paulxstretch/deps/juce/modules/juce_box2d/box2d/Collision/Shapes/b2PolygonShape.cpp

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/*
* Copyright (c) 2006-2009 Erin Catto http://www.box2d.org
*
* This software is provided 'as-is', without any express or implied
* warranty. In no event will the authors be held liable for any damages
* arising from the use of this software.
* Permission is granted to anyone to use this software for any purpose,
* including commercial applications, and to alter it and redistribute it
* freely, subject to the following restrictions:
* 1. The origin of this software must not be misrepresented; you must not
* claim that you wrote the original software. If you use this software
* in a product, an acknowledgment in the product documentation would be
* appreciated but is not required.
* 2. Altered source versions must be plainly marked as such, and must not be
* misrepresented as being the original software.
* 3. This notice may not be removed or altered from any source distribution.
*/
#include "b2PolygonShape.h"
b2Shape* b2PolygonShape::Clone(b2BlockAllocator* allocator) const
{
void* mem = allocator->Allocate(sizeof(b2PolygonShape));
b2PolygonShape* clone = new (mem) b2PolygonShape;
*clone = *this;
return clone;
}
void b2PolygonShape::SetAsBox(float32 hx, float32 hy)
{
m_vertexCount = 4;
m_vertices[0].Set(-hx, -hy);
m_vertices[1].Set( hx, -hy);
m_vertices[2].Set( hx, hy);
m_vertices[3].Set(-hx, hy);
m_normals[0].Set(0.0f, -1.0f);
m_normals[1].Set(1.0f, 0.0f);
m_normals[2].Set(0.0f, 1.0f);
m_normals[3].Set(-1.0f, 0.0f);
m_centroid.SetZero();
}
void b2PolygonShape::SetAsBox(float32 hx, float32 hy, const b2Vec2& center, float32 angle)
{
m_vertexCount = 4;
m_vertices[0].Set(-hx, -hy);
m_vertices[1].Set( hx, -hy);
m_vertices[2].Set( hx, hy);
m_vertices[3].Set(-hx, hy);
m_normals[0].Set(0.0f, -1.0f);
m_normals[1].Set(1.0f, 0.0f);
m_normals[2].Set(0.0f, 1.0f);
m_normals[3].Set(-1.0f, 0.0f);
m_centroid = center;
b2Transform xf;
xf.p = center;
xf.q.Set(angle);
// Transform vertices and normals.
for (int32 i = 0; i < m_vertexCount; ++i)
{
m_vertices[i] = b2Mul(xf, m_vertices[i]);
m_normals[i] = b2Mul(xf.q, m_normals[i]);
}
}
int32 b2PolygonShape::GetChildCount() const
{
return 1;
}
static b2Vec2 ComputeCentroid(const b2Vec2* vs, int32 count)
{
b2Assert(count >= 3);
b2Vec2 c; c.Set(0.0f, 0.0f);
float32 area = 0.0f;
// pRef is the reference point for forming triangles.
// It's location doesn't change the result (except for rounding error).
b2Vec2 pRef(0.0f, 0.0f);
#if 0
// This code would put the reference point inside the polygon.
for (int32 i = 0; i < count; ++i)
{
pRef += vs[i];
}
pRef *= 1.0f / count;
#endif
const float32 inv3 = 1.0f / 3.0f;
for (int32 i = 0; i < count; ++i)
{
// Triangle vertices.
b2Vec2 p1 = pRef;
b2Vec2 p2 = vs[i];
b2Vec2 p3 = i + 1 < count ? vs[i+1] : vs[0];
b2Vec2 e1 = p2 - p1;
b2Vec2 e2 = p3 - p1;
float32 D = b2Cross(e1, e2);
float32 triangleArea = 0.5f * D;
area += triangleArea;
// Area weighted centroid
c += triangleArea * inv3 * (p1 + p2 + p3);
}
// Centroid
b2Assert(area > b2_epsilon);
c *= 1.0f / area;
return c;
}
void b2PolygonShape::Set(const b2Vec2* vertices, int32 count)
{
b2Assert(3 <= count && count <= b2_maxPolygonVertices);
m_vertexCount = count;
// Copy vertices.
for (int32 i = 0; i < m_vertexCount; ++i)
{
m_vertices[i] = vertices[i];
}
// Compute normals. Ensure the edges have non-zero length.
for (int32 i = 0; i < m_vertexCount; ++i)
{
int32 i1 = i;
int32 i2 = i + 1 < m_vertexCount ? i + 1 : 0;
b2Vec2 edge = m_vertices[i2] - m_vertices[i1];
b2Assert(edge.LengthSquared() > b2_epsilon * b2_epsilon);
m_normals[i] = b2Cross(edge, 1.0f);
m_normals[i].Normalize();
}
#ifdef _DEBUG
// Ensure the polygon is convex and the interior
// is to the left of each edge.
for (int32 i = 0; i < m_vertexCount; ++i)
{
int32 i1 = i;
int32 i2 = i + 1 < m_vertexCount ? i + 1 : 0;
b2Vec2 edge = m_vertices[i2] - m_vertices[i1];
for (int32 j = 0; j < m_vertexCount; ++j)
{
// Don't check vertices on the current edge.
if (j == i1 || j == i2)
{
continue;
}
b2Vec2 r = m_vertices[j] - m_vertices[i1];
// If this crashes, your polygon is non-convex, has colinear edges,
// or the winding order is wrong.
float32 s = b2Cross(edge, r);
b2Assert(s > 0.0f && "ERROR: Please ensure your polygon is convex and has a CCW winding order");
}
}
#endif
// Compute the polygon centroid.
m_centroid = ComputeCentroid(m_vertices, m_vertexCount);
}
bool b2PolygonShape::TestPoint(const b2Transform& xf, const b2Vec2& p) const
{
b2Vec2 pLocal = b2MulT(xf.q, p - xf.p);
for (int32 i = 0; i < m_vertexCount; ++i)
{
float32 dot = b2Dot(m_normals[i], pLocal - m_vertices[i]);
if (dot > 0.0f)
{
return false;
}
}
return true;
}
bool b2PolygonShape::RayCast(b2RayCastOutput* output, const b2RayCastInput& input,
const b2Transform& xf, int32 childIndex) const
{
B2_NOT_USED(childIndex);
// Put the ray into the polygon's frame of reference.
b2Vec2 p1 = b2MulT(xf.q, input.p1 - xf.p);
b2Vec2 p2 = b2MulT(xf.q, input.p2 - xf.p);
b2Vec2 d = p2 - p1;
float32 lower = 0.0f, upper = input.maxFraction;
int32 index = -1;
for (int32 i = 0; i < m_vertexCount; ++i)
{
// p = p1 + a * d
// dot(normal, p - v) = 0
// dot(normal, p1 - v) + a * dot(normal, d) = 0
float32 numerator = b2Dot(m_normals[i], m_vertices[i] - p1);
float32 denominator = b2Dot(m_normals[i], d);
if (denominator == 0.0f)
{
if (numerator < 0.0f)
{
return false;
}
}
else
{
// Note: we want this predicate without division:
// lower < numerator / denominator, where denominator < 0
// Since denominator < 0, we have to flip the inequality:
// lower < numerator / denominator <==> denominator * lower > numerator.
if (denominator < 0.0f && numerator < lower * denominator)
{
// Increase lower.
// The segment enters this half-space.
lower = numerator / denominator;
index = i;
}
else if (denominator > 0.0f && numerator < upper * denominator)
{
// Decrease upper.
// The segment exits this half-space.
upper = numerator / denominator;
}
}
// The use of epsilon here causes the assert on lower to trip
// in some cases. Apparently the use of epsilon was to make edge
// shapes work, but now those are handled separately.
//if (upper < lower - b2_epsilon)
if (upper < lower)
{
return false;
}
}
b2Assert(0.0f <= lower && lower <= input.maxFraction);
if (index >= 0)
{
output->fraction = lower;
output->normal = b2Mul(xf.q, m_normals[index]);
return true;
}
return false;
}
void b2PolygonShape::ComputeAABB(b2AABB* aabb, const b2Transform& xf, int32 childIndex) const
{
B2_NOT_USED(childIndex);
b2Vec2 lower = b2Mul(xf, m_vertices[0]);
b2Vec2 upper = lower;
for (int32 i = 1; i < m_vertexCount; ++i)
{
b2Vec2 v = b2Mul(xf, m_vertices[i]);
lower = b2Min(lower, v);
upper = b2Max(upper, v);
}
b2Vec2 r(m_radius, m_radius);
aabb->lowerBound = lower - r;
aabb->upperBound = upper + r;
}
void b2PolygonShape::ComputeMass(b2MassData* massData, float32 density) const
{
// Polygon mass, centroid, and inertia.
// Let rho be the polygon density in mass per unit area.
// Then:
// mass = rho * int(dA)
// centroid.x = (1/mass) * rho * int(x * dA)
// centroid.y = (1/mass) * rho * int(y * dA)
// I = rho * int((x*x + y*y) * dA)
//
// We can compute these integrals by summing all the integrals
// for each triangle of the polygon. To evaluate the integral
// for a single triangle, we make a change of variables to
// the (u,v) coordinates of the triangle:
// x = x0 + e1x * u + e2x * v
// y = y0 + e1y * u + e2y * v
// where 0 <= u && 0 <= v && u + v <= 1.
//
// We integrate u from [0,1-v] and then v from [0,1].
// We also need to use the Jacobian of the transformation:
// D = cross(e1, e2)
//
// Simplification: triangle centroid = (1/3) * (p1 + p2 + p3)
//
// The rest of the derivation is handled by computer algebra.
b2Assert(m_vertexCount >= 3);
b2Vec2 center; center.Set(0.0f, 0.0f);
float32 area = 0.0f;
float32 I = 0.0f;
// s is the reference point for forming triangles.
// It's location doesn't change the result (except for rounding error).
b2Vec2 s(0.0f, 0.0f);
// This code would put the reference point inside the polygon.
for (int32 i = 0; i < m_vertexCount; ++i)
{
s += m_vertices[i];
}
s *= 1.0f / m_vertexCount;
const float32 k_inv3 = 1.0f / 3.0f;
for (int32 i = 0; i < m_vertexCount; ++i)
{
// Triangle vertices.
b2Vec2 e1 = m_vertices[i] - s;
b2Vec2 e2 = i + 1 < m_vertexCount ? m_vertices[i+1] - s : m_vertices[0] - s;
float32 D = b2Cross(e1, e2);
float32 triangleArea = 0.5f * D;
area += triangleArea;
// Area weighted centroid
center += triangleArea * k_inv3 * (e1 + e2);
float32 ex1 = e1.x, ey1 = e1.y;
float32 ex2 = e2.x, ey2 = e2.y;
float32 intx2 = ex1*ex1 + ex2*ex1 + ex2*ex2;
float32 inty2 = ey1*ey1 + ey2*ey1 + ey2*ey2;
I += (0.25f * k_inv3 * D) * (intx2 + inty2);
}
// Total mass
massData->mass = density * area;
// Center of mass
b2Assert(area > b2_epsilon);
center *= 1.0f / area;
massData->center = center + s;
// Inertia tensor relative to the local origin (point s).
massData->I = density * I;
// Shift to center of mass then to original body origin.
massData->I += massData->mass * (b2Dot(massData->center, massData->center) - b2Dot(center, center));
}