Scale2x (EPX)
A rule-based upscaler for pixel graphics that doubles the resolution and rounds off the staircases — without inventing a single new colour.
Each source pixel becomes four. The algorithm looks at the neighbours above, below, left and right: if, say, the top and left ones match each other but differ from the other two, the top-left corner takes their colour. Staircases turn into diagonals, and the image still contains only the original palette.
Eric Johnston devised the method around 1992 at LucasArts as EPX, to port SCUMM games to the Macintosh's higher resolution. Andrea Mazzoleni rediscovered the same rule in 2001 for AdvanceMAME and called it Scale2x. Successors such as hqx and xBR compare more neighbours and blend colours, which buys smoother curves at the cost of fidelity to the pixel art.