Seamless Tile Maker

This tool automates the "set up the tile" step of making a seamless pattern: it swaps opposite quadrants (top-left ⇄ bottom-right, top-right ⇄ bottom-left) so the original edges meet in the middle and the outer edges of your image now line up.

After downloading, you'll likely still want to fill the gap in the center by hand in your art program — the swap pushes your artwork to the edges and leaves the middle to be connected. Best results come from a source image with artwork concentrated near the center and blank margins at the edges.