Strange behaviour with CGContext - iOS - ios

Question

The following code splits an image into 2. It seems working fine with non-retina devices, however it gives a different output with retina devices. Could someone please help me fix it? Thanks..
My Code
UIImage *img = [UIImage imageNamed:#"apple.png"];
CGSize sz = [img size];
UIGraphicsBeginImageContextWithOptions(CGSizeMake(sz.width/2, sz.height), NO, 0);
[img drawAtPoint:CGPointMake(-sz.width/2, 0)];
UIImage *right = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
rightView = [[[UIImageView alloc] initWithImage:right] autorelease];
rightView.frame = CGRectMake(self.view.frame.size.width/2, 0, self.view.frame.size.width/2, self.view.frame.size.height);
CGImageRef leftRef = CGImageCreateWithImageInRect([img CGImage],CGRectMake(0,0,sz.width/2,sz.height));
UIGraphicsBeginImageContextWithOptions(CGSizeMake(sz.width/2, sz.height), NO, 0);
CGContextRef con = UIGraphicsGetCurrentContext();
CGContextDrawImage(con, CGRectMake(0,0,sz.width/2.0,sz.height), leftRef);
UIImage *left = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
UIImage *rotatedImage = [left imageRotatedByDegrees:180.0];
leftView = [[[UIImageView alloc] initWithImage:rotatedImage] autorelease];
leftView.frame = CGRectMake(0, 0, self.view.frame.size.width/2, self.view.frame.size.height);
leftView.transform = CGAffineTransformMake(-1,0,0,1,0,0);
CGImageRelease(leftRef);
[self.view addSubview:leftView];
[self.view addSubview:rightView];
non-retina
retina
PS: I don't know if this is important but apple.png has a #2x version..

Answer 1

The [-UIImage size] property returns the size in points, not in pixels. You probably need to also call [-UIImage scale] to figure out how the image is scaled.

Answer 2

When you create the left view with
leftView = [[[UIImageView alloc] initWithImage:rotatedImage] autorelease];,
you're not specifying the correct scale. Rather than creating your UIImage this way:
UIImage *left = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
UIImage *rotatedImage = [left imageRotatedByDegrees:180.0];
try creating a CGImageRef and then initializing the UIImage using
[UIImage imageWithCGImage:scale:orientation:]
while specifying the correct scale. There are a number of ways to convert the raw image data from the context to a CGImageRef, or you can use the image you've created and use the CGImage property of UIImage.