Crop, rotate and flip an image

These are one job on one image, which is why they sit on one page: each is the same canvas draw with a different transform in front of it, and the file is read in, drawn and read back without ever going to a server. What differs between them is what they cost.

Cropping costs nothing. Resizing resamples; every output pixel is interpolated from several input ones, which is why an enlarged image looks soft. Cropping discards the pixels outside the box and keeps the rest exactly as they were, so a crop of a sharp photo is still sharp. That is worth knowing because it means you should crop first and resize second, never the other way round: resizing to a target and then cropping throws away resolution you had. The ratio presets exist because most places you post an image want a specific one, and a picture cropped to the wrong ratio gets cropped again by the platform, usually badly and usually through someone’s face. 1:1 for most profile pictures, 16:9 for video thumbnails and slides, 9:16 for stories and short video, 4:3 for the classic camera frame.

Rotating costs nothing at 90, 180 and 270 degrees, because every pixel lands exactly on another pixel. Any other angle has to interpolate, so fine detail softens slightly and hard edges pick up a faint stair-step; make the correction once from the original and not in several nudges. The detail most rotate tools get wrong is the canvas size. A quarter turn just swaps width and height, but at 45° a square needs a canvas about 1.41 times as wide or the corners are cut off, and because the clipping happens at the edges it often is not noticed until later. The canvas here is sized from the rotated bounding box, so the whole image survives at any angle. A related thing worth knowing: a photo that looks rotated on one device and upright on another is usually not rotated at all. Cameras store the orientation as an EXIF tag and leave the pixels alone, so software that ignores the tag shows it sideways. Saving from here bakes the current orientation in and drops the tag, which is what makes the rotation stick everywhere.

Flipping costs nothing either, because it only reorders pixels. The usual reason people want it is a selfie: front cameras show a mirrored preview because that is what you are used to seeing, but most save the unmirrored version, so the photo looks subtly wrong to you and completely normal to everyone else. The one thing to check first is text. Anything written in the image — a sign, a label, a watermark, a number plate — becomes backwards, and it is easy to miss when the text is small or at the edge. Flipping is also used to double a training set for machine learning, and there it is safe for most subjects and wrong for a few: anything where handedness matters is corrupted by a horizontal flip, not augmented.

Rounding cuts the corners with a clipping path instead of filling them with a colour, which is the difference between an image that works anywhere and one that only works on white. A filled corner looks fine on the page you designed it for and shows as an obvious white wedge on a dark background. A circle needs a square source: the radius is measured against the shorter side, so at 50% a rectangle comes out as a stadium shape with rounded ends. The edits do not stack — every tab draws from the file you opened, not from the result of the last one — so squaring a rectangle means cropping it to 1:1, saving that, and opening the saved file again before rounding it.

A border grows the canvas instead of painting over the edges, so the whole original picture survives; a border drawn inside the frame would cover a strip of the image, which is what a naive implementation does. The width is the same on all four sides, so it changes the size and barely the shape: a 300 × 400 photo with a 20-pixel border comes out 340 × 440, still taller than it is wide. It will not letterbox a portrait photo into a square, which needs padding on two sides only. What it is for is margin — breathing room around a picture dropped on a slide, or a visible edge so a photo with a pale background does not bleed into a white page. Bear in mind the border becomes part of the image: a platform that crops to a circle will crop through it, and a dark-mode reader will see a white frame that does not belong.

On saving: rounding, bordering and off-axis rotation either keep transparency or create it, and JPEG has no alpha channel, so those are written as PNG. Cropping and flipping cannot introduce transparency, so those let you choose, and the choice matters more than it looks. PNG stores every kept pixel exactly, and a photograph stored that way is routinely two or three times the size of the JPEG it came from — you threw pixels away and the file grew. JPEG costs one more compression pass, which you will not see on a photograph at high quality, and is the right answer for anything going back on the web; PNG is right for a screenshot, a logo or anything with flat colour and hard edges. If the result still needs to be smaller than the format alone makes it, compress it afterwards; cropping tighter to save bytes is the wrong lever.

One side effect worth knowing about before you post a photo you did not take: none of these edits touch metadata directly, but drawing to a canvas drops the EXIF block, so location, camera and timestamp do not survive. That is a genuine privacy improvement here, but do not rely on any tool to strip metadata unless it says so.