When a file describes an image it should work somewhat like this: color value of the pixel in the upper left corner = whatever. next: the color of the second pixel = whatever. and so on. so that in the end I have a description of say 1000x1000 pixels. now how can this image description have a ppi value? if I have a screen that has 1000x1000 pixels and is 22" in size, the dpi will be different from a screen with 1000x1000 pixels and 24". so the dpi depends on the screen that I use to view the image, it's not a property of the image. but images do have a ppi value, I just don't understand the logic behind that?
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You are right, it is largely irrelevant... until you come to print it. Then the printer needs to know how big to print your 720 pixels across and if the resolution is 72dpi it knows that the width of the image on paper must be 10". – Mark Setchell Feb 28 '18 at 19:33
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okay that makes sense. so the ppi is effectively the proposed size of the image. – user3182532 Mar 01 '18 at 08:23
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Yes, that's correct. – Mark Setchell Mar 01 '18 at 09:04