A book and examples on collision detection
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A book and examples on collision detection
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Collision Detection

View the book, including interactive examples:

UPDATE! This site has been getting lots of visits which is awesome. I've made some changes so it will look better on mobile devices, make navigation easier, and tried to look over everything for mistakes. If you have any problems or suggestions, please post an issue on the project repository. Thanks!

The collision of objects underlies most game experiences and user-interfaces. Baseball bats collide with balls, zombies bump into walls, and Mario lands on platforms and stomps turtles. Even something as simple as clicking a button (a rectangle) with your mouse (a point) is a collision.

This book explains the algorithms behind those collisions using basic shapes like circles, rectangles, and lines so you can implement them into your own projects.


This book covers collisions between points, circles, rectangles, lines, polygons, and triangles. These examples are meant to be as readable and easily understood as possible. There are definitely faster, more efficient ways to detect these collisions, but this book is intended to be friendly and teach the principles with minimal math.

Each section includes a description of the collision algorithm and an interactive example built using processing.js.


As with any book, there's a lot more useful material than could be covered here. Things that aren't discussed are mostly left out because the math gets too complicated. Three-dimensional space isn't touched on. Ellipses, which seem like they should be pretty easy, are actually very difficult.

If there's a specific collision not covered that would be helpful, please please submit an issue with a request or, better yet, submit a working example that you've built!


If you find code that doesn't run correctly, an algorithm that isn't explained quite right, or a typo, please report them. Thanks for your help!


This book's entire contents, including the code examples and this text, is released under a Creative Commons Attribution, Non-Commercial, Share-Alike license. This means:

  1. You're welcome to use this book and the examples to make great stuff, but please cite this book somewhere in your project or its documentation.
  2. You can only use the book's text and/or code vertbatim for non-commercial projects. That means you can remix or make your own version of the book, and you can fork and create new libraries based on the code here, so long as they are not commercial projects (ie publishing a book with a publisher). I'm very happy to talk options if you have a paid gig and you'd like to use some of the materials.
  3. If you do make a project that forks or remixes this book or code-base, it must be released under this same license or a looser one. Pay it forward!

But you're free to use the ideas and code from this book inside your projects, so long as this is just a part of what you're working on. In other words: you're welcome to build a super cool game that relies on the code you've seen here, even if you're going to sell it!

If you have any questions about what you can and can't do with these examples, please get in touch.


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.

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Collision Detection