etc1tool is a command line utility that lets you encode PNG
 images to the ETC1 compression standard and decode ETC1 compressed images back to PNG.
The usage for etc1tool is:
etc1tool infile [--help | --encode | --encodeNoHeader | --decode] [--showDifference diff-file] [-o outfile]
| Option | Description | 
|---|---|
infile | 
      The input file to compress | 
--help | 
      Print usage information | 
--encode | 
      Create an ETC1 file from a PNG file. This is the default mode for the tool if nothing is specified. | 
--encodeNoHeader | 
      Create a raw ETC1 data file (without a header) from a PNG file. | 
--decode | 
      Create a PNG file from an ETC1 file | 
--showDifference diff-file | 
      Write the difference between the original and encoded image to
      diff-file (only valid when encoding). | 
    
-o outfile | 
      Specify the name of the output file.
      If outfile is not specified, the output file is constructed
      from the input filename with the appropriate suffix (.pkm or .png).
       |