The All-Important 'Resample Image' Option

Finally, we come to one of the most important options in the Image Size dialog box, Resample Image. Remember at the beginning of this tutorial when I said that the difference between resizing and resampling an image is controlled by nothing more than a single checkbox option? This is it! This is the option that determines whether we're resizing our image or resampling it. Again, resizing keeps the pixel dimensions (the number of pixels in the image) the same and simply changes the size at which the image will print, while resampling physically changes the number of pixels in the image. By default, the Resample Image option is checked, which means that the Image Size dialog box is now essentially the Image Resample dialog box, even though it still just says "Image Size" at the top.

To better understand the difference between what we'll call the "Image Resize" version and the "Image Resample" version of the Image Size dialog box, here's a screenshot of what the Image Size dialog box looks like with the "Resample Image" option checked, and below it is an screenshot of what Image Size looks like with "Resample Image" unchecked:

First, with the "Resample Image" option checked, as it is by default:

And here it is with "Resample Image" unchecked:

Can you spot the difference? With "Resample Image" unchecked, if you look up at the Pixel Dimensions section of the dialog box, you'll see that while Photoshop is still telling us how many pixels are in the image for the width and height, we're no longer able to change those numbers. They're listed for information only, and all we can do at this point is change how large our image will print using the Document Size section. With "Resample Image" checked though, the pixel dimensions are shown inside white input boxes which we can click inside of and type in new values, effectively changing how many pixels are in our image.

Also notice that the Scale Styles and Constrain Proportions options at the bottom of the Image Size dialog box are grayed out when Resample Image is not checked:

Scale Styles is grayed out because it's only a concern when the number of pixels in an image is being changed. Since we don't have the ability to change the number of pixels in the image with Resample Image unchecked, the Scale Styles option is of no concern to us or to Photoshop.

Constrain Proportions is grayed out when Resample Image is unchecked because we can no longer physically change the number of pixels in the image, therefore the proportions of the image are fixed. If the image is 20 pixels wide and 10 pixels tall, then no matter how large or small we print it, the width will always be twice the size of the height. If you change the width value in the Document Size section, both the height and the resolution values will also change. Change the height value and the width and resolution values will change. Change the resolution value, and you guessed it, the width and height values will change. All we can do now is change how large or small the image will print, either by entering new values for the document size or by changing the resolution. Again, just as with the Scale Styles option, the Constrain Proportions option is of no concern to us or to Photoshop when all we're doing is resizing the print size of the image.

Image Interpolation Options When Resampling Images

Finally, there's one more option that's grayed out with Resample Image unchecked, and that's the drop-down box to the right of the Resample Image option:

This drop down box is where you can select from Photoshop's "interpolation algorithms". That's a fancy phrase for what essentially means how Photoshop handles throwing away pixels when you resample the image to a smaller size, and how it handles adding pixels when you resample the image to a larger size. As of Photoshop CS2, there's three main options to choose from - Bicubic, Bicubic Sharper and Bicubic Smoother, and knowing when to use which option can make a big difference with image quality. We'll look at the differences between them in detail in another tutorial, but for now, here's the general rule:

• Bicubic Sharper: Use this option when resampling your image smaller for best image quality
• Bicubic Smoother: Use this option when resampling your image larger for best image quality
• Bicubic: Not really used much now that Bicubic Sharper and Bicubic Smoother are available

Again, these options are grayed out here because they only apply to images that are being resampled. If all you're doing is changing how large or small the image will print on paper, these options don't apply.

Summary

• Image resizing keeps the number of pixels in your image the same and affects only how large your image will print (the Document Size).
• Image resampling physically changes the number of pixels in your image (the Pixel Dimensions).
• The Resample Image option at the bottom of the Image Size dialog box controls whether you're resizing or resampling an image.
• With Resample Image checked, you're resampling the image. With it unchecked, you're simply resizing the image.
• Resampling images by changing the width and height values in the Pixel Dimensions section of the Image Size dialog box is primarily used when optimizing images for the web.
• Resizing images by changing the width, height and/or resolution values in the Document Size section of the Image Size dialog box is used for print.

And there we have it!

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