Flipping mirrors an image across an axis. Horizontal is by far the more common: it swaps left and right, which is what you want for undoing a selfie, matching the direction a subject faces, or producing a mirrored pair of graphics.
The selfie problem
Front-facing cameras usually show you a mirrored preview, because that is how you are used to seeing yourself. Some phones then save the mirrored version and some save the true one, and manufacturers disagree about which is correct. The result is that a photo can look subtly wrong to you but correct to everyone else, or the reverse. Flipping horizontally swaps between the two.
The giveaway is text. If there is any writing in the shot — a sign, a logo on clothing, a book — it reads correctly in exactly one of the two versions, and that is the true one regardless of which looks more familiar.
Where flipping is genuinely useful
- Making a subject face into a layout rather than out of it, which is a basic rule of page composition.
- Producing a mirrored pair from one asset — bookends, symmetrical decorations, opposing arrows.
- Correcting scans and photographs taken through a mirror or a viewfinder that inverts.
- Preparing artwork for iron-on transfers and other reverse-printing processes, which need a mirrored source.
- Vertical flips, which are rare in photography but normal for correcting an image saved upside down or a texture with the wrong origin.
When not to flip a photograph
Anything with legible text becomes nonsense, which is the obvious one. Less obvious: photographs of real places become subtly false — traffic drives on the wrong side, buildings are the wrong way round, people wear watches and part their hair on the wrong side. For editorial, documentary or evidential images that matters, and flipping is generally considered a misrepresentation rather than a stylistic choice.
It costs nothing in quality
Mirroring moves each pixel to a new position without recalculating it, so the result contains exactly the same pixel values in a different arrangement. Combined with PNG output, which is lossless, the flipped file is a perfect copy of the original in every respect except orientation. Dimensions are unchanged too — unlike a quarter-turn rotation, a flip does not swap width and height.
Selecting both axes at once produces the same result as rotating 180 degrees, which is worth knowing if that is actually what you meant. Everything runs in this tab, with nothing uploaded.