EPSILON (in all capitals, though this does not appear to stand for anything) is an raster image compression format and program created by Alexander Simakov, and based on the Asad Islam and William Pearlman's SPECK algorithm. It is composed of a number of independent blocks, each encoding a region of the original image, concatenated together, which means that images can be encoded in parallel. The EPSILON program can only convert to and from PPM and PGM.
EPSILON apparently did its own encryption before version 0.3.1.
As of 2020, the last release was in 2011.
EPSILON blocks, and therefore EPSILON files, will begin with
EPSILON uses the extension ".psi", for "ePSIlon".