Vertical flip of CGContext

Here is some code that I wrote based on this answer, in case it is helpful to anyone:

#import <QuartzCore/QuartzCore.h>
...
+ (UIImage *) flipImageVertically:(UIImage *)originalImage {
    UIImageView *tempImageView = [[UIImageView alloc] initWithImage:originalImage];

    UIGraphicsBeginImageContext(tempImageView.frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGAffineTransform flipVertical = CGAffineTransformMake(
            1, 0, 0, -1, 0, tempImageView.frame.size.height
    );
    CGContextConcatCTM(context, flipVertical);  

    [tempImageView.layer renderInContext:context];

    UIImage *flipedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [tempImageView release];

    return flipedImage;
}   

The CTM affects future drawing; you're capturing what you have already drawn. You need to concat that transformation before you draw, not after.

Here is the same code as above (benvolioT's answer) but in SWIFT 4.0

import QuartzCore
...
func flipImageVertically(originalImage:UIImage) -> UIImage{

    let tempImageView:UIImageView = UIImageView(image: originalImage)
    UIGraphicsBeginImageContext(tempImageView.frame.size)
    let context:CGContext = UIGraphicsGetCurrentContext()!
    let flipVertical: CGAffineTransform = CGAffineTransform(a: 1, b: 0, c: 0, d: -1, tx: 0, ty: tempImageView.frame.size.height)

    context.concatenate(flipVertical)
    tempImageView.layer.render(in: context)

    let flippedImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()

    return flippedImage
}

And here is an even better way in SWIFT:

func flipImageVertically(originalImage:UIImage) -> UIImage {
    let image:UIImage = UIImage(CGImage: originalImage.CGImage, scale: 1.0, orientation: UIImageOrientation.RightMirrored)!
    return image
}

Swift 4:

let flipVertical = CGAffineTransform(a: 1, b: 0, c: 0, d: -1, tx: 0, ty: originalImage.size.height)
ctx.concatenate(flipVertical)