How to use it
- Open a PNG with a transparent background.
- Set the Outline width (0 to 60 px) and pick a colour; white is the classic sticker look.
- Keep Soft shadow on for a lifted look, or switch it off.
- Download the PNG; the transparent area around the sticker stays transparent.
How it works
The picture is placed on a larger transparent canvas. A distance map from the visible pixels is computed in two passes with weights 1 for sideways steps and the square root of 2 for diagonal steps. Pixels within the outline width get the outline colour, the optional shadow is drawn under it, and the original picture is drawn on top.
Worked example
One visible pixel followed by transparent pixels in a row has distances 0, 1, 2, 3 and so on. With an outline width of 2, pixels at distance 1 are fully filled, the pixel at distance 2 is half filled (the soft edge) and the one at distance 3 is empty.
Limitations
The tool does not cut subjects out of photos. Only the transparency already in the file defines the shape, and semi-transparent pixels below about half opacity count as empty.