How do I rotate an image from a file with Xamarin on iOS - ios

Question

I have been trying to rotate an image for a couple days now, but the best I get is still a black image.
I suspect it may have something to do with the point I'm rotating around but I'm not sure. I say that because I tried the whole solution proposed here and translated in Xamarin terms, but that didn't work.
Here's my code:
public void Rotate (string sourceFile, bool isCCW){
using (UIImage sourceImage = UIImage.FromFile(sourceFile))
{
var sourceSize = sourceImage.Size;
UIGraphics.BeginImageContextWithOptions(new CGSize(sourceSize.Height, sourceSize.Width), true, 1.0f);
CGContext bitmap = UIGraphics.GetCurrentContext();
// rotating before DrawImage didn't work, just got the image cropped inside a rotated frame
// bitmap.RotateCTM((float)(isCCW ? Math.PI / 2 : -Math.PI / 2));
// swapped Width and Height because the image is rotated
bitmap.DrawImage(new CGRect(0, 0, sourceSize.Height, sourceSize.Width), sourceImage.CGImage);
// rotating after causes the resulting image to be just black
bitmap.RotateCTM((float)(isCCW ? Math.PI / 2 : -Math.PI / 2));
var resultImage = UIGraphics.GetImageFromCurrentImageContext();
UIGraphics.EndImageContext();
if (targetFile.ToLower().EndsWith("png"))
resultImage.AsPNG().Save(sourceFile, true);
else
resultImage.AsJPEG().Save(sourceFile, true);
}
}

Answer 1

It looks like you want to take a UIImage, and then rotate it either 90 clockwise or 90 degrees counter clockwise. You can actually do this with just a few lines of code:
public void RotateImage(ref UIImage imageToRotate, bool isCCW)
{
var imageRotation = isCCW ? UIImageOrientation.Right : UIImageOrientation.Left;
imageToRotate = UIImage.FromImage(imageToRotate.CGImage, imageToRotate.CurrentScale, imageRotation);
}
We use UIImage.FromImage() that accepts 3 parameters. The first is a CGImage, from which we can get from the UIImage you're trying to rotate. The 2nd parameter is the scale of the image. The 3rd parameter is the important one. We can rotate it using UIImageOrientation.Right (90 degrees CCW) or UIImageOrientation.Left (90 degrees CW). You can check out the Apple documentation for the meaning of the other UIImageOrientation constants:
https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIImage_Class/index.html#//apple_ref/c/tdef/UIImageOrientation
UPDATE:
Note that the code above only changes EXIF flags and calling it twice doesn't rotate the image 180deg.
Add this code to make the result cumulative:
UIGraphics.BeginImageContextWithOptions(new CGSize((float)h, (float)w), true, 1.0f);
imageToRotate.Draw(new CGRect(0, 0, (float)h, (float)w));
var resultImage = UIGraphics.GetImageFromCurrentImageContext();
UIGraphics.EndImageContext();
imageToRotate = resultImage;