function gradientKey(imf){
          var hash = ""
          for(var i = 0; i < imf.args.length; i++){
            var rgb = imf.args[i].rgb;
            hash += rgb[0] + ":" + rgb[1] + ":" + rgb[2];
          }
          return hash;
        }
get active points for a step in active zoom
returns a list of bounding boxes [[sw, ne] , [], []] where ne is a {lat: .., lon: ...} obj
empty list if there is no active pixels