UIGraphicsBeginImageContext doesn't return retina image - ios

Question

I want to capture current screen in an image. I'm doing this:
UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, self.view.isOpaque, UIScreen.main.scale)
self.view.drawHierarchy(in: self.view.bounds, afterScreenUpdates: false)
let snapshot = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
Problem is the scale parameter. If I understand correctly 0.0 represents non-retina, 2.0 represents retina and 3.0 represents retina for 6 Plus and 7 Plus. No matter what I input into the scale parameter, the output is always image with 375x667 resolution. I also tried different approach:
UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, self.view.isOpaque, UIScreen.main.scale)
self.view.layer.render(in: UIGraphicsGetCurrentContext()!)
let snapshot: UIImage? = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
Again, same scenario. I'm even using
UIScreen.main.scale
Which in fact returns value 2.0. What am I doing wrong? How do I get a higher resolution image?

Answer 1

This code will do the trick
let contextSize = CGSize(width: self.view.bounds.size.width * UIScreen.main.scale, height: self.view.bounds.size.height * UIScreen.main.scale)
UIGraphicsBeginImageContextWithOptions(contextSize, self.view.isOpaque, UIScreen.main.scale)
self.view.layer.render(in: UIGraphicsGetCurrentContext()!)
let snapshot: UIImage? = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
CGContext don't care about scale, it care about size only.
EDIT
You may want to use the newer API available in iOS 10 and later
let renderer = UIGraphicsImageRenderer(bounds: self.view.bounds)
let snapshot = renderer.image(actions: { context in
self.view.layer.render(in: context.cgContext)
})

Answer 2

snapshot is a UIImage which has two properties, size and scale; much like the screen. To determine the actual size in pixels of the image you must multiply the the size by the scale. I think your issue is that you're assuming the size property is pixels not points.
size
The logical dimensions of the image, measured in points.
You can test this in a very definitive way by creating a JPG using UIImageJPEGRepresentation and saving it to disk to inspect using image tools you're familiar with.