使用UIImageOrientationDownMirrored/UIImageOrientationUpMirrored翻转图片无效问题

Let me break down what's likely going wrong here and how to fix it—this is a common pitfall with UIImageOrientation because it's easy to misunderstand how the property works.

The Root Problem #

UIImageOrientation doesn't represent a transformation applied to the current image state—it describes how the image's pixel data relates to the standard "Up" orientation (the original, unmodified image). When you apply UpMirrored or DownMirrored, you're not altering the pixel data; you're just setting a flag that tells the system how to render the pixels.

If you're trying to "reverse" the flip by applying what you think is the opposite orientation (e.g., using DownMirrored to undo UpMirrored), that's where the error happens. Those orientations aren't direct opposites—UpMirrored is a horizontal flip relative to the original image, while DownMirrored is a 180° rotation plus a horizontal flip relative to the original. Applying the wrong "reverse" orientation will leave you with a distorted image, not the original.

The Correct Approach #

There are two reliable ways to handle this workflow:

1. Always Keep a Reference to the Original Image

This is the simplest solution. Instead of overwriting your original image when you apply flips, store the unmodified, orientation-Up image in a separate variable (e.g., originalImage).

When you need to apply a new flip:

• Ditch the currently flipped image entirely
• Create a new flipped image directly from originalImage using your desired new orientation

This eliminates the need to "restore" anything—you're always starting fresh from the source.

2. Calculate the Proper Inverse Orientation (If You Can't Keep the Original)

If you must work with transformed images and track orientations, you need to use the correct inverse orientation for each flip type. Here's how each orientation maps to its inverse (the orientation that will return you to the original image):

• Up ↔ Up (no change needed)
• UpMirrored ↔ UpMirrored (flipping horizontally twice returns to original)
• Down ↔ Down (rotating 180° twice returns to original)
• DownMirrored ↔ DownMirrored
• Left ↔ Right (counter-clockwise 90° reversed by clockwise 90°)
• LeftMirrored ↔ RightMirrored
• Right ↔ Left
• RightMirrored ↔ LeftMirrored

You can wrap this logic in a helper function to avoid mistakes:

func inverseOrientation(for orientation: UIImage.Orientation) -> UIImage.Orientation {
    switch orientation {
    case .up: return .up
    case .upMirrored: return .upMirrored
    case .down: return .down
    case .downMirrored: return .downMirrored
    case .left: return .right
    case .leftMirrored: return .rightMirrored
    case .right: return .left
    case .rightMirrored: return .leftMirrored
    @unknown default: return .up
    }
}

When you need to restore the original, apply this inverse orientation to your current image, then apply the new flip orientation to the restored image.

Example Workflow #

Let's say you start with originalImage (orientation Up):

1. Apply UpMirrored to get flippedImage (orientation UpMirrored)
2. When you want to switch to DownMirrored:
   • Option 1 (preferred): Create newFlippedImage directly from originalImage with orientation DownMirrored
   • Option 2: Restore flippedImage using inverseOrientation(.upMirrored) (which is UpMirrored) to get back to the original, then apply DownMirrored

Key Takeaway #

Stop thinking of UIImageOrientation as a transformation applied to the current image—it's always relative to the original pixel data. Keeping the original image handy is the most foolproof way to avoid these reverse-flip errors.

内容的提问来源于stack exchange，提问作者Sudhanshu Monga