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

Try the following:

UIImage* myimage = [UIImage imageNamed: @"myimage.png"];
myimage = [UIImage imageWithCGImage: myimage.CGImage scale: 1.0f orientation: UIImageOrientationUpMirrored];

Answer 2

This is merely a slightly fixed and 'updated to Swift' version of the old answer by @ultramiraculous. Just in case it helps someone.

Vertical Flip (reflection over x-axis):

func flipV(im:UIImage)->UIImage {
    var newOrient:UIImageOrientation
    switch im.imageOrientation {
    case .Up:
        newOrient = .DownMirrored
    case .UpMirrored:
        newOrient = .Down
    case .Down:
        newOrient = .UpMirrored
    case .DownMirrored:
        newOrient = .Up
    case .Left:
        newOrient = .LeftMirrored
    case .LeftMirrored:
        newOrient = .Left
    case .Right:
        newOrient = .RightMirrored
    case .RightMirrored:
        newOrient = .Right
    }
    return UIImage(CGImage: im.CGImage, scale: im.scale, orientation: newOrient)
}

Horizontal Flip (reflection over y-axis)

func flipH(im:UIImage)->UIImage {
    var newOrient:UIImageOrientation
    switch im.imageOrientation {
    case .Up:
        newOrient = .UpMirrored
    case .UpMirrored:
        newOrient = .Up
    case .Down:
        newOrient = .DownMirrored
    case .DownMirrored:
        newOrient = .Down
    case .Left:
        newOrient = .RightMirrored
    case .LeftMirrored:
        newOrient = .Right
    case .Right:
        newOrient = .LeftMirrored
    case .RightMirrored:
        newOrient = .Left
    }
    return UIImage(CGImage: im.CGImage, scale: im.scale, orientation: newOrient)
}

Answer 3

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.

Answer 4

I finally was able to figure this out. Here is the code that works for anyone else who might need it.

- (UIImage *)flippedImageByAxis:(MVImageFlip)axis{
    UIGraphicsBeginImageContext(self.size);
    CGContextRef context = UIGraphicsGetCurrentContext();

    if(axis == MVImageFlipXAxis){
        // Do nothing, X is flipped normally in a Core Graphics Context
    } else if(axis == MVImageFlipYAxis){
        // fix X axis
        CGContextTranslateCTM(context, 0, self.size.height);
        CGContextScaleCTM(context, 1.0f, -1.0f);

        // then flip Y axis
        CGContextTranslateCTM(context, self.size.width, 0);
        CGContextScaleCTM(context, -1.0f, 1.0f);
    } else if(axis == MVImageFlipXAxisAndYAxis){
        // just flip Y
        CGContextTranslateCTM(context, self.size.width, 0);
        CGContextScaleCTM(context, -1.0f, 1.0f);
    }

    CGContextDrawImage(context, CGRectMake(0.0, 0.0, self.size.width, self.size.height), [self CGImage]);

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

    return flipedImage;
}

Answer 5

Swift 3 version:

func flipV(im:UIImage)->UIImage {
        var newOrient:UIImageOrientation
        switch im.imageOrientation {
        case .up:
            newOrient = .downMirrored
        case .upMirrored:
            newOrient = .down
        case .down:
            newOrient = .upMirrored
        case .downMirrored:
            newOrient = .up
        case .left:
            newOrient = .leftMirrored
        case .leftMirrored:
            newOrient = .left
        case .right:
            newOrient = .rightMirrored
        case .rightMirrored:
            newOrient = .right
        }
        return UIImage(cgImage: im.cgImage!, scale: im.scale, orientation: newOrient)
    }

    func flipH(im:UIImage)->UIImage {
        var newOrient:UIImageOrientation
        switch im.imageOrientation {
        case .up:
            newOrient = .upMirrored
        case .upMirrored:
            newOrient = .up
        case .down:
            newOrient = .downMirrored
        case .downMirrored:
            newOrient = .down
        case .left:
            newOrient = .rightMirrored
        case .leftMirrored:
            newOrient = .right
        case .right:
            newOrient = .leftMirrored
        case .rightMirrored:
            newOrient = .left
        }
        return UIImage(cgImage: im.cgImage!, scale: im.scale, orientation: newOrient)
    }