#### Overview of this book

Physics is really important for game programmers who want to add realism and functionality to their games. Collision detection in particular is a problem that affects all game developers, regardless of the platform, engine, or toolkit they use. This book will teach you the concepts and formulas behind collision detection. You will also be taught how to build a simple physics engine, where Rigid Body physics is the main focus, and learn about intersection algorithms for primitive shapes. You’ll begin by building a strong foundation in mathematics that will be used throughout the book. We’ll guide you through implementing 2D and 3D primitives and show you how to perform effective collision tests for them. We then pivot to one of the harder areas of game development—collision detection and resolution. Further on, you will learn what a Physics engine is, how to set up a game window, and how to implement rendering. We’ll explore advanced physics topics such as constraint solving. You’ll also find out how to implement a rudimentary physics engine, which you can use to build an Angry Birds type of game or a more advanced game. By the end of the book, you will have implemented all primitive and some advanced collision tests, and you will be able to read on geometry and linear Algebra formulas to take forward to your own games!
Game Physics Cookbook
Credits
Acknowledgements
Acknowledgements
www.PacktPub.com
Customer Feedback
Preface
Free Chapter
Vectors
Matrices
Matrix Transformations
2D Primitive Shapes
2D Collisions
2D Optimizations
3D Primitive Shapes
3D Point Tests
3D Shape Intersections
3D Line Intersections
Triangles and Meshes
Models and Scenes
Camera and Frustum
Constraint Solving
Manifolds and Impulses
Springs and Joints
Index

## Circle

A circle is defined by a point in space and a Radius. The circle is an extremely simple shape as shown in the following diagram:

Intersection algorithms for the circle are as simple as its definition. For this reason, a circle is often the first choice to approximate the bounding volume of objects. Arguably, the circle is the most commonly used 2D primitive.

### How to do it…

Follow these steps to implement a two-dimensional circle:

1. Start the declaration of the `Circle `structure in `Geometry2D.h` by creating the variables that make up a circle:

```typedef struct Circle {
Point2D position;
`   inline Circle() : radius(1.0f) {}`
3. Finish the declaration of the `Circle` structure by creating an inline constructor that lets us specify the position and radius of the circle being created:
```   inline Circle(const Point2D& p, float r):