dabblet.css

/**
 * Clip image to hexagon
 */

img {
	max-width: 200px;
	max-height: 200px;
}

dabblet.html

<h1>Clip image to hexagon</h1>
<p>Edit the HTML source and run the JS.
Right click the hexagon and Save Image As… (Firefox lets you do that)</p>

<img src="http://w3.org/conf/img/speakers/lea.jpg" alt="lea" />
<img src="http://w3.org/conf/img/speakers/lea.jpg" alt="lea" class="hexagon" />

dabblet.js

function $$(expr, con) {
	var elements = (con || document).querySelectorAll(expr);
	
	return Array.prototype.slice.call(elements);
}

// Clip the speaker images to hexagons
(function() {

var hexagon = new Image();
hexagon.src = 'http://w3.org/conf/img/hexagon.svg';

hexagon.onload = function() {
	hexagon.aspectRatio = hexagon.width / hexagon.height;
	
	$$('img.hexagon').forEach(function(img) {
		img.onload = function() {
			var canvas = document.createElement('canvas'),
			    ctx = canvas.getContext('2d');
			
			canvas.width = this.offsetHeight * hexagon.aspectRatio;
			canvas.height = this.offsetHeight;
			
			['className', 'title', 'alt'].forEach(function(property) {
				if (this[property]) {
					canvas[property] = this[property];
				}
			}, this);
			
			var args = [this];
			
			var aspectRatio = this.naturalWidth / this.naturalHeight;
			
			if (aspectRatio >= hexagon.aspectRatio) {
				// Clip a rectangle from the center with the maximum height and
				// the same aspect ratio as the hexagon
				var clipWidth = this.naturalHeight * hexagon.aspectRatio;
				args.push((this.naturalWidth - clipWidth)/2, 0, clipWidth, this.naturalHeight);
			}
			else {
				var clipHeight = this.naturalWidth / hexagon.aspectRatio;
				args.push(0, (this.naturalHeight - clipHeight)/2, this.naturalWidth, clipHeight);
			}
			
			args.push(0, 0, canvas.width, canvas.height);
			ctx.drawImage.apply(ctx, args);
			
			ctx.globalCompositeOperation = 'destination-atop';
			
			var width = canvas.width * hexagon.aspectRatio;
			ctx.drawImage(hexagon, 0, 0, canvas.width, canvas.height);
			
			this.parentNode.replaceChild(canvas, this);
		}
		
		if(img.complete) {
			img.onload();
		}
	});
};

})();

settings.json

{"view":"split-vertical","fontsize":"100","seethrough":"","prefixfree":"1","page":"html"}