PNGJ: A simple library for reading/writing PNG images.

Focused on high resolution images, both huge in size (and hence not appropiate to be loaded in memory, eg. as a BufferedImage) and quality (the library is dedicated to truecolor images, with 8 and 16 bits per sample, with or without alpha). It provides basic line-oriented reading and writing capabilities.

A quick example: this code reads a PNG image file (true colour, 8-16 bpp, RGB-RGBA) and rewrites it cutting the red channel by two.

    public static void decreaseRed(String origFilename, String destFilename) {
      PngReader pngr = new PngReader(origFilename);
      PngWriter pngw = new PngWriter(destFilename, pngr.imgInfo);
      pngw.setOverrideFile(true); // allows to override writen file if it already exits
      System.out.println(pngr.toString());
      pngw.prepare(pngr); // not necesary; but this can copy some informational chunks from original 
      int channels = pngr.imgInfo.channels;
      if(channels<3) throw new RuntimeException("Only for truecolour images");
      for (int row = 0; row < pngr.imgInfo.rows; row++) {
        ImageLine l1 = pngr.readRow(row);
        for(int j=0;j<pngr.imgInfo.cols;j++)
          l1.scanline[j*channels]/=2;
        pngw.writeRow(l1);
      }
      pngr.end();
      pngw.end();
   }
  

See the docs and source of the samples for more.