What's wrong with the way I have tried to flip (mirror) a UIImage?

Question

I have been attempting this for a few days now. I'm creating a sprite sheet loader, however I must also be able to load the sprites facing the opposite direction. This involves flipping the images that I already have loaded.

I have already attempted to do this using the UIImageOrientation / UIImageOrientationUpMirrored method however this has absolutely no effect and simply draws the frame with the exact same orientation as before.

I have since attempted a slightly more complicated way which I will include below. But still, simply draws the image in exactly the same way as it is loaded into the application. (Not mirrored).

I've included the method below (along with my comments so that you can maybe follow my thought pattern), can you see what I am doing wrong?

- (UIImage*) getFlippedFrame:(UIImage*) imageToFlip
{
//create a context to draw that shizz into
UIGraphicsBeginImageContext(imageToFlip.size);
CGContextRef currentContext = UIGraphicsGetCurrentContext();



//WHERE YOU LEFT OFF. you're attempting to find a way to flip the image in imagetoflip. and return it as a new UIimage. But no luck so far.
[imageToFlip drawInRect:CGRectMake(0, 0, imageToFlip.size.width, imageToFlip.size.height)];

//take the current context with the old frame drawn in and flip it.
CGContextScaleCTM(currentContext, -1.0, 1.0);

//create a UIImage made from the flipped context. However will the transformation survive the transition to UIImage? UPDATE: Apparently not.

UIImage* flippedFrame = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return flippedFrame;
}

Thank you, Guy.

Answer 1

I would have expected that you have to change the transform of the context and then draw. Also, you would need to translate because you are flipping to negative coordinates, so, replace

[imageToFlip drawInRect:CGRectMake(0, 0, imageToFlip.size.width, imageToFlip.size.height)];
CGContextScaleCTM(currentContext, -1.0, 1.0);

with (edited based on comments)

CGContextTranslateCTM(currentContext, imageToFlip.size.width, 0);      
CGContextScaleCTM(currentContext, -1.0, 1.0);
[imageToFlip drawInRect:CGRectMake(0, 0, imageToFlip.size.width, imageToFlip.size.height)];

NOTE: From comments, a category to use

@implementation UIImage (Flip) 
  - (UIImage*)horizontalFlip { 
     UIGraphicsBeginImageContext(self.size); 
     CGContextRef current_context = UIGraphicsGetCurrentContext();                           
     CGContextTranslateCTM(current_context, self.size.width, 0);
     CGContextScaleCTM(current_context, -1.0, 1.0); 
     [self drawInRect:CGRectMake(0, 0, self.size.width, self.size.height)]; 
     UIImage *flipped_img = UIGraphicsGetImageFromCurrentImageContext(); 
     UIGraphicsEndImageContext(); 
     return flipped_img; 
  } 
@end