svg-path-to-polygons.js

module.exports = {
	pathDataToPolys:svgPathToPolygons,
	compare:compare
};
const { parseSVG, makeAbsolute } = require('svg-path-parser');

function svgPathToPolygons(svgPathString, opts={}) {
	if (!opts.tolerance) opts.tolerance=1;
	const polys = [];
	const tolerance2 = opts.tolerance*opts.tolerance;
	let poly = [];
	let prev;
	makeAbsolute(parseSVG(svgPathString)).forEach( cmd => {
		switch(cmd.code) {
			case 'M':
				polys.push(poly=[]);
				// intentional flow-through
			case 'L':
			case 'H':
			case 'V':
			case 'Z':
				add(cmd.x,cmd.y);
				if (cmd.code==='Z') poly.closed = true;
			break;

			case 'C':
				sampleCubicBézier(cmd.x0,cmd.y0,cmd.x1,cmd.y1,cmd.x2,cmd.y2,cmd.x,cmd.y);
				add(cmd.x,cmd.y);
			break;

			case 'S':
				let x1=0, y1=0;
				if (prev) {
					if (prev.code==='C') {
						x1 = prev.x*2 - prev.x2;
						y1 = prev.y*2 - prev.y2;
					} else {
						x1 = prev.x;
						y1 = prev.y;
					}
				}
				sampleCubicBézier(cmd.x0,cmd.y0,x1,y1,cmd.x2,cmd.y2,cmd.x,cmd.y);
				add(cmd.x,cmd.y);
			break;

			default:
				console.error('Our deepest apologies, but '+cmd.command+' commands ('+cmd.code+') are not yet supported.');
				process.exit(2);
		}
		prev = cmd;
	});
	return polys;

	// http://antigrain.com/research/adaptive_bezier/
	function sampleCubicBézier(x0, y0, x1, y1, x2, y2, x3, y3) {
		// Calculate all the mid-points of the line segments
		const x01   = (x0 + x1) / 2,
		      y01   = (y0 + y1) / 2,
		      x12   = (x1 + x2) / 2,
		      y12   = (y1 + y2) / 2,
		      x23   = (x2 + x3) / 2,
		      y23   = (y2 + y3) / 2,
		      x012  = (x01 + x12) / 2,
		      y012  = (y01 + y12) / 2,
		      x123  = (x12 + x23) / 2,
		      y123  = (y12 + y23) / 2,
		      x0123 = (x012 + x123) / 2,
		      y0123 = (y012 + y123) / 2;

		// Try to approximate the full cubic curve by a single straight line
		const dx = x3-x0,
		      dy = y3-y0;

		const d1 = Math.abs(((x1-x3)*dy - (y1-y3)*dx)),
		      d2 = Math.abs(((x2-x3)*dy - (y2-y3)*dx));

		if (((d1+d2)*(d1+d2)) < (tolerance2 * (dx*dx + dy*dy))) add(x0123,y0123);
		else { // Continue subdivision
		  sampleCubicBézier(x0, y0, x01, y01, x012, y012, x0123, y0123); 
		  sampleCubicBézier(x0123, y0123, x123, y123, x23, y23, x3, y3); 
		}
   }

   function add(x,y){
   	if (opts.decimals && opts.decimals>=0) {
   		x = x.toFixed(opts.decimals)*1;
   		y = y.toFixed(opts.decimals)*1;
   	}
   	poly.push([x,y]);
   }
}

// OMG YOU FOUND THE SECRET UNDOCUMENTED FEATURE
function compare(pathData,opts={}) {
	var polys = svgPathToPolygons(pathData,opts);
	var minX=Infinity, maxX=-Infinity, minY=Infinity, maxY=-Infinity;
	polys.forEach(poly => {
		poly.forEach(pt => {
			if (pt[0]<minX) minX=pt[0];
			if (pt[1]<minY) minY=pt[1];
			if (pt[0]>maxX) maxX=pt[0];
			if (pt[1]>maxY) maxY=pt[1];
		});
	});
	let dx=maxX-minX, dy=maxY-minY;
	console.log(`
<svg xmlns="http://www.w3.org/2000/svg" width="${dx}px" height="${dy}px" viewBox="${minX} ${minY} ${dx*2} ${dy}">
<style>path,polygon,polyline { fill-opacity:0.2; stroke:black }</style>
<path d="${pathData}"/>
<g transform="translate(${dx},0)">
${polys.map(poly => `  <${poly.closed ? 'polygon' : 'polyline'} points="${poly.join(' ')}"/>`).join("\n")}
</g>
</svg>
	`.trim());
};