Flip UIImage Along Either Axis

Question

I\'m trying to create a method which flips a UIImage along the X axis, Y axis, or both. I keep getting close but my transform knowledge isn\'t good enough to get all the way

Answer 1

If you're intending to just display the image and not save it, you probably don't want to create a second CGContext. It's far more efficient to just use the already-loaded CGImage in the UIImage, and change the orientation like so:

- (UIImage *)flippedImageByFlippingAlongXAxis:(BOOL)flipOnX andAlongYAxis:(BOOL)flipOnY
{
    UIImageOrientation currentOrientation = self.imageOrientation;
    UIImageOrientation newOrientation = currentOrientation;

    if (flipOnX == YES) {
        switch (newOrientation) {
            case UIImageOrientationUp:
                newOrientation = UIImageOrientationUpMirrored;
                break;
            case UIImageOrientationDown:
                newOrientation = UIImageOrientationMirrored;
                break;
            case UIImageOrientationUpMirrored:
                newOrientation = UIImageOrientationUp;
                break;
            case UIImageOrientationDownMirrored:
                newOrientation = UIImageOrientationDown;
                break;
            case UIImageOrientationLeft:
                newOrientation = UIImageOrientationLeftMirrored;
                break;
            case UIImageOrientationRight:
                newOrientation = UIImageOrientationRightMirrored;
                break;
            case UIImageOrientationLeftMirrored:
                newOrientation = UIImageOrientationLeft;
                break;
            case UIImageOrientationRightMirrored:
                newOrientation = UIImageOrientationRight;
                break;
        }
    }

    if (flipOnY == YES) {
        switch (newOrientation) {
            case UIImageOrientationUp:
                newOrientation = UIImageOrientationDownMirrored;
                break;
            case UIImageOrientationDown:
                newOrientation = UIImageOrientationUpMirrored;
                break;
            case UIImageOrientationUpMirrored:
                newOrientation = UIImageOrientationDown;
                break;
            case UIImageOrientationDownMirrored:
                newOrientation = UIImageOrientationUp;
                break;
            case UIImageOrientationLeft:
                newOrientation = UIImageOrientationRightMirrored;
                break;
            case UIImageOrientationRight:
                newOrientation = UIImageOrientationLeftMirrored;
                break;
            case UIImageOrientationLeftMirrored:
                newOrientation = UIImageOrientationRight;
                break;
            case UIImageOrientationRightMirrored:
                newOrientation = UIImageOrientationLeft;
                break;
        }
    }

    return [UIImage imageWithCGImage:self.CGImage scale:self.scale orientation:newOrientation];
}

When you start a new CGContext and doing flip using CGContextDrawImage, you're allocating another block of memory to hold the same bytes in a different order. By changing the UIImage orientation, you're able to avoid another allocation. The same image data is used, just drawn in a different orientation.