How to screenshot website in JavaScript client-side / how Google did it? (no need to access HDD) [duplicate]

I'm working on web application that needs to render a page and make a screenshot on the client (browser) side.

I don't need the screenshot to be saved on the local HDD though, just kept it in RAM and send it to the application server later.

I researched:

1. BrowserShots alike services...
2. Mechanized browsers...
3. wkhtmltoimage...
4. Python WebKit2PNG...

But none of those gives me all I need, which is:

1. Processing at browser side (generate screenshot of page). Don't need to be saved on HDD! Just...
2. ...send image to Server for further processing.
3. Capturing whole page (not only visible part)

Eventually I came upon Google's Feedback Tool (click "feedback" on YouTube footer to see this). It contains JavaScript for JPG encoding and two other huge scripts which I can't determine what exactly they do...

But it's processed on the Client side - otherwise there would be no point putting this huge JPEG encoder in the code!

Anyone have any idea how did they made it / how I can make it?

Here is an example of the feedback (report a bug on some screens)

This answers your problem.

You can use JavaScript/Canvas to do the job but it is still experimental.

Update:

There is a library for this now https://html2canvas.hertzen.com/

I needed to snapshot a div on the page (for a webapp I wrote) that is protected by JWT's and makes very heavy use of Angular.

I had no luck with any of the above methods.

I ended up taking the outerHTML of the div I needed, cleaning it up a little (*) and then sending it to the server where I run wkhtmltopdf against it.

This is working very well for me.

(*) various input devices in my pages didn't render as checked or have their text values when viewed in the pdf... So I run a little bit of jQuery on the html before I send it up for rendering. ex: for text input items -- I copy their .val()'s into 'value' attributes, which then can be seen by wkhtmlpdf