Rotate UIImage produces a flipped image

Question

I'm just trying to rotate a UIImage around its center using the following piece of code:

- (UIImage *) rotateImage: (UIImage *)image angle:(CGFloat)angleInRadian
    {
        float newSide = MAX([image size].width, [image size].height);
        CGSize size =  CGSizeMake(newSide, newSide);
        UIGraphicsBeginImageContext(size);
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        CGContextTranslateCTM(ctx, newSide/2, newSide/2);
        CGContextRotateCTM(ctx, angleInRadian);
        CGContextDrawImage(UIGraphicsGetCurrentContext(),CGRectMake(-[image size].width/2,-[image size].height/2,size.width, size.height),image.CGImage);
        //CGContextTranslateCTM(ctx, [image size].width/2, [image size].height/2);
        
        UIImage *i = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return i;
    }

Here is a sample of the results I got:

Input image:

Result Image for angle = 0 radians :

The result image is flipped but I'm not sure why? Any idea about what I'm doing wrong?

Thanks in advance.

Answer 1

Add this line after CGContextRotateCTM

CGContextScaleCTM(ctx, 1.0, -1.0);

UIKit coordinate system is upside down to Quartz coordinate system. See here for more details

https://developer.apple.com/library/ios/qa/qa1708/_index.html

Also check image orientation.