A helper script for use with phantomJS (http://phantomjs.org/) similar to rasterize.js allowing you to rasterize elements of a web page by specifying a selector.

rasterizeElement.js

/*
  rasterizeElement.js
  
  This file is a helper script for use with phantomJS (http://phantomjs.org/) similar to rasterize.js
  allowing you to rasterize elements of a web page by specifying a selector.

  Author - Stephen James 
  Twitter - @stephenhjames
  GitHub - https://github.com/stephen-james
  
*/

console.log("");
console.log("== rasterizeElement.js =====================================================");

var page = require('webpage').create(),
	
	defaults = {
		// default values for rasterisation if not passed in as parameters
		viewPortSize : { 
			width : 1280,
			height : 768
		},
		zoomFactor : 1,
		paperSize : {
			format : "A4",
			orientation : "portrait",
			margin : "1cm"
		} 
	},
	
    options = parseScriptArguments();

function extendObject(targetObjectToExtend, sourceObject) {
	// extends the target object by copying properties that don't already exist (by key) from the source object
	for(var propertyToCheck in sourceObject) {
		if (!targetObjectToExtend.hasOwnProperty(propertyToCheck)) {
			targetObjectToExtend[propertyToCheck] = sourceObject[propertyToCheck];
		}
	}
}
	
function raiseErrorAndExit(message, showOptions) {
	// helper method to write errors to the console and terminate the process
	console.log(message);
	if (showOptions != false) {
		console.log("[info] - you called rasterizeElement with these options : " + JSON.stringify(options));
	}
	phantom.exit();
};
	
function parseScriptArguments() {
	// parses the script arguments and builds up the options for the rasterization operation
	var system = require('system'),
		clonedArguments = system.args.slice(),
		options;

	// clean up args to not include the initial argument to phantomjs (which is this script name)		
	clonedArguments.shift();
	
	// validate...
	if (clonedArguments.length < 3 || clonedArguments.length > 6) {
		console.log("    Usage: rasterizeElement.js fromUrl renderElementBySelector toDestinationFilename [viewPortSize] [zoomFactor] [paperSize]");
		console.log("");		
		console.log("    eg. (basic usage)");	
		console.log("");		
		console.log("    phantomjs rasteriseElement.js http://www.awebsite.com \"#idOfElementToSelect\" output.png");				
		console.log("");				
		raiseErrorAndExit("", false);
	}
	else {
		
		options = {
			addressOfPageToRender : clonedArguments[0],
			elementSelector : clonedArguments[1],
			outputFileName : clonedArguments[2]			
		};
		
		// handle optional arguments
		if (clonedArguments.length > 3) { 
			console.log("[info] - interpretting viewPortSize");		
			try {
				options["viewPortSize"] = JSON.parse(clonedArguments[3])
			}
			catch (err) {
				raiseErrorAndExit("[error] - Invalid value passed for viewPortSize (" + clonedArguments[3] + ")");
			}
		}
		if (clonedArguments.length > 4) { options["zoomFactor"] = clonedArguments[4]; }
		if (clonedArguments.length > 5) { 
			console.log("[info] - interpretting paperSize");				
			try {
				options["paperSize"] = JSON.parse(clonedArguments[5])
			}
			catch (err) {
				raiseErrorAndExit("[error] - Invalid value passed for paperSize (" + clonedArguments[5] + ")");
			}
		}		
		
		extendObject(options, defaults);

		// if we are rendering to pdf, we must consider this a paper render
		options.paperRender = options.outputFileName.substr(-4) === ".pdf";
		
		return options;
	}
}

page.open(options.addressOfPageToRender, function (loadStatus) {
	
	if (loadStatus !== 'success') {
		raiseErrorAndExit("[error] - Unable to load the address! (" + options.addressOfPageToRender + ")");
	} 

	page.zoomFactor = options.zoomFactor;
	page.viewportSize = options.viewPortSize;
	
	// evaluate the clipping rectangle of the element we would like to select and rasterise
	var elementClipRect = page.evaluate(function(selector) {
		var elementToRasterize = document.querySelector(selector);
		
		if (!elementToRasterize) {
			return;
		}
		else {
			var elementRect = elementToRasterize.getBoundingClientRect();
		
			return {
				top:    elementRect.top,
				left:   elementRect.left,
				width:  elementRect.width,
				height: elementRect.height
			};
		}
	}, options.elementSelector);
	
	if (!elementClipRect) {
		raiseErrorAndExit("[error] - Unable to locate element for provided selector! (" + options.elementSelector + ")");
	}
	else
	{
		page.clipRect = elementClipRect;
		console.log("[info] - applying clipping rectangle for selection " + JSON.stringify(elementClipRect));
	}

	
	if (options.paperRender) {
		page.paperSize = options.paperSize;
	}
	
	page.render(options.outputFileName);
	
	console.log("[info] - done");		

	phantom.exit();

});

for a bit of background and usage info, here is a link to a blog article for this gist

http://stephenjamescode.blogspot.co.uk/2013/01/automating-rasterization-of-html.html