Processing in java is a really nice project and it would be really nice to be able to run processing visualization in the browser without having to run java applets, but the current lack of support for Internet Explorer (I do not know if it would even be possible to realize, BTW) is really preventing me to invest too much time in it. I have been using Raphael.js instead.
excanvas is pretty slow, I would personally prefer to just drop support for IE <9 than to go through the trouble of making sure it works with excanvas.
Also, canvas is just one of the html5 features it uses (there's e.g. also audio).
You could also add Google Chrome Frame to the page instead of outright dropping IE <9 support. At least it gives them another option, especially while IE9 is still in beta :)
Processing is a graphics language that gives you a great abstraction for writing time-based animations with useful primitives. Neither Canvas nor SVG provide the same high-level starting point, meaning it takes more effort to achieve the same effects.
I think that is also what the website says, but when I experimented with both it didn't seem like drawing shapes/blitting bitmaps on canvas with js was all that different from doing it with processing, although I didn't try doing any animations.
One downside that turned me off about processing is that the available tools (IDEs, debuggers, profilers) are even worse than what js has - you basically have to use the js tools on the generated js.
Another down side is that the language is less powerful than js, but then again you can also code in js and use processing.js as a drawing library, which alleviates both issues.
It is pretty much a direct port of the Processing language (originally written in Java). So you can bring your Processing projects to the browser without the need for an applet.
Oh, I know that - was just wondering why I'd be better off starting a new project in processing.js instead of in plain js/canvas.
It's a lot easier to see the point with java/processing, since java2d is quite a bit harder to work with (or rather, it has many more gotcha's) than js/canvas.
Personally, I use processing.js because I can use all the existing Processing examples as a point of reference. It is easier to get the ball rolling as opposed to starting from scratch.
As a plus, the experience I get with processing.js, I will be able to apply to Processing (as you probably know, Processing is often used to graph output from an Arduino board, something else I have as a hobby).
