Question

I have a programmatically created UIImage image, using this kind of code:

UIGraphicsBeginImageContextWithOptions(CGSizeMake(128, 128), NO, 0.0f);
// Render in context
UIImage *resultImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Since my context options specify scale of 0, on retina devices it will be set to 2, and I can confirm that on resulting UIImage scale property.

Now, the problem is that this image size is 128×128, scale 2. When I am putting it into UIImageView of size 64×64 and contentMode = Center, it renders my image outside imageview, presumably rendering into 128×128 box without any scaling.

My understanding of retina graphics was that if an image has scale factor 2.0, the it should be rendered at 1/2 size, thus resulting in higher DPI.
So I was expecting the image view to render 64×64 image at retina quality. Where am I wrong?

Answer 1

The image will be rendered at the size you give it – 128 x 128. The scale factor means that you will have better rendered curves etc, but the image will still be 128 x 128 points. As stated in the documentation, the size parameter is:

The size (measured in points) of the new bitmap context. This represents the size of the image returned by the UIGraphicsGetImageFromCurrentImageContext function.

If you want a retina-quality 64×64 image, use a 64×64 size for your context.