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When I run an iPhone app which supports retina displays on an iPad, does it have increased clarity when shown at 2x size? I've noticed text appears pixelated, but photos do not. Is there a complete list of how iOS handles this scaling?

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No, and given that apple could easily handle most things correctly (particularly text and UI elements!) it appears they've made a deliberate choice to use pixel doubling to encourage application developers to update their apps specifically for the iPad. This question, however, could serve as a good start on such a list. – Adam Davis Mar 20 '12 at 21:23
@AdamDavis Good point, I hadn't thought of that. – timothymh Mar 20 '12 at 21:25
up vote 1 down vote accepted

I haven't found a detailed list of the exact behaviors. For the most part, it seems that everything is simply pixel-scaled (which is why text looks bad), except for images which were scaled down to begin with. I presume that the UIImageView class has some behavior to handle this automatically.

On Retina devices, Apple's built-in text and image rendering are aware of the scaling factor of the screen, and will render text and photos in high quality. But this doesn't seem to apply to iPhone apps run on an iPad. (See Apple's developer documentation on high-resolution screens for more information on that.)

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What do you mean by "for images which were scaled down to begin with". Why they are scaled down ? – Paul Brewczynski Dec 24 '14 at 21:01
@PaulBrewczynski To run in Retina resolution. – timothymh Oct 10 '15 at 1:18

App interfaces are scaled up from from an image usually a PNG of a set resolution when photos are usually scaled down to fit the current interfaces. So that's why photos usually look better.

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If you run an application with iPhone user interface on iPad, it simple doubles all pixels when you zoom it, it does not reload and resize the PNG from the resources of the application as Lyken suggest. Images usually do not have strong contours that's why the "pixelation" is less noticeable. If you have some black text on white background as an image, you will see it same pixeled as text. iPad does not use either the retina variant of the application UI and this I find ridiculous: if the application supports retina, the OS has every information in the application bundle to give much better looking UIs but it does not care about it.

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I would up vote this but it isn't entirely true, as images are still iPad-quality. – timothymh Mar 20 '12 at 10:55
I mean, images shown in iPhone apps are displayed as though they're an iPad-resolution image, not an iPhone-resolution image blown up 2x. – timothymh Mar 20 '12 at 11:56

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