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I am sure this is a simple question, but the answers I have received from people are never simple. I am trying to apply an answer to this question for scaling images for the iOS Retina display.

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As DA01 implies: for computer images, inches don't exist except as a suggestion to the software. The inches only come into play the moment the image is shown on a screen or printed on paper. If I take a 100x100px image and change it's ppi/dpi number, it is still 100x100px. Some software will assume you want to change the pixel dimensions instead, which may or may not be helpful: if you add pixels, they are faked. You can play with this a little: photoshop if you copy the height in px, double the dpi and then reset the height to its original px, the file size stays exactly the same. – horatio Oct 11 '11 at 20:58

ppi = pixels per inch = typically used as a measurement for screens (the iPhone 4 has twice the ppi as the iphone 3)

dpi = dots per inch = typically used as a print measurement and refers to the number of pixels in the image that will be used to render 1" on paper

scaling = this is a loaded term and why the answer isn't simple. For raster images, you can scale an image a number of ways in Photoshop. Some of these ways may change the ppi settings. Some of these may change the actual number of pixels.

All that said, for working with screen graphics, discard the whole concept of ppi and dpi for your own images. All that matters are your image's pixel dimensions.

If you have an icon that is 100px x 100px and want to make a crisper, retina display version, then that version of the image would need to be 200px x 200px.

So, technically, an image that will be displayed at the same physical size on an iPhone 4 as the iPhone 3 version appears would have to have 4 times the number of pixels, as each axis of the image now is twice the pixel dimensions.

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