get average color of an imagedata object (from an html5 canvas) in javascript

average-rgba.js

/*global define*/
define(
	function()
	{
		var i;
		var len;
		var multiplicator = 20;
		var count;
		var rgba;

		function getAverageRGBA( image_data, resolution )
		{
			multiplicator = parseInt( resolution, 10 ) > 1 ? parseInt( resolution, 10 ) : 10;
			len = image_data.data.length;
			count = 0;
			rgba = [ 0, 0, 0, 0 ];

			for ( i = 0; i < len; i += multiplicator * 4 )
			{
				rgba[0] = rgba[0] + image_data.data[i];
				rgba[1] = rgba[1] + image_data.data[i + 1];
				rgba[2] = rgba[2] + image_data.data[i + 2];
				rgba[3] = rgba[3] + image_data.data[i + 3];

				count++;
			}

			rgba[0] = ~~ ( rgba[0] / count );
			rgba[1] = ~~ ( rgba[1] / count );
			rgba[2] = ~~ ( rgba[2] / count );
			rgba[3] = ~~ ( rgba[3] / count );

			return rgba;
		}

		return getAverageRGBA;
	}
);