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Game Physics Cookbook
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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.
Follow these steps to implement a two-dimensional circle:
Start the declaration of the Circle structure in Geometry2D.h by creating the variables that make up a circle:
typedef struct Circle {
Point2D position;
float radius;Next, declare an inline constructor that will create a circle at origin with a radius of 1:
inline Circle() : radius(1.0f) {}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):
position(p), radius(r) {}
} Circle;