Rotate & Flip Images

Why quarter turns are free and other angles are not

A quarter turn or a mirror moves every pixel to a position another pixel already occupied. Nothing is averaged, nothing is invented, and the result contains exactly the same pixel values in a different arrangement — the operation is mathematically lossless before the file is written. Any other angle is not: rotating a picture by two degrees to straighten a horizon means every new pixel falls between four old ones and has to be calculated from them, which softens the whole image slightly and leaves wedge-shaped gaps at the corners that must be cropped or filled. That is a different job, done deliberately, and not what this page does.

There is a second reason a photograph can look sideways, and it has nothing to do with the pixels. Phones and cameras usually store the picture in the sensor's own orientation and record which way up it should be shown as an orientation tag in the file. Software that reads the tag displays it correctly; software that ignores it shows the same file rotated — which is why a photo can appear upright on your phone and sideways after being emailed or posted. Rotating here writes the turn into the pixels themselves, so the file is right whether or not anything reads the tag. This is the fix for a picture that keeps coming out wrong in one particular program.

Mirroring is the operation people reach for less often and misjudge more. Flipping horizontally is fine for a portrait or an abstract shape and immediately wrong for anything containing text, a clock face, a number plate, a road sign or a logo — all of which read as backwards even when the viewer cannot say why. It is genuinely useful for correcting a selfie, which most phone cameras have already mirrored on your behalf.

The file is written back in its original format. For a PNG that is lossless end to end; for a JPG the re-encoding is a second generation of compression, invisible at a sensible quality but real. Metadata, including the orientation tag that may have caused the problem, does not survive the rewrite.

When you want something else

For a JPG there is a strictly better method available outside a browser. jpegtran -rotate 90 -copy all in.jpg out.jpg turns the picture by rearranging its compressed blocks without ever decoding it, so the result is bit-for-bit the same quality as the original — no second generation at all. exiftran -a does the same for a whole folder and reads each file's own orientation tag to work out which way to turn it.

When the picture needs straightening rather than turning, this is the wrong tool and says so. magick in.jpg -deskew 40% out.jpg finds the dominant angle of a scanned page and corrects it, and any photo editor will do an arbitrary rotation with a crop. For a stack of crooked scans, unpaper was written for exactly that and is free.