Take screenshot at single viewport size using CasperJS

capture.js

var screenshotUrl = 'http://example.com/'

var casper = require("casper").create({
    viewportSize: {
        width: 1024,
        height: 768
    }
});

if (casper.cli.args.length < 1) {
  casper
    .echo("Usage: $ casperjs capture.js http://example.com")
    .exit(1)
  ;
} else {
  screenshotUrl = casper.cli.args[0];
}

casper.start(screenshotUrl, function() {
    this.capture('example.png', {
        top: 0,
        left: 0,
        width: 1024,
        height: 768
    });
});

casper.run();