requires okio.jar in your classpath. see: https://github.com/square/okio

nudge4j.PNGEncoder.java

//
// PNG Encoder. This script requires okio.jar 
//

var URL =        Java.type('java.net.URL'); 
var ByteString = Java.type('okio.ByteString');
var Okio =       Java.type('okio.Okio');
var Buffer =     Java.type('okio.Buffer');

var url = new URL('http://i936.photobucket.com/albums/ad204/0_Marathon_0/Abstract.png');

var PNG_HEADER = ByteString.decodeHex("89504e470d0a1a0a");
var pngSource = Okio.buffer(Okio.source(url.openStream()));

var header = pngSource.readByteString(PNG_HEADER.size());
if (!header.equals(PNG_HEADER)) {
    throw "Not a PNG.";
} 

var str =''
while (true) {
    
    var chunk = new Buffer();

    // Each chunk is a length, type, data, and CRC offset.
    var length = pngSource.readInt();
    var type = pngSource.readUtf8(4);
    pngSource.readFully(chunk, length);
    var crc = pngSource.readInt();
    
    if (type == "IHDR" ) {
      var width = chunk.readInt();
      var height = chunk.readInt();
      str+=  chunk.size()+' '+ type+' '+ width+' '+height +'\n';
    } else {
      str += chunk.size()+' '+ type+'\n';
      // System.out.printf("%08x: %s%n", chunk.size(), type);
    }
    
    if (type.equals("IEND")) break;
}


pngSource.close();
str