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Actual Size vs Phone Resolution


Arend Pryor

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Hi there. I have an app I'm working on where I'd like to display the "actual size" of a ring on the screen. There are multiple sizes of rings and each has a very specific size (some in inches, some in millimeters).


I found a sizing chart online (pdf document) that included the "actual size" of the rings I was interested in using. The printable pdf allows you to take a ring and match it up with one on the sheet, in order to determine the actual size. I opened the pdf in Photoshop, which converted each ring image to an image, which was great, and gave me the size in pixels.


I moved this image to xCode, then to an iPhone 5 to test it out and it was much smaller than expected. I then drew an oval with a stroke and through trial and error, I increased the size of the oval until the size displayed on the iPhone 5 screen matched with a physical ring of the expected size (used for testing), by placing it on the screen. Awesome! But then, I tested it out on an iPhone 7 Plus and the ring image was larger than expected.


Now, I'm aware that each phone has a different resolution and this is what's causing the problem. What I'd like to figure out, is the best way to approach this problem. If I have the exact measurement of an item/ring, is there a way to calculate what the measurement in pixels should/will be for each resolution of phone so it displays correctly? Any help is much appreciated. Some measurements are included below.


Physical size: 11/16 inch
Pixel measurement from pdf import: W: 49.68px x H: 49.8px
Trial & Err - Correct Measurement on iPhone 5: W: 115px x H: 115px (Resolution = 326ppi)
iPhone 7 Plus: Haven't gone through the trial and error to get the right measurement yet. (401ppi)
 

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