Resizing and Resampling Images

People frequently get confused concerning what constitutes image size and document size. Briefly, image size is determined by factors such as the number of pixels, the color mode, bits per channel, embedded profiles, and the type and amount of compression applied. Document size is determined by how the pixels are resolved per inch/cm when output to a printer. For example, a document containing 1200 X 1200 pixels and one that has its resolution set to 300 pixels/inch will create a print that's 4 X 4 inches: 1200 pixels divided by 300 pixels/inch equals 4 inches.

Although the Image Size dialog box is laid out very logically, it still perplexes many people (Figure 11-13). If you look carefully, you will see that it's divided into two halves by etched lines. The top half is devoted to Pixel Dimensions and the bottom half to Document Size. Perhaps if Document Size were called Print Size, there would be less confusion (the term Document Size is used elsewhere to indicate file size, and open files are usually referred to as documents). Nevertheless, the Resolution setting refers to print size only and should not be confused with the size of the document when displayed on-screen. That's because on-screen resolution is determined by your video card and your monitor's screen size, whereas the resolution of the print is determined by the Resolution setting in Image Size.

Figure 11-13: The Image Size dialog box. If you observe the etch lines, you can see the dialog box delineated between Pixel Dimensions (top) and Document Size (bottom).

■ With Resample selected—Changing the pixel dimensions changes Document Size, and vice versa, but not the Resolution. Changing the Resolution changes the Pixel Dimensions but not the Document Size.

■ With Resample deselected—Changing Width/Height changes the Resolution (as one drops, the other rises) and the Pixel Dimensions are unaffected.

TIP The default Resample Image algorithm that you see in the Image Size dialog box can be set in the Preferences dialog box. It's a good idea to keep an eye on it because, although all transforms make use of it, the default is not suitable for all transforms. For example, transforms applied to photos benefit if Bicubic Smoother is used when resized upward but Bicubic Sharper if resized downward.

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