Il seguente metodo funziona perfettamente per me, quindi ecco la mia piena implementazione:
<img id="my_image" style="display: none;" src="my.png" width="924" height="330" border="0" usemap="#map" />
<map name="map" id="map">
<area shape="poly" coords="774,49,810,21,922,130,920,222,894,212,885,156,874,146" href="#mylink" />
<area shape="poly" coords="649,20,791,157,805,160,809,217,851,214,847,135,709,1,666,3" href="#myotherlink" />
</map>
<script>
$(function(){
var image_is_loaded = false;
$("#my_image").on('load',function() {
$(this).data('width', $(this).attr('width')).data('height', $(this).attr('height'));
$($(this).attr('usemap')+" area").each(function(){
$(this).data('coords', $(this).attr('coords'));
});
$(this).css('width', '100%').css('height','auto').show();
image_is_loaded = true;
$(window).trigger('resize');
});
function ratioCoords (coords, ratio) {
coord_arr = coords.split(",");
for(i=0; i < coord_arr.length; i++) {
coord_arr[i] = Math.round(ratio * coord_arr[i]);
}
return coord_arr.join(',');
}
$(window).on('resize', function(){
if (image_is_loaded) {
var img = $("#my_image");
var ratio = img.width()/img.data('width');
$(img.attr('usemap')+" area").each(function(){
console.log('1: '+$(this).attr('coords'));
$(this).attr('coords', ratioCoords($(this).data('coords'), ratio));
});
}
});
});
</script>