Add a border to an image

The border grows the canvas rather than 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 almost never what people want and is what a naive implementation does.

The practical use most people have is padding a photo to a required ratio without cropping it. A tall photo that must be square for a grid can take white bars at the sides instead of losing its top and bottom, which is the same trick as letterboxing a film. Bear in mind that the border becomes part of the image: a platform that crops to a circle will crop through the border, and a dark-mode reader will see a white frame that does not belong.

On colour choice: matching the border to the background it will sit on makes it invisible and is the point when you are padding for a ratio. A contrasting border is a design decision and works best when it is either much lighter or much darker than the photo’s edges — a mid-grey border against a mid-grey photo edge reads as a mistake.

The output is PNG so the border colour is exact. A JPEG would introduce compression artefacts along the hard edge between border and photo, which is exactly the kind of boundary JPEG handles worst.