WHAT YOU SHOULD ALREADY KNOW

This book's examples are written in Processing, a wrapper for the Java programming language. While you need only a little programming experience, you should understand how a basic Processing sketch is structured, how to use variables, how to draw shapes and get input from the mouse, and how if/else statements work.

It may be helpful to understand using PVector objects to store positions, but we'll cover the basics if you haven't used them before.

At the end, we will talk about using collision in object-oriented code. Understanding object-oriented programming will not be required to use this book, but it will be helpful for using these topics in larger projects with lots of objects hitting each other.

FUNCTIONS

The core of the collision examples are functions, so you will need to be familiar with them to use this book. If you have never created a function, please read this section carefully; if you already understand this topic, you can skip ahead.

A function is a re-usable, self-contained piece of code. Functions are used for operations that you want to perform more than once, like checking for collision between two objects.

A function returns (sends back) a variable type (like int or boolean). For example, here's a function that returns the string "Hello!"

function sayHi() {
    return "Hello!";
}

Note that when we declare the function, we list the variable type to be returned. The function does something useful, then sends data back using the return command. If the function doesn't need to return anything (for example, if all it does is draw a rectangle), the type is void. Sound familiar? The setup() and draw() sections of Processing are actually functions!

Functions can also receive arguments, or parameters that are fed into them. An argument is given a type and name (which exists only inside the function); multiple arguments are separated by commas. Here's a simple function that adds two numbers:

function sum(a, b) {
    return a + b;
}

Once finished, you can use the function elsewhere in your code. For example, our sum() function above can be used like this:

var result = sum(2, 2);
print(result);

>> 4

All of the examples in this book are functions. They are fed parameters of the objects to be tested (such as position or size) and return a boolean value whether or not a collision is happening. They could also be modified to return the position of the collision, such as in the Line/Line example. Be sure to look at the full code at the end of each example to see how the function is structured and called.