One of the most important issues in working with images—and, unfortunately, one which few people seem to understand—is how the resolution can change relative to (or independently of) the physical size of your image.
There are two ways you can change resolution: scaling and resampling. Figure 2-5 is an example of simple scaling—it doesn’t change the pixel dimensions, just the resolution. On the other hand, resampling changes the pixel dimensions, as shown in Figure 2-10.
Figure 2-10 Upsampling creates more resolution but can’t create more detail or sharpness. Resampling from the high-quality original image always works best.
Photoshop gives you both options. If you scale an image down without changing the resolution, Photoshop has to throw away a bunch of pixels; that’s called downsampling. If you double the size to 4 by 4 inches by upsampling, the program has to add more pixels by interpolating between the other pixels in the image.
Upsampling vs. Downsampling
We used to avoid upsampling when our images mostly came from scanned film, but in the digital age there is no longer a hard-and-fast rule. Digital captures have no film grain, and excellent noise-reduction software is available, which makes digital camera images more amenable to upsampling than film scans ever were. We often upsample digital camera captures by 200 percent, and sometimes more. Upsampling never adds details that weren’t there in the capture, so when possible it’s still better to have enough pixels for the job in the first place.
Downsampling is simpler, because it just throws away data in a more or less intelligent manner. In fact, it’s a common and necessary practice: Today’s digital cameras can record more pixels than you’d need for an image that’s less than a full page in size, or for a Web page. We downsample to the optimal resolution before printing to save processing time and storage space.
Resampling methods. Photoshop offers five resampling methods. You choose one in the General pane of the Preferences dialog or in the Image Size dialog (see Figure 2-11). The resampling method you choose in the Preferences dialog affects when you resample outside the Image Size dialog, such as when you use the Free Transform tool. Each method has its strengths and weaknesses, and we use each of them in different situations.
- Nearest Neighbor is the most basic, and it’s very fast: To create a new pixel, Photoshop simply looks at the pixel next to it and copies its value. Unfortunately, the results are usually lousy for photographic content. Nearest Neighbor works best when the image has hard, straight edges and when you only want to make the existing pixels look bigger.
- Bilinear is more complex and produces better quality: The program sets the color or gray value of each pixel according to the pixels surrounding it. Some pictures can be upsampled pretty well with bilinear interpolation, but one of the bicubic options usually looks better.
- Bicubic creates better effects than Nearest Neighbor or Bilinear. Like Bilinear, it looks at surrounding pixels, but its equation is much more complex and calculation-intensive, producing smoother gradations.
- Bicubic Smoother is specifically designed for upsampling. As its name suggests, it gives a smoother result that handles subsequent sharpening better than the Bicubic sampling method does.
- Bicubic Sharper is designed for downsampling. It does a better job of preserving detail than the Bicubic method does.
Figure 2-11 Where to find resampling (interpolation) methods in Photoshop
The differences between these resampling methods are often subtle. As a starting point, use Bicubic Smoother for upsampling (but don’t expect miracles) and Bicubic Sharper for downsampling, but always test different resampling options when working with critical images.