson printers, I use an image resolution of 360 ppi.)If you’re printing something that will be viewed only at a distance, such as a banner to be hung above the crowd or a poster that hangs on a wall, you can print at a substantially lower resolution to save ink and print faster. Banners, for example, can often be printed with a resolution of 100 ppi.
Web images: Ignore resolution (including “72 ppi”). Consider only the image’s pixel dimensions. Determine what area of the web page the image will occupy and then resize to exactly those pixel dimensions. Remember, too, that some social media have specific guidelines for images uploaded to their sites. Check the site’s info before changing the image dimensions.
Page layout programs and commercial printing: If your image is to be placed into a page layout program’s document and sent to a commercial printing facility, you need to know the line screen frequency (the resolution, so to speak) of the printing press on which the job will be run. Ask the print shop or the person handling the page layout. Your image resolution should be either exactly 1.5 times or exactly twice the line screen frequency. (You shouldn’t notice any difference in the final printed product with either resolution.)
Presentation programs and word processing documents: Generally speaking, 72 ppi is appropriate for images that you place into a presentation or Word document. You should resize to the exact dimensions of the area on the page or slide that the image fills.
FIGURE 2-12: The X to the left shows inkjet printer droplets and to the right, pixels.
CONTENT-AWARE SCALING
The Edit ⇒ Content-Aware Scale command is designed to be used when an image needs to be resampled to a new aspect ratio but can’t be cropped. It tries (very hard) to keep the subject of the photo undistorted while stretching or shrinking the background. Here’s how to use it:
1 Open an image or make a selection. Make a selection if you need to scale only part of an image. If you need to resize the entire image, don’t make any selection.
2 Convert the Background layer. You can’t use Content-Aware Scale on a flattened image (an image that doesn’t support transparency). If your image has a layer named Background, click the lock icon to the right of the layer name in the Layers palette.
3 Choose Image ⇒ Canvas Size. If increasing the pixel dimensions, resize the canvas as required. If you’re reducing the size of the image, skip this step.
4 Choose Edit ⇒ Content-Aware Scale. Hold down the Shift key and drag the anchor points in the center of the four sides of the bounding box that appears to resize to fill the new canvas, and then press Return/Enter. (If you want to retain the original aspect ratio, don’t use the Shift key.) Hold down the Option/Alt key to scale from the center. If you’re resizing an image of one or more people, click the little “man” button to the right in the Options bar to protect skin tones. Before selecting Content-Aware Scale, you can also create an alpha channel (a saved selection) to identify areas of the image you want to protect. Make a selection, choose Select ⇒ Save Selection, and then select that alpha channel on Content-Aware Scale’s Options bar in the Protect menu, immediately to the left of the “man” icon. See Chapter 7 for more on alpha channels.
5 Flatten (optional). If desired, choose Layer ⇒ Flatten Image.
In this example, the original image is at the bottom. To the left, the image has been resampled from 6.67 x 10 inches to 8 x 10 inches using Image Size (with Constrain Proportions deselected). To the right, Content-Aware Scale does a much better job — in this particular case — of scaling the image to 8 x 10 inches, minimizing distortion of the subject.
Is Content-Aware Scale a substitute for properly composing in-camera before shooting? Absolutely not! Is it preferable to cropping to a new aspect ratio? Rarely. Is it an incredibly powerful tool for certain difficult challenges? Now we’re talking!
File Formats: Which Do You Need?
After working with your image in Photoshop, you need to save the changes. Choosing File ⇒ Save updates the current file on your hard drive, maintaining the current file format when possible. If you added a feature to the file that isn’t supported by the original file format, Photoshop automatically opens the Save As dialog box and shows you which features are not supported by the selected file format. In Figure 2-13, the lower part of the Save As dialog box shows the yellow warning triangles that identify options being used in the image that are not available when saving as a JPEG.
You can go ahead and save the image in that format, but your file will no longer contain those unsupported features. In the example shown in Figure 2-13, I can click the Save button and create a JPEG file, but that JPEG won’t have the alpha channel (a saved selection) or the spot colors (a custom printing color) and it will be flattened to a single layer. If I want to retain those features in the file, I need to choose a different file format, such as Photoshop’s own PSD format. (Read more about alpha channels in Chapter 7 and about spot channels in Chapter 5.)
FIGURE 2-13: Photoshop shows you which image features are not available in your selected file format.
In Figure 2-13,