specialized markup language for representing PNG contents

 SNG (Scriptable Network Graphics) is a minilanguage designed
 specifically to represent the entire contents of a PNG (Portable
 Network Graphics) file in an editable form. Thus, SNGs representing
 elaborate graphics images and ancillary chunk data can be readily
 generated or modified using only text tools.
 .
 SNG is implemented by a compiler/decompiler called sng that
 losslessly translates between SNG and PNG.