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Retina display and [UIImage initWithData]

Answer 1

A:

AFAIK you don't need to set the scale value yourself. The OS will handle the points to pixel translation for you.

Rengers 2010-07-20 11:16:08

If I just initialise a UIImage from data, it is up-scaled on the iPhone 4. (eg, a 320x480 image will be fullscreen on iPhone 4). I need to prevent this somehow so I can use high resolution images

Sam 2010-07-20 11:20:11

Ah ok didn't know that, thanks!

Rengers 2010-07-20 11:35:07

Answer 2

+5 A:

I'm not aware of anything you can embed in the image data itself to tell the phone that it's a @2x image, but something like this should work:

UIImage * img = ...;
img = [UIImage imageWithCGImage:img.CGImage scale:2 orientation:img.imageOrientation];

tc. 2010-07-20 11:24:29

Perfect, thank you

Sam 2010-07-20 11:28:37

This fixed it for me too, thanks! I was drawing text on an image and needed this to keep the scale correct on the resulting image.

Jason 2010-09-28 02:44:58

Perfekt for me too... But why do we have to implement such low level stuff if we store an image with UIImagePNGrepresentation and want it back correctly?? That's not Apple-like. CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData((CFDataRef)<storedPNGData (NSData)>); CGImageRef cgHeaderImage= CGImageCreateWithPNGDataProvider(dataProvider,NULL,NO, kCGRenderingIntentDefault); CGDataProviderRelease(dataProvider); headerCell.headerImage.image = [UIImage imageWithCGImage:cgHeaderImage scale:2.0 orientation:UIImageOrientationUp];CGImageRelease(cgHeaderImage);

Gerd 2010-10-11 10:34:10

Because the PNGRepresentation has no notion of "scale" (similarly, the JPEGRepresentation has no notion of transparency); the image is simply loaded by CGImage. While in theory you could use image DPI, I'm pretty sure that CGImage simply ignores DPI information, so UIImage would have to do additional parsing. Additionally, DPI information is *almost always meaningless*: Macs default to 72, Windows defaults to 96, and most digital cameras default to 300 (and the "DPI" of a photo is completely meaningless; I want angular resolution/FOV and lens distortion curve).

tc. 2010-10-13 20:13:29

I'd also just use `UIImage * img = [UIImage imageWithData:...]; img = [UIImage imageWithCGImage:img.CGImage, scale:2 orientation:img.imageOrientation];`. And finally, `img = [UIImage imageWithData:UIImagePNGRepresentation(img)]` isn't an indentity transform; UIImagePNGRepresentation will often have to convert from premultiplied alpha to "normal" alpha.

tc. 2010-10-13 20:16:25

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Retina display and [UIImage initWithData]

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