SNG (Scriptable Network Graphics) is a textual mini-language designed
specifically to reflect the data and structure of PNG (Portable
Network Graphics) files in an editable form.

The sng program translates between SNG and PNG. It can be useful
especially for viewing and editing data in obscure ancillary chunks
that graphics editors won't handle. Also, since SNG is easy to
generate from scripts, sng may be useful at the end of a pipeline
that programmatically generates PNG images.