private void computeCrcForWriting(){
  crcengine=new CRC32();
  crcengine.update(idbytes,0,4);
  if (len > 0)   crcengine.update(data,0,len);
  PngHelperInternal.writeInt4tobytes((int)crcengine.getValue(),crcval,0);
}

private void computeCrcForWriting ( ) { crcengine = new CRC32 ( ) ; crcengine . update ( idbytes , 0 , 4 ) ; if ( len > 0 ) crcengine . update ( data , 0 , len ) ; PngHelperInternal . writeInt4tobytes ( ( int ) crcengine . getValue ( ) , crcval , 0 ) ; }
this is called after setting data , before writing to os
