magnum/doc/collision-detection.dox

namespace Magnum { namespace Physics {
/** @page collision-detection Collision detection
@brief Collection of simple shapes for high performance collision detection.

The essential thing in collision detection is to define a complex object with
collection of simple shapes, for which it is easy to detect collisions. These
shapes can be either one-, two- or three-dimensional and they can be grouped
together using various operations.

@tableofcontents

@section collision-detection-shape-collection Available shapes

@subsection collision-detection-shapes1D One-dimensional shapes

- @ref Physics::Point "Physics::Point*D" - @copybrief Physics::Point
- @ref Physics::Line "Physics::Line*D" - @copybrief Physics::Line
- @ref Physics::LineSegment "Physics::LineSegment*D" - @copybrief Physics::LineSegment

Because of numerical instability it's not possible to detect collisions of
line and point. Collision of two lines can be detected only in 2D.

@subsection collision-detection-shapes2D Two-dimensional shapes

- Physics::Plane - @copybrief Physics::Plane

@subsection collision-detection-shapes3D Three-dimensional shapes

- @ref Physics::Sphere "Physics::Sphere*D" - @copybrief Physics::Sphere
- @ref Physics::Capsule "Physics::Capsule*D" - @copybrief Physics::Capsule
- @ref Physics::AxisAlignedBox "Physics::AxisAlignedBox*D" - @copybrief Physics::AxisAlignedBox
- @ref Physics::Box "Physics::Box*D" - @copybrief Physics::Box

The easiest (and most efficient) shape combination for detecting collisions
is point and sphere, followed by two spheres. Computing collision of two boxes
is least efficient.

@section collision-detection-shape-groups Creating hierarchic groups of shapes

Shapes can be grouped together using one of three available logical
operations: AND, OR and NOT. These operations are mapped to operator&&(),
operator||() and operator!(), so for example creating negation of logical OR
of line segment and point is simple as this:
@code
Physics::LineSegment3D segment;
Physics::Point3D point;

Physics::ShapeGroup3D group = !(segment || point);
@endcode

@note Logical operations are not the same as set operations -- intersection of
    two spheres will not generate any collision if they are disjoint, but
    logical AND will if the object collides with both of them.

The resulting object internally stores copies of both shapes, so the original
instances can be destroyed. For simple combinations appropriate resulting
shape is generated (e.g. logical OR of point and sphere can be only the sphere,
if the point lies inside) and stored inside ShapeGroup instead of two original
objects.

@subsection collision-detection-shape-reference Referencing the shapes for later changes

Sometimes you may want to modify the shape based on changes of the object
itself. In previous example all the shapes were copied into ShapeGroup, so it
was not possible to change their properties such as sphere radius without
recreating the group again. You can, however, explicitly pass a reference to
original object, so you can change it later:
@code
Physics::LineSegment3D segment;
Physics::Point3D point;

Physics::ShapeGroup3D group = !(segment || std::ref(point));

point.setPosition({1.0f, -6.0f, 0.5f});
@endcode

Note that passing a reference implies that you must not destroy the original
instance (in this case the point). Also because the referenced instance could
change, there are no shape optimizations done, unlike above.

@subsection collision-detection-shape-simplification Providing simplified version of shape for better performance

If there are many shapes grouped together, it might hurt performance of
collision detection, because it might be testing collision with more shapes
than necessary. It's then good to specify simplified version of such shape,
so the collision detection is done on the original if and only if collision
was detected with the simplified shape. It is in fact logical AND using
operator&&() - the collision is initially detected on first (simplified) shape
and then on the other:
@code
Physics::Sphere3D sphere;
Physics::Box3D box;
Physics::AxisAlignedBox3D simplified;

Physics::ShapeGroup3D object = simplified && (sphere || box);
@endcode

@section collision-detection-shape-collisions Detecting shape collisions

Shape pairs which have collision detection implemented can be tested for
collision using operator%(), for example:
@code
Physics::Point3D point;
Physics::Sphere3D sphere;

bool collide = point % sphere;
@endcode

*/
}}}