Right angles cost nothing
Rotating by ninety or a hundred and eighty degrees moves whole pixels to new positions. Nothing is resampled, nothing is interpolated, and the result is pixel-for-pixel identical to the original with the coordinates rearranged.
That is why the [msm_tool slug="image-rotate"] offers only quarter and half turns. An arbitrary angle would require resampling, which softens the image and leaves triangular gaps at the corners that have to be filled or cropped away. Those are real costs, and for the common case — a photograph that came out sideways — they are unnecessary.
For a quarter turn the canvas swaps its width and height first, or the image would be cut off by its own frame.
Your camera may have already done it
Phones and cameras frequently store a photograph in the orientation the sensor captured it, along with an EXIF tag saying which way up it should be displayed. Browsers and most modern software apply that tag automatically.
So an image that already looks correct is correct, and rotating it will make it wrong. If a photograph looks sideways in one program and upright in another, the tag is present and one of them is ignoring it — the fix is to bake in the rotation, not to guess.
Flipping is not rotating
A flip mirrors; a rotation turns. They are different operations with different consequences.
Text in a flipped image reads backwards. A face becomes subtly wrong — human faces are not symmetrical, and a mirrored portrait looks like a different person to anyone who knows the subject, even if they cannot say why. Number plates, signage and logos all become nonsense.
Use rotation to correct orientation. Use flipping only when a mirror image is genuinely what you want — matching a subject's gaze direction to a layout, for instance, and only when the frame contains no text.
Vertical flip in two steps
A horizontal flip is offered directly. A vertical mirror is a half turn followed by a horizontal flip — the two together produce exactly that.
Doing it in one pass rather than saving between the two steps means one encode rather than two, which matters if the output format is lossy.
PNG output
The result is lossless and any transparency survives. It is also larger — a rotated photograph will be considerably bigger than the JPEG it came from, despite containing exactly the same pixels.
Convert afterwards if file size matters. One re-compression at the end costs less than compressing at every step of a workflow.
Quarter turns change the shape
A landscape image becomes portrait. Anything downstream that expects a fixed aspect ratio — a CSS grid cell, a social card, a print layout — needs that accounted for.
It sounds obvious and it is a routine surprise when rotating a batch of images that then no longer fit the template they were prepared for.
When you do need an arbitrary angle
Straightening a horizon or a scanned document requires a small rotation, which requires resampling and leaves corners to crop. That is a job for an editor with a straightening tool, which will handle the crop for you.
This is for the ninety-degree case, which is most of them.