CollisionB2ShapeEdge.go

package box2d

// A line segment (edge) shape. These can be connected in chains or loops
// to other edge shapes. Edges created independently are two-sided and do
// no provide smooth movement across junctions.
type B2EdgeShape struct {
	B2Shape
	// These are the edge vertices
	M_vertex1, M_vertex2 B2Vec2

	// Optional adjacent vertices. These are used for smooth collision.
	M_vertex0, M_vertex3 B2Vec2

	// Uses m_vertex0 and m_vertex3 to create smooth collision.
	M_oneSided bool
}

func MakeB2EdgeShape() B2EdgeShape {
	return B2EdgeShape{
		B2Shape: B2Shape{
			M_type:   B2Shape_Type.E_edge,
			M_radius: B2_polygonRadius,
		},
		M_vertex0:  MakeB2Vec2(0, 0),
		M_vertex3:  MakeB2Vec2(0, 0),
		M_oneSided: false,
	}
}

func NewB2EdgeShape() *B2EdgeShape {
	res := MakeB2EdgeShape()
	return &res
}

///////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////
// B2EdgeShape.cpp
///////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////

// Set this as a part of a sequence. Vertex v0 precedes the edge and vertex v3
// follows. These extra vertices are used to provide smooth movement
// across junctions. This also makes the collision one-sided. The edge
// normal points to the right looking from v1 to v2.
func (edge *B2EdgeShape) SetOneSided(v0 B2Vec2, v1 B2Vec2, v2 B2Vec2, v3 B2Vec2) {
	edge.M_vertex0 = v0
	edge.M_vertex1 = v1
	edge.M_vertex2 = v2
	edge.M_vertex3 = v3
	edge.M_oneSided = true
}

// Set this as an isolated edge. Collision is two-sided.
func (edge *B2EdgeShape) SetTwoSided(v1 B2Vec2, v2 B2Vec2) {
	edge.M_vertex1 = v1
	edge.M_vertex2 = v2
	edge.M_oneSided = false
}

func (edge B2EdgeShape) Clone() B2ShapeInterface {
	clone := NewB2EdgeShape()
	clone.M_vertex0 = edge.M_vertex0
	clone.M_vertex1 = edge.M_vertex1
	clone.M_vertex2 = edge.M_vertex2
	clone.M_vertex3 = edge.M_vertex3
	clone.M_oneSided = edge.M_oneSided
	return clone
}

func (edge *B2EdgeShape) Destroy() {}

func (edge B2EdgeShape) GetChildCount() int {
	return 1
}

func (edge B2EdgeShape) TestPoint(xf B2Transform, p B2Vec2) bool {
	return false
}

// p = p1 + t * d
// v = v1 + s * e
// p1 + t * d = v1 + s * e
// s * e - t * d = p1 - v1
func (edge B2EdgeShape) RayCast(output *B2RayCastOutput, input B2RayCastInput, xf B2Transform, childIndex int) bool {
	// Put the ray into the edge's frame of reference.
	p1 := B2RotVec2MulT(xf.Q, B2Vec2Sub(input.P1, xf.P))
	p2 := B2RotVec2MulT(xf.Q, B2Vec2Sub(input.P2, xf.P))
	d := B2Vec2Sub(p2, p1)

	v1 := edge.M_vertex1
	v2 := edge.M_vertex2
	e := B2Vec2Sub(v2, v1)

	// Normal points to the right, looking from v1 at v2
	normal := MakeB2Vec2(e.Y, -e.X)
	normal.Normalize()

	// q = p1 + t * d
	// dot(normal, q - v1) = 0
	// dot(normal, p1 - v1) + t * dot(normal, d) = 0
	numerator := B2Vec2Dot(normal, B2Vec2Sub(v1, p1))
	if edge.M_oneSided && numerator > 0.0 {
		return false
	}

	denominator := B2Vec2Dot(normal, d)

	if denominator == 0.0 {
		return false
	}

	t := numerator / denominator
	if t < 0.0 || input.MaxFraction < t {
		return false
	}

	q := B2Vec2Add(p1, B2Vec2MulScalar(t, d))

	// q = v1 + s * r
	// s = dot(q - v1, r) / dot(r, r)
	r := B2Vec2Sub(v2, v1)
	rr := B2Vec2Dot(r, r)
	if rr == 0.0 {
		return false
	}

	s := B2Vec2Dot(B2Vec2Sub(q, v1), r) / rr
	if s < 0.0 || 1.0 < s {
		return false
	}

	output.Fraction = t
	if numerator > 0.0 {
		output.Normal = B2RotVec2Mul(xf.Q, normal).OperatorNegate()
	} else {
		output.Normal = B2RotVec2Mul(xf.Q, normal)
	}

	return true
}

func (edge B2EdgeShape) ComputeAABB(aabb *B2AABB, xf B2Transform, childIndex int) {

	v1 := B2TransformVec2Mul(xf, edge.M_vertex1)
	v2 := B2TransformVec2Mul(xf, edge.M_vertex2)

	lower := B2Vec2Min(v1, v2)
	upper := B2Vec2Max(v1, v2)

	r := MakeB2Vec2(edge.M_radius, edge.M_radius)
	aabb.LowerBound = B2Vec2Sub(lower, r)
	aabb.UpperBound = B2Vec2Sub(upper, r)
}

func (edge B2EdgeShape) ComputeMass(massData *B2MassData, density float64) {
	massData.Mass = 0.0
	massData.Center = B2Vec2MulScalar(0.5, B2Vec2Add(edge.M_vertex1, edge.M_vertex2))
	massData.I = 0.0
}