PNG Encoder based on LibGX's Pixmap class

import java.io.ByteArrayOutputStream;
import java.io.DataOutputStream;
import java.io.IOException;

import com.badlogic.gdx.graphics.Pixmap;

/*
 * Minimal PNG encoder to create PNG streams (and MIDP images) from RGBA arrays.
 *
 * Copyright 2006-2009 Christian Fröschlin
 *
 * www.chrfr.de
 *
 *
 * Changelog:
 *
 * 09/22/08: Fixed Adler checksum calculation and byte order
 *           for storing length of zlib deflate block. Thanks
 *           to Miloslav Ruzicka for noting this.
 *
 * 05/12/09: Split PNG and ZLIB functionality into separate classes.
 *           Added support for images > 64K by splitting the data into
 *           multiple uncompressed deflate blocks.
 *
 * Terms of Use:
 *
 * You may use the PNG encoder free of charge for any purpose you desire, as long
 * as you do not claim credit for the original sources and agree not to hold me
 * responsible for any damage arising out of its use.
 *
 * If you have a suitable location in GUI or documentation for giving credit,
 * I'd appreciate a mention of
 *
 *  PNG encoder (C) 2006-2009 by Christian Fröschlin, www.chrfr.de
 *
 * but that's not mandatory.
 *
 */

public class PNG {
   public static byte[] toPNG(Pixmap pixmap) throws IOException {
      byte[] signature = new byte[] { (byte) 137, (byte) 80, (byte) 78,
            (byte) 71, (byte) 13, (byte) 10, (byte) 26, (byte) 10 };
      byte[] header = createHeaderChunk(pixmap.getWidth(), pixmap.getHeight());
      byte[] data = createDataChunk(pixmap);
      byte[] trailer = createTrailerChunk();

      ByteArrayOutputStream png = new ByteArrayOutputStream(signature.length
            + header.length + data.length + trailer.length);
      png.write(signature);
      png.write(header);
      png.write(data);
      png.write(trailer);
      return png.toByteArray();
   }

   public static byte[] createHeaderChunk(int width, int height)
         throws IOException {
      ByteArrayOutputStream baos = new ByteArrayOutputStream(13);
      DataOutputStream chunk = new DataOutputStream(baos);
      chunk.writeInt(width);
      chunk.writeInt(height);
      chunk.writeByte(8); // Bitdepth
      chunk.writeByte(6); // Colortype ARGB
      chunk.writeByte(0); // Compression
      chunk.writeByte(0); // Filter
      chunk.writeByte(0); // Interlace
      return toChunk("IHDR", baos.toByteArray());
   }

   public static byte[] createDataChunk(Pixmap pixmap) throws IOException {
      int width = pixmap.getWidth();
      int height = pixmap.getHeight();
      int source = 0;
      int dest = 0;
      byte[] raw = new byte[4 * (width * height) + height];
      for (int y = 0; y < height; y++) {
         raw[dest++] = 0; // No filter
         for (int x = 0; x < width; x++) {

            // 32-bit RGBA8888
            int pixel = pixmap.getPixel(x, y);

            int mask = pixel & 0xFFFFFFFF;
            int rr = (mask >> 24) & 0xff;
            int gg = (mask >> 16) & 0xff;
            int bb = (mask >> 8) & 0xff;
            int aa = (mask) & 0xff;

            if (rr < 0 || rr > 255 || gg < 0 || gg > 255 || bb < 0
                  || bb > 255) {
               // break ! (assert doesn't always kick-in with the
               // Eclipse
               // debugger...)
               int divide_by_zero = 0 / 0;
            }

            raw[dest++] = (byte) rr;
            raw[dest++] = (byte) gg;
            raw[dest++] = (byte) bb;
            raw[dest++] = (byte) aa;
            source++;
         }
      }
      return toChunk("IDAT", toZLIB(raw));
   }

   public static byte[] createTrailerChunk() throws IOException {
      return toChunk("IEND", new byte[] {});
   }

   public static byte[] toChunk(String id, byte[] raw) throws IOException {
      ByteArrayOutputStream baos = new ByteArrayOutputStream(raw.length + 12);
      DataOutputStream chunk = new DataOutputStream(baos);

      chunk.writeInt(raw.length);

      byte[] bid = new byte[4];
      for (int i = 0; i < 4; i++) {
         bid[i] = (byte) id.charAt(i);
      }

      chunk.write(bid);

      chunk.write(raw);

      int crc = 0xFFFFFFFF;
      crc = updateCRC(crc, bid);
      crc = updateCRC(crc, raw);
      chunk.writeInt(~crc);

      return baos.toByteArray();
   }

   static int[] crcTable = null;

   public static void createCRCTable() {
      crcTable = new int[256];

      for (int i = 0; i < 256; i++) {
         int c = i;
         for (int k = 0; k < 8; k++) {
            c = ((c & 1) > 0) ? 0xedb88320 ^ (c >>> 1) : c >>> 1;
         }
         crcTable[i] = c;
      }
   }

   public static int updateCRC(int crc, byte[] raw) {
      if (crcTable == null) {
         createCRCTable();
      }

      for (int i = 0; i < raw.length; i++) {
         crc = crcTable[(crc ^ raw[i]) & 0xFF] ^ (crc >>> 8);
      }

      return crc;
   }

   /*
    * This method is called to encode the image data as a zlib block as
    * required by the PNG specification. This file comes with a minimal ZLIB
    * encoder which uses uncompressed deflate blocks (fast, short, easy, but no
    * compression). If you want compression, call another encoder (such as
    * JZLib?) here.
    */
   public static byte[] toZLIB(byte[] raw) throws IOException {
      return ZLIB.toZLIB(raw);
   }
}

class ZLIB {
   static final int BLOCK_SIZE = 32000;

   public static byte[] toZLIB(byte[] raw) throws IOException {
      ByteArrayOutputStream baos = new ByteArrayOutputStream(raw.length + 6
            + (raw.length / BLOCK_SIZE) * 5);
      DataOutputStream zlib = new DataOutputStream(baos);

      byte tmp = (byte) 8;
      zlib.writeByte(tmp); // CM = 8, CMINFO = 0
      zlib.writeByte((31 - ((tmp << 8) % 31)) % 31); // FCHECK
                                          // (FDICT/FLEVEL=0)

      int pos = 0;
      while (raw.length - pos > BLOCK_SIZE) {
         writeUncompressedDeflateBlock(zlib, false, raw, pos,
               (char) BLOCK_SIZE);
         pos += BLOCK_SIZE;
      }

      writeUncompressedDeflateBlock(zlib, true, raw, pos,
            (char) (raw.length - pos));

      // zlib check sum of uncompressed data
      zlib.writeInt(calcADLER32(raw));

      return baos.toByteArray();
   }

   private static void writeUncompressedDeflateBlock(DataOutputStream zlib,
         boolean last, byte[] raw, int off, char len) throws IOException {
      zlib.writeByte((byte) (last ? 1 : 0)); // Final flag, Compression type 0
      zlib.writeByte((byte) (len & 0xFF)); // Length LSB
      zlib.writeByte((byte) ((len & 0xFF00) >> 8)); // Length MSB
      zlib.writeByte((byte) (~len & 0xFF)); // Length 1st complement LSB
      zlib.writeByte((byte) ((~len & 0xFF00) >> 8)); // Length 1st complement
                                          // MSB
      zlib.write(raw, off, len); // Data
   }

   private static int calcADLER32(byte[] raw) {
      int s1 = 1;
      int s2 = 0;
      for (int i = 0; i < raw.length; i++) {
         int abs = raw[i] >= 0 ? raw[i] : (raw[i] + 256);
         s1 = (s1 + abs) % 65521;
         s2 = (s2 + s1) % 65521;
      }
      return (s2 << 16) + s1;
   }
}

/**To use**/
/*
FileHandle image = Gdx.files.external("directory/filename.png");
OutputStream stream = image.write(false);
try {
byte[] bytes = PNG.toPNG(pixmap);
stream.write(bytes);
stream.close();
} catch (IOException e) {
// TODO: fallback if fail
}
*/