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Apropos the perennial retina vs PPI questions:

We have an answer to a recent question about setting up an image which, in my opinion, correctly states: "pixels are all that matter." But I have dealt, in a non-professional setting, with web pages where various devices (especially mobile) completely ignored specified units. Are there any caveats that a designer should know about?

I know that the pixel dimensions are all that matters and that PPI is merely a flag in the file header, rather than pixel data.

However in practice, are there times when the PPI flag is honored by iphone/retina/screen-density-aware software if it finds one?

If one provides a 100px square image, one with 10ppi flag set, and one set as 1200ppi, in the absence of implicit style sheets or specification (etc) will the software treat them differently solely due to the ppi flag?

  • In general, "No" web formats and web applications customarily ignore any PPI marker entirely. Now, open the 10ppi and 1200ppi image in Photoshop, and yes Photoshop will use the PPI data - but web/iOS stuff doesn't. – Scott Apr 7 '15 at 20:52
  • Its about working with images in a design environment, IMO – Yorik Apr 7 '15 at 20:53
  • @Yorik hard to say... – joojaa Apr 7 '15 at 20:53
  • Note PPI is on screen.. DPI is on paper. – Scott Apr 7 '15 at 20:55
  • @Ryan given the number of PPI questions on this site, I'd say it actually is on-topic. Designers do have to know about both screen and image resolution, after all. – DA01 Apr 7 '15 at 21:21
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are there times when the PPI flag is honored by iphone/retina/screen-density-aware software if it finds one?

No.

In fact, I can't think of any place it's really honored outside some very select software.

For example, I believe, in Photoshop, if the PPI is set, it will affect the size the image is printed at, if you print directly from Photshop. But even then, you can change things up with the print driver's settings.

Specialized pre-press software such as RIPs and PDF print workflows may also use the PPI setting to calculate things like line screens as well.

But, for the most part, the PPI setting is usually more confusing than helpful in most situations--and is all but irrelevant when it comes to creating images for screen.

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