Factorio Prototype DocsVersion 1.1.107

Icons of reduced size will be used at decreased scale. 0 or 1 mipmaps is a single image. The file must contain half-size images with a geometric-ratio, for each mipmap level. Each next level is aligned to the upper-left corner, with no extra padding. Example sequence: 128x128@(0,0), 64x64@(128,0), 32x32@(192,0) is three mipmaps.

See here for more about the visual effects of icon mipmaps.