Drawing retina versus non-retina images

Question

UIImage 1: Loaded from a file with the @2x modifier with size 400x400, thus UIImage 1 will report its size as 200x200

UIImage 2: Loaded from a file without the @2x modifier with size 400x400, thus UIImage 2 will report its size as 400x400

I then create 2 images from the above applying the code below to each

UIGraphicsBeginImageContextWithOptions(CGSizeMake(400,400), YES, 1.0);
[image drawInRect:CGRectMake(0, 0, 400, 400)];
UIImage *rescaledI = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Considering the above, can I expect the image quality for both resulting images to be exactly the same? (I am trying to determine if drawing a 200x200 retina image to a 400x400 non-retina context will degrade quality versus drawing the same image not loaded as a retina image)

user-44651 user-44651 · Accepted Answer · 2013-04-04T13:56:47

Just return the current image's size.

   UIImage *image1 = [UIImage imagedNamed:@"myimage.png"];
    //access width and height like this
    image1.size.width;
    image1.size.height;

UIGraphicsBeginImageContextWithOptions(CGSizeMake(image1.size.width,  image1.size.height), YES, 1.0);
[image drawInRect:CGRectMake(0, 0, image1.size.width, image1.size.height)];
UIImage *rescaledI = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Of course if you should replace image1 with whatever image you are trying to get the size of. Some switch statement or if statement should do the trick for you.

Never hardcode sizes/dimensions/locations etc. You should always pull that info dynamically by asking your image its size. Then you can change the size of your image without fear of having to locate the hardcoded size in your application.

Drawing retina versus non-retina images

2 Answers