Creating an online collaborative drawing: what could the building blocks be?

I was wondering about creating an online collaborative drawing environment. Basically, what I have in mind is that two or more people can access the same sheet through their browsers and draw something together.

My question here is how this could be accomplished. Specifically, I understand开发者_如何学JAVA that there might be a zillion solutions, but I'm definitely not an expert in this field and I would love to know what these solutions are and why a solution should be preferred to another.

First of all, some "constraints"

• no Flash/Silverlight
• interface should be point and click (that is, not like here)

So far, the potential solutions I've come up with are the following:

• processing.js - It seems to me that its not really widespread. Is it a valid alternative anyway?
• HTML5 - is it mature enough? From what I see (e.g. here), I've got the feeling it's still in too early a stage. (BTW, a parallel discussion about HTML5/js is going on here)
• Anything else?

Any input is most welcome! :)

You would have to make some concessions in what browsers you'd support. The actual drawing itself is fairly easy, just use the Canvas element for that, google even has a library that will make canvas work in < IE8 too.

The collaboration/multiuser part is the tricky part, you could look into HTML Websockets, but they are relatively new, and not supported in the mainstream browsers at the moment. Whats left is either sockets through flash, or a technique called long polling. Basicly you have an AJAX call which will last very long, and the server just ends the connection when some new data is aquired. See also: Comet