Processing.js for Processing Devs

Processing.js Quick Start - Processing Developer Edition

Introduction

This quick start guide is written from the standpoint of a Processing developer. No HTML or JavaScript knowledge is assumed, and only basic Processing knowledge is necessary.

Why Processing.js?

The Web: from Java to JavaScript

Processing is built using Java. It was created at much the same time that the web was starting. At this time the choice of the Java language, and Java Runtime, as implementation targets for Processing made a lot of sense. In the mid-1990s, Java was poised to become the language of the web, with Applets and other Java technologies being used broadly on the client-side. Even Netscape, who created the language which would eventually become the lingua franca of the web with JavaScript, named their language so as to align themselves with the growing hype around Java.

In the end, Java became an important server side technology, receding from the client-side and browser. Today, most web browsers still support Java Applets, by means of a binary plugin. However, few web developers deploy Java-based web applications now, due to long load and startup times and difficulties relying on Java (or compatible Java versions) being installed. This trend is not isolated to Java, but is happening to all browser plugins (e.g., Flash), which are becoming less popular as issues of security, installation, deployment, etc. make them inconvenient or risky.

Another reason that plugins like Java and Flash have fallen out of favour is that recent advances in standard web technologies, specifically HTML5 and JavaScript, have made it possible to do things that previously depended on native (i.e., faster, compiled) code. Companies like Google, with GMail and Google Docs, or Scribd (see http://www.scribd.com/doc/30964170/Scribd-in-HTML5) have shown that HTML, CSS, and JavaScript alone are enough to build fast, full featured web applications.

Processing.js uses Modern Web Standards

Processing.js is built using JavaScript and HTML5. Processing.js is really two things: a Processing-to-JavaScript translator; and an implementation of the Processing API (e.g., functions like line(), stroke(), etc.) written in JavaScript instead of Java. It might seem odd at first to imagine your Processing sketches running in a browser, usually without modification. But this is exactly what Processing.js enables.

Processing.js automatically converts your Processing code to JavaScript. This means that you don't have to learn JavaScript in order to run your code in a browser. You can, quite literally, write your code using the Processing IDE like you always have, and follow the steps below to get it running on the web. There's nothing new to learn, beyond getting a simple web page created.

Under the hood, Processing.js uses the new HTML5 canvas element to create your sketch's graphics. The canvas element is a new feature of the web, and is either implemented or will be implemented by all major web browsers. All Processing drawing features have been reimplemented in Processing.js to use canvas, so any browser that supports canvas will also support Processing.js.

Here's a sample of a Processing.js sketch running in the browser. If you can see it working, your browser supports everything you need already, and you can move on to instructions below.

Create simple sketch here

1. Writing a Processing.js Sketch

There's nothing you should do differently here: you should write your Processing sketches exactly the same way you already do. For most people, this will mean using the Processing IDE, which has the nice benefit of letting you write, run, and test your code all in once place. Remember, any valid Processing sketch should also be a valid Processing.js sketch.

If you want to experiment with web-based Processing.js code editors, you can also try these:

• http://sketch.processing.org/
• http://sketchpad.cc/
• http://hascanvas.com/

Let's make a simple sketch that is 200 by 200 in size, sets the background to gray, draws a small white circle, and prints a message to the debug console:

void setup() {
  size(200, 200);
  background(100);
  stroke(255);
  ellipse(50, 50, 25, 25);
  println('hello web!');
}

I'll assume below that you saved this to a file called hello-web.pde.

2. Obtaining Processing.js

Processing.js is a JavaScript library that is meant to be included in a web page. You don't have to compile it, tell your web server about it, etc. It simply has to be included in a web page, and the browser will do the rest.

You can download Processing.js at http://processingjs.org/download. The library comes in a number of forms, for example:

• processing-0.9.7.js
• processing-0.9.7.min.js

The version numbers may be different as you read this, but note the file extensions. Both end in .js, but one also has .min. The .min version is the other file in a minified form, which means it will be smaller to download (minified JavaScript uses tricks like removing whitespace and renaming long variable names to single letters). File sizes, and download times, matter on the web in a way they don't with normal Processing sketches.

3. Creating a Processing.js Web Page

It's easy to get overwhelmed with the amount there is to learn about modern web technologies. But there's a secret: you can ignore 95% of it as you start, and add more later as you have time and interest. Unlike compilers or programming languages, web browsers are designed to accept almost any input you throw at them, whether valid or not, whether complete or not. Here's your first Processing.js web page:

  <script src="processing-0.9.7.min.js"></script>
  <canvas data-processing-sources="hello-web.pde"></canvas>

That's it! No <html> or <body> tags, no title, no CSS, just the processing.js script, and a canvas. While there isn't much here, it's important to understand what is. First, the script tag has a src attribute, which is the file to load. This could be a full url or a relative path. In this case the browser is going to look for a file named processing-0.9.7.min.js in the same directory as your web page.

The second thing in this web page is a canvas tag. Notice that it too has an attribute, data-processing-sources. This is a list of filenames (or just one filename if you only have 1 file) separated by spaces (filenames and URLs on the web can't include spaces, so space is a safe choice for separating lists). In this case, the browser is going to look for a file named hello-web.pde located in the same directory as your page page.

How does the browser know how to load a Processing *.pde file? Processing.js takes care of this, and will download, parse (i.e., translate to JavaScript), then run it automatically when the page is loaded.

And that's it! Save this file to the same directory as hello-web.pde and processing-0.9.7.min.js and call it hello-web.html.

If you're the kind of person who doesn't like taking shortcuts, here's what a more complete web page might look like:

<!DOCTYPE html>
<html>
  <head>
    <title>Hello Web - Processing.js Test</title>
    <script src="processing-0.9.7.min.js"></script>
  </head>
  <body>
    <h1>Processing.js Test</h1>
    <p>This is my first Processing.js web-based sketch:</p>
    <canvas data-processing-sources="hello-web.pde"></canvas>
  </body>
</html>

Both ways work, and you shouldn't let yourself get burdened by HTML and other web syntax until you feel you want to do other things with your web pages.

3. Running your Processing.js Web Page

In case it isn't obvious, you run your hello-web.pde sketch by loading your hello-web.html web page in a compatible browser. Web browsers will provide you a way to load a local file, usually using the File menu and then Open File.... If you've saved the files above on a web server, you can use the remote URL instead.

Things to Know as a Processing Developer using Processing.js

While Processing.js is compatible with Processing, Java is not JavaScript, and canvas has some differences from Java's graphics classes. Here are some tricks and tips as you start working on more complex sketches in Processing.js.

Processing.js has no data directory

Processing uses the concept of a data directory, where images and other resources are located. Processing.js does not include this. As a result, you should always provide file pages (e.g., images) that are relative to your web page, which is the norm on the web.

Processing.js implements Processing, but not all of Java

Processing.js is compatible with Processing, but is not, and will never be, fully compatible with Java. If your sketch uses functions or classes not defined as part of Processing, they are unlikely to work with Processing.js. Similarly, libraries that are written for Processing, which are written in Java instead of Processing, will most likely not work.

Processing.js has to cheat to simulate Processing's synchronous I/O

Processing uses a synchronous I/O model, which means that functions like loadImage() take time to execute, and while they are running, nothing else happens: the program waits until loadImage() is done before moving on to the next statement. This means that you can count on the value returned by a function like loadImage() being usable in the next line of code.

Web browsers don't work like this. The web uses an asynchronous I/O model, which means that functions which load external resources can't make the program wait until they finish. In order to replicate Processing's load* functions, you have to use a special Processing.js Directive.

The Processing.js Directives are hints to the browser that are written in comments rather than in the Processing code itself. Here's a typical Processing sketch that loads an image synchronously and then draws it:

PImage img;

void setup() {
  img = loadImage("picture.jpg");
  image(img, 0, 0);
}

This code will not work in the browser with Processing.js, because the call to image() will happen before the file picture.jpg has been downloaded. The fix is to ask Processing.js to download the image before the sketch starts, and cache it--a technique known as preloading. Here is the modified code:

/* @pjs preload="picture.jpg"; */
PImage img;

void setup() {
  img = loadImage("picture.jpg");
  image(img, 0, 0);
}

Notice the extra comment line at the top of the code. The @pjs directive is for Processing.js, and not the developer. Think of it as an extra in of code that will be executed before the program begins.

If you have multiple images to load, use a list like so:

/* @pjs preload="picture.jpg,picture2.jpg,picture3.png"; */

Processing.js requires more care with variable naming than Processing

One of the powerful features of JavaScript is its dynamic, typeless nature. Where typed languages like Java, and therefore Processing, can reuse names without fear of ambiguity (e.g., method overloading), Processing.js cannot. Without getting into the inner-workings of JavaScript, the best advice for Processing developers is to not use function/class/etc. names from Processing as variable names. For example, a variable named line might seem reasonable, but it will cause issues with the similarly named line() function built-into Processing and Processing.js.