MoonTools.Bonk/Bonk/NarrowPhase/GJK2D.cs

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2019-09-06 08:11:58 +00:00
/*
* Implementation of the GJK collision algorithm
* Based on some math blogs
* https://blog.hamaluik.ca/posts/building-a-collision-engine-part-1-2d-gjk-collision-detection/
* and some code from https://github.com/kroitor/gjk.c
*/
using Microsoft.Xna.Framework;
using MoonTools.Core.Structs;
using System;
namespace MoonTools.Core.Bonk
{
public static class GJK2D
{
private enum SolutionStatus
{
NoIntersection,
Intersection,
StillSolving
}
public static ValueTuple<bool, SimplexVertices> TestCollision(IShape2D shapeA, Transform2D Transform2DA, IShape2D shapeB, Transform2D Transform2DB)
{
var vertices = new SimplexVertices(new Vector2?[] { null, null, null, null });
const SolutionStatus solutionStatus = SolutionStatus.StillSolving;
var direction = Transform2DB.Position - Transform2DA.Position;
var result = (solutionStatus, direction);
while (result.solutionStatus == SolutionStatus.StillSolving)
{
result = EvolveSimplex(shapeA, Transform2DA, shapeB, Transform2DB, vertices, result.direction);
}
return ValueTuple.Create(result.solutionStatus == SolutionStatus.Intersection, vertices);
}
private static (SolutionStatus, Vector2) EvolveSimplex(IShape2D shapeA, Transform2D Transform2DA, IShape2D shapeB, Transform2D Transform2DB, SimplexVertices vertices, Vector2 direction)
{
switch(vertices.Count)
{
case 0:
if (direction == Vector2.Zero)
{
direction = Vector2.UnitX;
}
break;
case 1:
direction *= -1;
break;
case 2:
var ab = vertices[1] - vertices[0];
var a0 = vertices[0] * -1;
direction = TripleProduct(ab, a0, ab);
if (direction == Vector2.Zero)
{
direction = Perpendicular(ab);
}
break;
case 3:
var c0 = vertices[2] * -1;
var bc = vertices[1] - vertices[2];
var ca = vertices[0] - vertices[2];
var bcNorm = TripleProduct(ca, bc, bc);
var caNorm = TripleProduct(bc, ca, ca);
// the origin is outside line bc
// get rid of a and add a new support in the direction of bcNorm
if (Vector2.Dot(bcNorm, c0) > 0)
{
vertices.RemoveAt(0);
direction = bcNorm;
}
// the origin is outside line ca
// get rid of b and add a new support in the direction of caNorm
else if (Vector2.Dot(caNorm, c0) > 0)
{
vertices.RemoveAt(1);
direction = caNorm;
}
// the origin is inside both ab and ac,
// so it must be inside the triangle!
else
{
return (SolutionStatus.Intersection, direction);
}
break;
}
return (AddSupport(shapeA, Transform2DA, shapeB, Transform2DB, vertices, direction) ?
SolutionStatus.StillSolving :
SolutionStatus.NoIntersection, direction);
}
private static bool AddSupport(IShape2D shapeA, Transform2D Transform2DA, IShape2D shapeB, Transform2D Transform2DB, SimplexVertices vertices, Vector2 direction)
{
var newVertex = shapeA.Support(direction, Transform2DA) - shapeB.Support(-direction, Transform2DB);
vertices.Add(newVertex);
return Vector2.Dot(direction, newVertex) >= 0;
}
private static Vector2 TripleProduct(Vector2 a, Vector2 b, Vector2 c)
{
var A = new Vector3(a.X, a.Y, 0);
var B = new Vector3(b.X, b.Y, 0);
var C = new Vector3(c.X, c.Y, 0);
var first = Vector3.Cross(A, B);
var second = Vector3.Cross(first, C);
return new Vector2(second.X, second.Y);
}
private static Vector2 Perpendicular(Vector2 v)
{
return new Vector2(v.Y, -v.X);
}
}
}