How to take a snapshot of a section of a web page

I have a section of a web page that I need to take a gif snapshot of at a given time interval. The snapshot needs to be full page size resolution, however as I said, it only goes to a certain place on the page (in this case it's after a table).

What would be the best way to grab a page snapshot image image like this? I'd like to just throw it into a cron job and forget it, but I'm not readily seeing a tool that would make quick work of this.

SOLUTION:

As per the @Eduardo's excellent direction I implemented a clean and quick solution based around phantomjs and imagemagick (Mac: brew install phantomjs & brew install imagemagick):

*NOTE: If you want to remove imagemagick altogether just add the following to rasterize.js: page.clipRect = { top: 10, left: 10, width: 500, height: 500 }

#! /usr/bin/env bash
# Used with PhantomJS - rasterize.js source: http://j.mp/xC7u1Z

refresh_seconds=30

while true; do
    date_now=`date +"%Y-%m-%d %H%M"` 

    phantomjs rasterize.js $1 "${date_now}-original.png"  # just sucking in the first arg from shell for the URL
    convert "${date_now}-original.png" -crop 500x610+8+16 "${date_now}.png" # crop args: WIDTHxHEIGHT+LEFT_MARGIN+TOP_MARGIN
    rm "${date_now}-original.png"

    echo "Got image: ${date_now}.png - Now waiting ${refresh_seconds} seconds for next image..."
    sleep ${refresh_seconds}
done

And here's the js used by phantomjs in the above:

// As explained here: http://code.google.com/p/phantomjs/wiki/QuickStart

var page = new WebPage(),
    address, output, size;

if (phantom.args.length < 2 || phantom.args.length > 3) {
    console.log('Usage: rasterize.js URL filename');
    phantom.exit();
} else {
    address = phantom.args[0];
    output = phantom.args[1];
    page.viewportSize = { width: 600, height: 600 };
    page.open(address, function (status) {
        if (status !== 'success') {
            console.log('Unable to load the address!');
        } else {
            window.setTimeout(function () {
                page.render(output);
                phantom.exit();
            }, 200);
        }
    });
}

This question has already been answered here: How can I take a screenshot/image of a website using Python?

It was answered on '09, but that option is still very valid. I'll try to extend with some more options.

Those tools will get you full page snapshots, which you can later clip using imagemagick easily.

Another option that you might have these days is Phantomjs. Phantom is a headless browser built to be run on node, it will allow you to take a picture of a whole page or just an area of the page.

Take a look at this example:

var page = new WebPage(),
    address, output, size;

if (phantom.args.length < 2 || phantom.args.length > 3) {
    console.log('Usage: rasterize.js URL filename');
    phantom.exit();
} else {
    address = phantom.args[0];
    output = phantom.args[1];
    page.viewportSize = { width: 600, height: 600 };
    page.open(address, function (status) {
        if (status !== 'success') {
            console.log('Unable to load the address!');
        } else {
            window.setTimeout(function () {
                page.render(output);
                phantom.exit();
            }, 200);
        }
    });
}