iOS中的UIImage中有imageOrientation屬性，該屬性決定了UIImage在手機上顯示時的方向。如果imageOrientation的值為left或right，那麼顯示出來的影象和實際影象在記憶體中的儲存就存在90度的旋轉問題。比如顯示出來的圖片是720*1280的，實際上該圖在記憶體中是按1280*720儲存的。
在stackoverflow上折騰了一番，找到了下面靠譜的解決辦法。

extension UIImage {
    func fixImageOrientation() -> UIImage? {

        guard let cgImage = self.cgImage
 
 else {
            return nil
        }

        if self.imageOrientation == UIImageOrientation.up {
            return self
        }

        let width  = self.size.width
        let height = self.size.height

        var transform = CGAffineTransform.identity

        switch self.imageOrientation
 
 {
        case .down, .downMirrored:
            transform = transform.translatedBy(x: width, y: height)
            transform = transform.rotated(by: CGFloat.pi)

        case .left, .leftMirrored:
            transform = transform.translatedBy(x: width, y: 0)
            transform = transform.rotated
 
(by: 0.5*CGFloat.pi)

        case .right, .rightMirrored:
            transform = transform.translatedBy(x: 0, y: height)
            transform = transform.rotated(by: -0.5*CGFloat.pi)

        case .up, .upMirrored:
            break
        }

        switch self.imageOrientation {
        case .upMirrored, .downMirrored:
            transform = transform.translatedBy(x: width, y: 0)
            transform = transform.scaledBy(x: -1, y: 1)

        case .leftMirrored, .rightMirrored:
            transform = transform.translatedBy(x: height, y: 0)
            transform = transform.scaledBy(x: -1, y: 1)

        default:
            break;
        }

        // Now we draw the underlying CGImage into a new context, applying the transform
        // calculated above.
        guard let colorSpace = cgImage.colorSpace else {
            return nil
        }

        guard let context = CGContext(
            data: nil,
            width: Int(width),
            height: Int(height),
            bitsPerComponent: cgImage.bitsPerComponent,
            bytesPerRow: 0,
            space: colorSpace,
            bitmapInfo: UInt32(cgImage.bitmapInfo.rawValue)
            ) else {
                return nil
        }

        context.concatenate(transform);

        switch self.imageOrientation {

        case .left, .leftMirrored, .right, .rightMirrored:
            // Grr...
            context.draw(cgImage, in: CGRect(x: 0, y: 0, width: height, height: width))

        default:
            context.draw(cgImage, in: CGRect(x: 0, y: 0, width: width, height: height))
        }

        // And now we just create a new UIImage from the drawing context
        guard let newCGImg = context.makeImage() else {
            return nil
        }

        let img = UIImage(cgImage: newCGImg)

        return img;
    }
 }

上面的程式碼能根據UIImage中的imageOrientation去翻轉影象的實際儲存佈局。

如果想任意角度旋轉影象，比如來個30度？下面的函式可以實現。

func imageRotatedByDegrees(oldImage: UIImage, deg degrees: CGFloat) -> UIImage {
    //Calculate the size of the rotated view's containing box for our drawing space

    var rotatedViewBox: UIView = UIView(frame: CGRect(x: 0, y: 0, width: oldImage.size.width, height: oldImage.size.height))

    print(oldImage.size.height)
    if (oldImage.imageOrientation == .right)
    {
        rotatedViewBox = UIView(frame: CGRect(x: 0, y: 0, width: oldImage.size.height, height: oldImage.size.width))
    }

    let t: CGAffineTransform = CGAffineTransform(rotationAngle: degrees * CGFloat.pi / 180)
    rotatedViewBox.transform = t
    let rotatedSize: CGSize = rotatedViewBox.frame.size
    //Create the bitmap context
    UIGraphicsBeginImageContext(rotatedSize)
    let bitmap: CGContext = UIGraphicsGetCurrentContext()!
    //Move the origin to the middle of the image so we will rotate and scale around the center.
    bitmap.translateBy(x: rotatedSize.width / 2, y: rotatedSize.height / 2)
    //Rotate the image context
    bitmap.rotate(by: (degrees * CGFloat.pi / 180))
    //Now, draw the rotated/scaled image into the context
    bitmap.scaleBy(x: 1.0, y: -1.0)
    bitmap.draw(oldImage.cgImage!, in: CGRect(x: -oldImage.size.height / 2, y: -oldImage.size.width / 2, width: oldImage.size.height, height: oldImage.size.width))
    let newImage: UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    return newImage
}