public static final int TARGA_HEADER_SIZE = 18;

void screenshot(GL gl, int width, int, height, File file) {
  try {
    RandomAccessFile out = new RandomAccessFile(file, "rw");
    FileChannel ch = out.getChannel();
    int fileLength = TARGA_HEADER_SIZE + width * height * 3;
    out.setLength(fileLength);
    MappedByteBuffer image = ch.map(FileChannel.MapMode.READ_WRITE, 0, fileLength);

    // write the TARGA header
   image.put(0, (byte) 0).put(1, (byte) 0);
   image.put(2, (byte) 2); // uncompressed type
   image.put(12, (byte) (width & 0xFF)); // width
   image.put(13, (byte) (width >> 8)); // width
   image.put(14, (byte) (height & 0xFF)); height
   image.put(15, (byte) (height >> 8)); height
   image.put(16, (byte) 24); // pixel size
              
   // go to image data position
   image.position(TARGA_HEADER_SIZE);
   // jogl needs a sliced buffer
   ByteBuffer bgr = image.slice();
   // read the BGR values into the image buffer
   gl.glReadPixels(0, 0, width, height, GL.GL_BGR,
                      GL.GL_UNSIGNED_BYTE, bgr);

   // close the file channel
   ch.close();
  } catch (Exception e) {
     e.printStackTrace();
  }
}

screenshot( canvas.getGL(), canvas.getWidth(), canvas.getHeight(), imageFile );