Take a screenshot of a DOM container and use it as a background image

I am currently developing a drag and drop interface for a course builder. Courses are split into lessons and lessons into slides. While switching between slides, I wanted to have a small thumbnail representing the current slide content.

One option would have been to use something like PhantomJS / CasperJS.  A headless browser , to take a screenshot of the slide. I have done that in the past and it works perfectly.

This time, I decided to go with another solution, that allows me to convert html to a canvas element.

The project is called html2canvas (GitHub).

“This script allows you to take screenshots of webpages or parts of it, directly on the users browser. The screenshot is based on the DOM and as such may not be 100% accurate to the real representation as it does not make an actual screenshot, but builds the screenshot based on the information available on the page.”

EXAMPLE

html2canvas(document.body, {
  onrendered: function(canvas) {
    document.body.appendChild(canvas);
  }
});

CANVAS TO BACKGROUND IMAGE

I have a slide container (#slideContainer), that holds the active slide for editing and a thumb navigation to switch between slides (.slide.thumb).

var current = $(".slide.thumb.active");

html2canvas($("#slideContainer").get(0), {
  onrendered: function(canvas) {

   var dataURL = canvas.toDataURL();
   current.css({"background" :"url("+dataURL+")",
                "background-size":"contain",
                "background-position": "center",
                "background-repeat":"no-repeat"});
  }
});

When switching between slides, I am doing a snapshot of the current slideContainer and add it as a new background image to the thumb navigation item for that slide. This is done with canvas.toDataURL() and by embedding the resulting image  using a data: url.

Works perfectly. html2canvas has no full css support, so the output will not always match 100%, but its close enough to get a glimpse of what is stored per slide ;)

html2canvas
PhantomJS
CasperJS

Enjoy coding …