to Figure 2-7 to see that you have eight different ways to calculate the change (called resampling algorithms).
When you deselect the Resample option (as shown in Figure 2-10), you’re changing the print dimensions without changing the pixel dimensions of the image.
When using Image Size without resampling, you’re simply changing the instructions recorded in the image for your printing device. When you enter one dimension, either width or height, Photoshop does the math and fills in both the other dimension and the new resolution.
FIGURE 2-9: Resizing an image without constraining proportions. Interesting, yes, but useful?
FIGURE 2-10: Clear the Resample Image check box to change print size, not pixel dimensions.
FIGURE 2-11: Enter a value, and Photoshop recalculates the fields automatically.
When you resample an image (change the pixel dimensions), Photoshop takes the image and maps it to the new size, attempting to preserve the image’s appearances as much as possible at the new size, using the new number of pixels. Of course, if you take an image that’s more than 3,000 pixels wide and resample it to 300 pixels wide, you’re going to lose some of the detail. And, conversely, resampling an image from 300 pixels wide to 3,000 pixels wide, even when using the Preserve Details resampling algorithm, is likely to introduce some softness to the image’s appearance.
TO RESAMPLE OR TO CROP: THAT IS THE QUESTION
To better understand the difference between resampling an image and cropping an image, consider this situation:
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
