Resampling refers to the process of changing a file's pixel dimensions to affect its display size and ultimately the file's resolution. When you resample down—called downsampling—you move from a higher file resolution to a lower one and typically gain quality in the process. However, when you resample up, you move from a lower file resolution to a higher one, and Photoshop is forced to "make up" the contents of those added pixels.
How Photoshop creates those new pixels (and for that matter, how it deletes them as well) is referred to as the interpolation method. Photoshop allows you to specify the default interpolation method in its preferences (choose Edit I Preferences I General), and also enables you to adjust that interpolation method for specific files from within the options for changing an image's size.
TRY IT To adjust an image's resolution after you've brought it into Photoshop, make sure the file is open and choose Image I Image Size to bring up the following window of options:
Under Document Size, you are given the opportunity to increase or decrease a file's resolution. As you change that image, you should see the Width and Height of the image also change under Pixel Dimensions. Photoshop adjusts those sizes according to the file resolution you specify.
To change the interpolation method for this particular image, click the pop-up menu next to Resample Image at the bottom of the options window to select one of the following methods:
• Bicubic Photoshop uses this as its default interpolation method. Although this method is the slowest in terms of processing time, it is the most accurate and produces the smoothest gradations.
• Bilinear This method is your average, middle-of-the-road interpolation method, offering midgrade quality and processing time.
• Nearest Neighbor This method is the fastest to process but may produce jagged edges. Adobe recommends using this method for files with non-anti-aliased edges as it helps preserve those hard edges and makes a smaller file size.
After making your adjustments, click OK to see how they affect your file. If you're unsatisfied with the changes, choose Edit I Undo before doing anything else in Photoshop.
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