Rotate Image

Two different jobs that share a name

Rotating by 90, 180, or 270 degrees is a quarter turn, and it moves every pixel to a new position without changing any of them. It is completely lossless and fully reversible, because rotating back restores the original exactly. Rotating by a small arbitrary angle is a different operation: the image has to be turned within a larger frame, and the triangular areas at the corners have to be filled with something. That step is where a small rotation becomes lossy, and it is the more common real need.

Fixing a tilted photo, and the crop that follows

Most tilted shots are only a few degrees out, and correcting that means rotating and then cropping away the empty corners, because rotating alone leaves triangular gaps. The sensible order is rotate to level the horizon or the vertical, then crop to the largest rectangle that fits inside the rotated frame. Doing it in that order keeps the maximum amount of the photo. Rotating and then trying to fill the corners with a colour rarely looks natural, because the fill is not part of the scene.

• Rotate first to level the subject, then crop to the largest rectangle that fits.
• Filling the corners with a colour looks artificial. Cropping is better.
• Rotate back and check the level before committing to a crop.
• A horizon or a door frame is a better reference than the subject itself.
• Crop generously enough to remove every corner, and no more.

NoteThe order matters. Rotating then cropping keeps more of the photo than cropping then rotating.

Why 90 degree turns happen at all

The most common rotation request is a phone photo that is sideways or upside down, because the camera did not record an orientation tag or the receiving program ignored it. That is a quarter turn and it costs nothing. The other common case is a scanned document that arrived rotated, which is usually 90 or 180 degrees. In both cases a quarter turn is the right tool and there is no quality consideration at all, which makes it one of the few edits where nothing is lost.

• Sideways phone photo: a 90 degree quarter turn, fully lossless.
• Upside down photo: 180 degrees, also fully lossless.
• Scanned document: usually 90 or 180 degrees.
• No quality cost at all for any quarter turn.

Metadata and orientation tags

Some images do not need rotating at all, because the pixels are already upright and the file merely carries a tag telling a program to turn them for display. If an image looks correct in one program and sideways in another, that is the tag being honoured inconsistently rather than the file being wrong. That is worth knowing because in that case rotating produces a file whose pixels are now genuinely turned, and which will display incorrectly in a program that also applies the original tag. Converting to JPG while rotating usually resolves the ambiguity.

• An orientation tag tells programs how to display the file. Not every program honours it.
• If an image is correct in one viewer and sideways in another, that is the likely cause.
• Rotating the pixels and saving as JPG removes the ambiguity.
• If the image displays sideways everywhere, the pixels really are turned and should be rotated.