UIImageFixOrientationExtension.swift

import Foundation

extension UIImage {
  
  func fixOrientation() -> UIImage {
    
    // No-op if the orientation is already correct
    if self.imageOrientation == .Up { return self }
    
    // We need to calculate the proper transformation to make the image upright.
    // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
    var transform = CGAffineTransformIdentity
    
    switch self.imageOrientation {
    case .Down, .DownMirrored:
      transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height)
      transform = CGAffineTransformRotate(transform, CGFloat(M_PI))
      
    case .Left, .LeftMirrored:
      transform = CGAffineTransformTranslate(transform, self.size.width, 0);
      transform = CGAffineTransformRotate(transform, CGFloat(M_PI_2))
      
    case .Right, .RightMirrored:
      transform = CGAffineTransformTranslate(transform, 0, self.size.height);
      transform = CGAffineTransformRotate(transform, CGFloat(-M_PI_2))
      
    default:
      break
    }
    
    switch self.imageOrientation {
    case .UpMirrored, .DownMirrored:
      transform = CGAffineTransformTranslate(transform, self.size.width, 0)
      transform = CGAffineTransformScale(transform, -1, 1)
      
    case .LeftMirrored, .RightMirrored:
      transform = CGAffineTransformTranslate(transform, self.size.height, 0)
      transform = CGAffineTransformScale(transform, -1, 1)
      
    default:
      break
    }
    
    // Now we draw the underlying CGImage into a new context, applying the transform
    // calculated above.
    let ctx = CGBitmapContextCreate(nil, UInt(self.size.width), UInt(self.size.height),
      CGImageGetBitsPerComponent(self.CGImage), 0,
      CGImageGetColorSpace(self.CGImage),
      CGImageGetBitmapInfo(self.CGImage))
    
    CGContextConcatCTM(ctx, transform)
    
    switch self.imageOrientation {
    case .Left, .LeftMirrored, .Right, .RightMirrored:
      // Grr...
      CGContextDrawImage(ctx, CGRectMake(0,0,self.size.height,self.size.width), self.CGImage)
      
    default:
      CGContextDrawImage(ctx, CGRectMake(0,0,self.size.width,self.size.height), self.CGImage)
    }
    
    // And now we just create a new UIImage from the drawing context
    let cgimg = CGBitmapContextCreateImage(ctx)
    let img = UIImage(CGImage: cgimg)
    return img!
  }
}