title: PNG to GeoJSON (or similiar format)
body: <p>I have PNG files wich have yellow, orange and red polygons on a white background . I would like to extract the polygons to GeoJSON. 
  I already did some research and found out that I can maybe use <a href="http://www.gdal.org/gdal_polygonize.html" rel="nofollow">gdal_polygonize</a> to extract the polygons and than use <a href="http://www.gdal.org/drv_geojson.html" rel="nofollow">drv_geojson</a> to make it to GeoJSON (both are part of Gdal)</p>
  
  <p>Here are my problems/questions: 
  1) What is the best way to call these scripts from java?
  2) What are the necessary arguments?
  3) Is there a better way you know? Maybe a Java library which does all this ?</p>