embedded_libs/box2d-2.4.1/src/collision/b2_polygon_shape.cpp
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// MIT License
// Copyright (c) 2019 Erin Catto
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
#include "box2d/b2_polygon_shape.h"
#include "box2d/b2_block_allocator.h"
#include <new>
b2Shape* b2PolygonShape::Clone(b2BlockAllocator* allocator) const
{
void* mem = allocator->Allocate(sizeof(b2PolygonShape));
b2PolygonShape* clone = new (mem) b2PolygonShape;
*clone = *this;
return clone;
}
void b2PolygonShape::SetAsBox(float hx, float hy)
{
m_count = 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(float hx, float hy, const b2Vec2& center, float angle)
{
m_count = 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_count; ++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(0.0f, 0.0f);
float area = 0.0f;
// Get a reference point for forming triangles.
// Use the first vertex to reduce round-off errors.
b2Vec2 s = vs[0];
const float inv3 = 1.0f / 3.0f;
for (int32 i = 0; i < count; ++i)
{
// Triangle vertices.
b2Vec2 p1 = vs[0] - s;
b2Vec2 p2 = vs[i] - s;
b2Vec2 p3 = i + 1 < count ? vs[i+1] - s : vs[0] - s;
b2Vec2 e1 = p2 - p1;
b2Vec2 e2 = p3 - p1;
float D = b2Cross(e1, e2);
float triangleArea = 0.5f * D;
area += triangleArea;
// Area weighted centroid
c += triangleArea * inv3 * (p1 + p2 + p3);
}
// Centroid
b2Assert(area > b2_epsilon);
c = (1.0f / area) * c + s;
return c;
}
void b2PolygonShape::Set(const b2Vec2* vertices, int32 count)
{
b2Assert(3 <= count && count <= b2_maxPolygonVertices);
if (count < 3)
{
SetAsBox(1.0f, 1.0f);
return;
}
int32 n = b2Min(count, b2_maxPolygonVertices);
// Perform welding and copy vertices into local buffer.
b2Vec2 ps[b2_maxPolygonVertices];
int32 tempCount = 0;
for (int32 i = 0; i < n; ++i)
{
b2Vec2 v = vertices[i];
bool unique = true;
for (int32 j = 0; j < tempCount; ++j)
{
if (b2DistanceSquared(v, ps[j]) < ((0.5f * b2_linearSlop) * (0.5f * b2_linearSlop)))
{
unique = false;
break;
}
}
if (unique)
{
ps[tempCount++] = v;
}
}
n = tempCount;
if (n < 3)
{
// Polygon is degenerate.
b2Assert(false);
SetAsBox(1.0f, 1.0f);
return;
}
// Create the convex hull using the Gift wrapping algorithm
// http://en.wikipedia.org/wiki/Gift_wrapping_algorithm
// Find the right most point on the hull
int32 i0 = 0;
float x0 = ps[0].x;
for (int32 i = 1; i < n; ++i)
{
float x = ps[i].x;
if (x > x0 || (x == x0 && ps[i].y < ps[i0].y))
{
i0 = i;
x0 = x;
}
}
int32 hull[b2_maxPolygonVertices];
int32 m = 0;
int32 ih = i0;
for (;;)
{
b2Assert(m < b2_maxPolygonVertices);
hull[m] = ih;
int32 ie = 0;
for (int32 j = 1; j < n; ++j)
{
if (ie == ih)
{
ie = j;
continue;
}
b2Vec2 r = ps[ie] - ps[hull[m]];
b2Vec2 v = ps[j] - ps[hull[m]];
float c = b2Cross(r, v);
if (c < 0.0f)
{
ie = j;
}
// Collinearity check
if (c == 0.0f && v.LengthSquared() > r.LengthSquared())
{
ie = j;
}
}
++m;
ih = ie;
if (ie == i0)
{
break;
}
}
if (m < 3)
{
// Polygon is degenerate.
b2Assert(false);
SetAsBox(1.0f, 1.0f);
return;
}
m_count = m;
// Copy vertices.
for (int32 i = 0; i < m; ++i)
{
m_vertices[i] = ps[hull[i]];
}
// Compute normals. Ensure the edges have non-zero length.
for (int32 i = 0; i < m; ++i)
{
int32 i1 = i;
int32 i2 = i + 1 < m ? 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();
}
// Compute the polygon centroid.
m_centroid = ComputeCentroid(m_vertices, m);
}
bool b2PolygonShape::TestPoint(const b2Transform& xf, const b2Vec2& p) const
{
b2Vec2 pLocal = b2MulT(xf.q, p - xf.p);
for (int32 i = 0; i < m_count; ++i)
{
float 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;
float lower = 0.0f, upper = input.maxFraction;
int32 index = -1;
for (int32 i = 0; i < m_count; ++i)
{
// p = p1 + a * d
// dot(normal, p - v) = 0
// dot(normal, p1 - v) + a * dot(normal, d) = 0
float numerator = b2Dot(m_normals[i], m_vertices[i] - p1);
float 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_count; ++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, float 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_count >= 3);
b2Vec2 center(0.0f, 0.0f);
float area = 0.0f;
float I = 0.0f;
// Get a reference point for forming triangles.
// Use the first vertex to reduce round-off errors.
b2Vec2 s = m_vertices[0];
const float k_inv3 = 1.0f / 3.0f;
for (int32 i = 0; i < m_count; ++i)
{
// Triangle vertices.
b2Vec2 e1 = m_vertices[i] - s;
b2Vec2 e2 = i + 1 < m_count ? m_vertices[i+1] - s : m_vertices[0] - s;
float D = b2Cross(e1, e2);
float triangleArea = 0.5f * D;
area += triangleArea;
// Area weighted centroid
center += triangleArea * k_inv3 * (e1 + e2);
float ex1 = e1.x, ey1 = e1.y;
float ex2 = e2.x, ey2 = e2.y;
float intx2 = ex1*ex1 + ex2*ex1 + ex2*ex2;
float 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));
}
bool b2PolygonShape::Validate() const
{
for (int32 i = 0; i < m_count; ++i)
{
int32 i1 = i;
int32 i2 = i < m_count - 1 ? i1 + 1 : 0;
b2Vec2 p = m_vertices[i1];
b2Vec2 e = m_vertices[i2] - p;
for (int32 j = 0; j < m_count; ++j)
{
if (j == i1 || j == i2)
{
continue;
}
b2Vec2 v = m_vertices[j] - p;
float c = b2Cross(e, v);
if (c < 0.0f)
{
return false;
}
}
}
return true;
}