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March
2003

Resizing and Resampling

One of the most confusing aspects of Adobe Photoshop and similar programs is the concept of resizing and resampling images. These tasks are accomplished in the Image Size dialog box in Photoshop (from the Image menu select Image Size) or Photoshop Elements (select Resize from the Image menu).

Photoshop’s Image Size dialog box has two basic functions: Resizing the image and resampling the image. It can be easy to confuse these two operations at first, but after you understand the process, the difference will be second nature.

First, let’s talk about resizing. When you resize an image, you change the physical, printed size in inches, centimeters or picas. This does not change the file size (i.e. how many kilobytes or megabytes the file takes up on the hard drive), but it does change the pixels-per-inch or resolution. Figure 1 shows an image that is 3.01 x 3.45 inches at 300 pixels-per-inch. Two things to notice in this dialog box are that the Resample Image box is not checked which means that the Pixel Dimensions are not editable and the file size (2.67M) will not change.

Fig. 1

Figure 2 shows what happens when the Resolution is changed to 30 (ten times less than the 300 in Figure 1). Notice that the file size and Pixel Dimensions have not changed, but the Document Size is now 30.1 x 34.5 inches (ten time larger than in Figure 1). You will also notice the link icon next to the Width, Height and Resolution under the Document Size section. These are all related, if you change one the others will change as well. The total number of pixels in the image has not changed; only the print size has changed, spreading out those same pixels over a wider area. When you click the OK button, the change happens immediately because Photoshop has to do very little work.

Fig. 2

You would resize an image for printing purposes. Say you want to print an image at a specific size, like 8x10 or 5x7; you would resize it to the target size.

Resampling is different. When you resample an image you change the actual Pixel Dimensions of the image. This will change the Document Size or the Resolution or both. Figure 3 shows the Image Size dialog box with the Resample Image box checked. There are three modes of resampling; Bicubic is used almost exclusively because it gives a smoother result although it takes longer. Nearest Neighbor is the fastest interpolation method, but the results may be quite jagged, Bilinear splits the difference. You should always use Bicubic interpolation. When Resample Image is checked the Constrain Proportions box becomes active. This should only be unchecked if you want to distort the image (i.e. squash or stretch it).

Fig. 3

Notice that the Pixel Dimensions are now editable. Also notice now that the Document Size Width and Height are still linked, but they are no longer linked to the Resolution. Following the example in Figure 2, now when the Resolution is changed to 30 pixels-per-inch the Document Size dimensions remain the same, but the Pixel Dimensions and file size change dramatically. Now when you hit the OK button, Photoshop has to do some serious calculations and it will take anywhere from a couple of seconds to several minutes (or more) depending on the size of the image, the amount of resampling and the speed of your computer.

When you reduce the number of pixels it is called downsampling; the opposite, adding more pixels, is called upsampling. Resampling is typically used for reducing an image’s file size, for emailing or web graphics for example. You rarely want to upsample an image more than about 10%. Adding additional pixels adds no additional detail to the image; it simply increases the file size. When you downsample an image you will typically lose some detail and the image may need to be sharpened again (select Unsharp Mask from the Sharpen submenu of the Filter menu).

When you send your cool digital camera images to your relatives, it is often a good idea to downsample them so that they transmit faster and can be viewed easily on the recipient’s screen, especially if the receiver has a small monitor. Likewise, it is very important when you are building web pages to resample the images to a specific pixel dimension to fit within your layout.

The Image Size dialog box is by far one of the most confusing areas of Adobe Photoshop. It takes a little time to master the details, but one you get it you will feel like a Photoshop pro.

Paul Vaughn is a freelance graphic artist, writer and web designer. If you would like to see the Graphics Guy address a specific topic email Paul Vaughn at paulv@mac.com.

 

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