Upsampling in Photoshop

Upsampling is an image-editing process that enlarges your original photo, making up (or interpolating) additional pixels to fill in the gaps. There are several upsampling techniques, and professionals are divided as to when and how the method should be used. In truth, the best technique for one image might not be ideal for another.

We'll look at a couple of techniques that can easily be performed with Adobe Photoshop.

It's important to realize at the outset that upsampling can't increase the detail in a image, and there are limits to its effectiveness. Our goal is to enlarge a low-resolution image so that we can obtain a smooth, focused print with no sign of jagged pixel edges.

 When you're in Photoshop, start by pulling up the Image Size dialog box. (choose Image > Image Size) This is where you'll make your first decision: whether or not to upsample.

To decide, uncheck the Resample Image check box and enter the desired print dimensions in the Width and Height fields of the Document Size section.

As an example, imagine we want to make a 4.5-by-6 inch print from a 480- by 640-pixel image. Entering these print dimensions reveals that as is, the image will print at a resolution of 106.667 pixels per inch. Three hundred ppi is the standard for professional printing; as a general rule, you'll need to achieve at least half that resolution (150 ppi) before you stop seeing jagged pixels in your prints.  Clearly, our image will benefit from upsampling, so we'll essentially double the image's resolution by raising it to 200 ppi.

The first step is to recheck the Resample Image box; notice that the fields in the Pixel Dimensions section become active. Next, make sure the Constrain Proportions check box is selected. Then enter 200 in the Resolution field. This roughly doubles the Width and Height values in the Pixel Dimensions section, raising them to 900 and 1,200 respectively. (Doubling the width and height quadruples the number of pixels in the image, raising this 0.3-megapixel image to a bit over 1MP.)

The Resample Image menu shows five different flavors of interpolation. For upsampling, just set the Resample Image menu to Bicubic Smoother, click OK, and the job is done.

A popular variation on this upsampling technique is stair interpolation, created by photographer and software developer Fred Miranda. His theory is that it's better to upsample several times by smaller amounts than all at once. To try this, enter the Image Size dialog box, activate Resample Image and Constrain Proportions, and under Document Size switch either Width or Height from inches to percent. Then enter 110 for the Width or Height and click OK. To achieve a rough doubling in size as before, you'll repeat this six times, then use Image Size once again (with Resample Image turned off) to set the desired print dimensions.

While no form of upsampling can work miracles, it can enable you to turn a low-resolution image, into something a bit more usable.

The Image Size dialog in Photoshop CS2