CentroidFinder.js

/* Center of Gravity Finder

This script will find the center of gravity (centroid) of all closed paths in the selection set. It was built for a specific purpose so does not have much error
handling. For example, if you get errors it may be the result of the selection set containing compound shapes or a self-intersecting polygon. 

References for the math:
http://paulbourke.net/geometry/polygonmesh/ (Calculating the area and centroid of a polygon)
https://en.wikipedia.org/wiki/Centroid#Centroid_of_polygon

Save this file with a jsx extension and place in your Illustrator/Presets/en_US/Scripts folder. You can then access it from the File > Scripts menu

Made by: Scott Swaaley, April 2, 2017

*/

var holeDiameter = 0.125 * 72; // in points

if (app.documents.length > 0 && app.activeDocument.selection.length > 0) {

	
	//initialize log file	
	var reportFile =new File(app.activeDocument.path+'/'+'AI_SCRIPT_LOG.txt');
	reportFile.open('w'); // open with write access
	reportFile.writeln(new Date().toLocaleString());
	
	for(var selectionIndex = 0; selectionIndex < app.activeDocument.selection.length; selectionIndex++) {
		var object =  app.activeDocument.selection[selectionIndex];
		reportFile.writeln("Selection Index:" + selectionIndex);
		
		// copy anchor points into array
		// The equations used below require that " the vertex ( xn, yn ) is assumed to be the
		// same as ( x0, y0 ), meaning i + 1 on the last case must loop around to i = 0."[https://en.wikipedia.org/wiki/Centroid#Centroid_of_polygon]
		var arrayOfAnchors = new Array();
		for(var i = 0; i < object.pathPoints.length;i++) arrayOfAnchors.push(object.pathPoints[i].anchor); // fill array with shape's anchor point coordinates
		arrayOfAnchors.push(arrayOfAnchors[0]);
		reportFile.writeln("Number of Anchor Points (exclusive):" + object.pathPoints.length);
		
		var shapeArea = 0;
		var Cx = 0;
		var Cy = 0;
		for(var i = 0; i < arrayOfAnchors.length-1;i++) {
			 
			shapeArea += (arrayOfAnchors[i][0]*arrayOfAnchors[i+1][1] - arrayOfAnchors[i+1][0]*arrayOfAnchors[i][1]);
			Cx += (arrayOfAnchors[i][0]+arrayOfAnchors[i+1][0]) * (arrayOfAnchors[i][0]*arrayOfAnchors[i+1][1]-arrayOfAnchors[i+1][0]*arrayOfAnchors[i][1]);
			Cy += (arrayOfAnchors[i][1]+arrayOfAnchors[i+1][1]) * (arrayOfAnchors[i][0]*arrayOfAnchors[i+1][1]-arrayOfAnchors[i+1][0]*arrayOfAnchors[i][1]);
		}
		// performing final steps of calculation [https://en.wikipedia.org/wiki/Centroid#Centroid_of_polygon]
		shapeArea = 0.5*shapeArea;
		Cx = Cx / (6*shapeArea);
		Cy = Cy / (6*shapeArea);
		
		//print results
		reportFile.writeln("Shape Area by New Script:" + shapeArea + " square points");
		reportFile.writeln("Center of Gravity (in points) at: " + Cx + "," + Cy);
		
		//alert(object.typename);
		
		for(var i = 0; i<arrayOfAnchors.length;i++) {
			reportFile.writeln(arrayOfAnchors[i]);
			//note += object.pathPoints[i].anchor; + '\n'
		}

		var newGroup = app.activeDocument.groupItems.add();
		object.move(newGroup,ElementPlacement.INSIDE);
		newGroup.pathItems.ellipse(Cy + holeDiameter/2,Cx - holeDiameter/2, holeDiameter, holeDiameter, false, true);	
		newGroup.selected=true; // ensures that entirety of new group is selected
		reportFile.writeln("- - - - - - - - - - - - -");

	}
}	else alert('Select a path first');