Xamarin UIImage rotation

Question

画像を回転して保存すると画像が保存されるときに動作しますが、画像を回転しようとすると回転は行われますが画像は空白になります。ここで

public UIImage RotateImage (UIImage originalImage, int rotationAngle) 
     { 
      UIImage rotatedImage = originalImage; 

      if (rotationAngle > 0) { 
       CGSize rotatedSize; 
       float angle = Convert.ToSingle ((Math.PI/180) * rotationAngle); 

       using (UIView rotatedViewBox = new UIView (new CGRect (0, 0, originalImage.Size.Width, originalImage.Size.Height))) { 
        CGAffineTransform t = CGAffineTransform.MakeRotation (angle); 
        rotatedViewBox.Transform = t; 
        rotatedSize = rotatedViewBox.Frame.Size; 

        UIGraphics.BeginImageContext (rotatedSize); 
        CGContext context = UIGraphics.GetCurrentContext(); 

        context.TranslateCTM (rotatedSize.Width/2, rotatedSize.Height/2); 
        context.RotateCTM (angle); 
        context.ScaleCTM ((nfloat)1.0, -(nfloat)1.0); 

        context.DrawImage (new CGRect (-originalImage.Size.Width/2, -originalImage.Size.Height/2, originalImage.Size.Width, originalImage.Size.Height), originalImage.CGImage); 

        rotatedImage = UIGraphics.GetImageFromCurrentImageContext(); 

        UIGraphics.EndImageContext(); 
       } 

      } 

      return rotatedImage; 
     } 

は私が得るものです：

Rotated image

とオリジナル画像：

original image

更新：小さなコード変更

Answer 1

rotate image using CGContextDrawImage

 CGImage imgRef = originalImage.CGImage; 
     float width = imgRef.Width; 
     float height = imgRef.Height; 
     CGAffineTransform transform = CGAffineTransform.MakeIdentity(); 
     RectangleF bounds = new RectangleF(0, 0, width, height); 

     float angle = Convert.ToSingle((rotationAngle/180f) * Math.PI); 
     transform = CGAffineTransform.MakeRotation(angle); 

     UIGraphics.BeginImageContext(bounds.Size); 

     CGContext context = UIGraphics.GetCurrentContext(); 

     context.TranslateCTM(width/2, height/2); 
     context.SaveState(); 
     context.ConcatCTM(transform); 
     context.SaveState(); 
     context.ConcatCTM(CGAffineTransform.MakeScale(1.0f, -1.0f)); 

     context.DrawImage(new RectangleF(-width/2, -height/2, width, height), imgRef); 
     context.RestoreState(); 

     UIImage imageCopy = UIGraphics.GetImageFromCurrentImageContext(); 
     UIGraphics.EndImageContext();