Game Development Reference
In-Depth Information
internal struct PickingRay
{
internal Vector3 Origin;
internal Vector3 Direction;
public PickingRay(Vector3 direction, Matrix transform)
{
Origin = new Vector3(0.0F, 0.0F, 0.0F);
Direction = direction;
Origin.TransformCoordinate(transform);
Direction.TransformNormal(transform);
Direction.Normalize();
}
}
Note
Vector3.TransformCoordinate() is used to transform points because it sets the fourth com-
ponent to W = 1 , whereas Vector3.TransformNormal() is used to transform vectors because
it sets the fourth component to W = 0 .
Bounding Sphere Intersection Tests
At this point, we are converting screen space coordinates to world space and com-
puting a ray that will be used for picking. Intersection tests will be performed
against objects in the scene using the computed ray to determine which objects the
user has selected.
In order to perform intersection tests, we need a 3D shape to test against.
Bounding spheres are common because their approximated nature makes them
fast to compute and use. Each object is represented by a bounding sphere that
describes the approximated volume of the object.
A sphere is represented by its center c and its radius r . Points can be tested for
whether they belong to a sphere if their distance from the center is equal to the
radius, shown by the following equation.
Intersecting a ray with a sphere can be found with the following equation, where
p is substituted with
to represent the ray.
 
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