Pixels per inch (PPI) and dots per inch (DPI)

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What is PPI? What does PPI/DPI stand for? Does PPI matter?

If you're searching for information on PPI (pixels per inch) or DPI (dots per inch) you're probably concerned about how your image will print. Perhaps you've even seen a low-resolution warning when making your book.

If you see a low-resolution warning

If you see a low-resolution warning then you must correct the problem. At that point it doesn't matter what the PPI or DPI of your image is. You must fix the cause of the low-res warning. You must place the image in a smaller container, or zoom out of the image until the low-res warning goes away, or use a different image that doesn't trigger the warning.

You can choose to ignore a low-resolution warning but your image may not print well. We do not recommend artificially manipulating the image to increase the resolution (known as upsampling). The printed results may not look good because this artificially creates new pixels that were not part of the original image.

Want to learn more about PPI and DPI?

If you're searching for a better understanding of what PPI is, how PPI differs from DPI, and what effect it has on your digital photographs in general, below are a few resources. Note that these are external sites with no affiliation with Blurb--we just found them useful.

DPI and PPI Explained The author does a good job of explaining things clearly and succinctly. His idea that "creating pixels is a bad idea" is spot on.

PPI vs DPI: what's the difference? If you'd like to delve deeper then this article (aimed at designers but useful here, too) offers additional insight and some helpful visuals).

Basics of Digital Camera Pixels If you want to go further down the rabbit hole of PPI, DPI, pixels, and resolution, then check out this article from the folks at Cambridge in Colour.

And don't forget

If you see a low-resolution image warning when making your Blurb book, you must do one of the following:

  • Place the image in a smaller container.
  • Zoom out of the image (i.e., make it smaller) until the warning goes away.
  • Use a different image that doesn't trigger the low-resolution warning.
  • Leave it as-is and take your chances that the image will not print well. We're unable to offer reprints/replacements for problems caused by low-resolution images.

And you might want to give any questionable images the 200% test, too. The low-res warning can't detect things like blurry images or too-dark photos. It takes a human eye to do that so be sure you look things over before you upload and order.

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