void ComputeShapeInformation(TransformableMeshData data, out ShapeDistributionInformation shapeInformation)
{
//Compute the surface vertices of the shape.
surfaceVertices.Clear();
try
{
ConvexHullHelper.GetConvexHull(data.vertices, surfaceVertices);
for (int i = 0; i < surfaceVertices.Count; i++)
{
AffineTransform.Transform(ref surfaceVertices.Elements[i], ref data.worldTransform, out surfaceVertices.Elements[i]);
}
}
catch
{
surfaceVertices.Clear();
//If the convex hull failed, then the point set has no volume. A mobile mesh is allowed to have zero volume, however.
//In this case, compute the bounding box of all points.
BoundingBox box = new BoundingBox();
for (int i = 0; i < data.vertices.Length; i++)
{
Vector3 v;
data.GetVertexPosition(i, out v);
if (v.X > box.Max.X)
box.Max.X = v.X;
if (v.X < box.Min.X)
box.Min.X = v.X;
if (v.Y > box.Max.Y)
box.Max.Y = v.Y;
if (v.Y < box.Min.Y)
box.Min.Y = v.Y;
if (v.Z > box.Max.Z)
box.Max.Z = v.Z;
if (v.Z < box.Min.Z)
box.Min.Z = v.Z;
}
//Add the corners. This will overestimate the size of the surface a bit.
surfaceVertices.Add(box.Min);
surfaceVertices.Add(box.Max);
surfaceVertices.Add(new Vector3(box.Min.X, box.Min.Y, box.Max.Z));
surfaceVertices.Add(new Vector3(box.Min.X, box.Max.Y, box.Min.Z));
surfaceVertices.Add(new Vector3(box.Max.X, box.Min.Y, box.Min.Z));
surfaceVertices.Add(new Vector3(box.Min.X, box.Max.Y, box.Max.Z));
surfaceVertices.Add(new Vector3(box.Max.X, box.Max.Y, box.Min.Z));
surfaceVertices.Add(new Vector3(box.Max.X, box.Min.Y, box.Max.Z));
}
shapeInformation.Center = new Vector3();
if (solidity == MobileMeshSolidity.Solid)
{
//The following inertia tensor calculation assumes a closed mesh.
shapeInformation.Volume = 0;
for (int i = 0; i < data.indices.Length; i += 3)
{
Vector3 v2, v3, v4;
data.GetTriangle(i, out v2, out v3, out v4);
//Determinant is 6 * volume. It's signed, though; this is because the mesh isn't necessarily convex nor centered on the origin.
float tetrahedronVolume = v2.X * (v3.Y * v4.Z - v3.Z * v4.Y) -
v3.X * (v2.Y * v4.Z - v2.Z * v4.Y) +
v4.X * (v2.Y * v3.Z - v2.Z * v3.Y);
shapeInformation.Volume += tetrahedronVolume;
shapeInformation.Center += tetrahedronVolume * (v2 + v3 + v4);
}
shapeInformation.Center /= shapeInformation.Volume * 4;
shapeInformation.Volume /= 6;
shapeInformation.Volume = Math.Abs(shapeInformation.Volume);
data.worldTransform.Translation -= shapeInformation.Center;
//Source: Explicit Exact Formulas for the 3-D Tetrahedron Inertia Tensor in Terms of its Vertex Coordinates
//http://www.scipub.org/fulltext/jms2/jms2118-11.pdf
//x1, x2, x3, x4 are origin, triangle1, triangle2, triangle3
//Looking to find inertia tensor matrix of the form
// [ a -b' -c' ]
// [ -b' b -a' ]
// [ -c' -a' c ]
float a = 0, b = 0, c = 0, ao = 0, bo = 0, co = 0;
float totalWeight = 0;
for (int i = 0; i < data.indices.Length; i += 3)
{
Vector3 v2, v3, v4;
data.GetTriangle(i, out v2, out v3, out v4);
//Determinant is 6 * volume. It's signed, though; this is because the mesh isn't necessarily convex nor centered on the origin.
float tetrahedronVolume = v2.X * (v3.Y * v4.Z - v3.Z * v4.Y) -
v3.X * (v2.Y * v4.Z - v2.Z * v4.Y) +
v4.X * (v2.Y * v3.Z - v2.Z * v3.Y);
totalWeight += tetrahedronVolume;
a += tetrahedronVolume * (v2.Y * v2.Y + v2.Y * v3.Y + v3.Y * v3.Y + v2.Y * v4.Y + v3.Y * v4.Y + v4.Y * v4.Y +
v2.Z * v2.Z + v2.Z * v3.Z + v3.Z * v3.Z + v2.Z * v4.Z + v3.Z * v4.Z + v4.Z * v4.Z);
b += tetrahedronVolume * (v2.X * v2.X + v2.X * v3.X + v3.X * v3.X + v2.X * v4.X + v3.X * v4.X + v4.X * v4.X +
v2.Z * v2.Z + v2.Z * v3.Z + v3.Z * v3.Z + v2.Z * v4.Z + v3.Z * v4.Z + v4.Z * v4.Z);
c += tetrahedronVolume * (v2.X * v2.X + v2.X * v3.X + v3.X * v3.X + v2.X * v4.X + v3.X * v4.X + v4.X * v4.X +
v2.Y * v2.Y + v2.Y * v3.Y + v3.Y * v3.Y + v2.Y * v4.Y + v3.Y * v4.Y + v4.Y * v4.Y);
ao += tetrahedronVolume * (2 * v2.Y * v2.Z + v3.Y * v2.Z + v4.Y * v2.Z + v2.Y * v3.Z + 2 * v3.Y * v3.Z + v4.Y * v3.Z + v2.Y * v4.Z + v3.Y * v4.Z + 2 * v4.Y * v4.Z);
bo += tetrahedronVolume * (2 * v2.X * v2.Z + v3.X * v2.Z + v4.X * v2.Z + v2.X * v3.Z + 2 * v3.X * v3.Z + v4.X * v3.Z + v2.X * v4.Z + v3.X * v4.Z + 2 * v4.X * v4.Z);
co += tetrahedronVolume * (2 * v2.X * v2.Y + v3.X * v2.Y + v4.X * v2.Y + v2.X * v3.Y + 2 * v3.X * v3.Y + v4.X * v3.Y + v2.X * v4.Y + v3.X * v4.Y + 2 * v4.X * v4.Y);
}
float density = 1 / totalWeight;
float diagonalFactor = density / 10;
float offFactor = -density / 20;
a *= diagonalFactor;
b *= diagonalFactor;
c *= diagonalFactor;
ao *= offFactor;
bo *= offFactor;
co *= offFactor;
shapeInformation.VolumeDistribution = new Matrix3x3(a, bo, co,
bo, b, ao,
co, ao, c);
}
else
{
shapeInformation.Center = new Vector3();
float totalWeight = 0;
for (int i = 0; i < data.indices.Length; i += 3)
{ //Configure the inertia tensor to be local.
Vector3 vA, vB, vC;
data.GetTriangle(i, out vA, out vB, out vC);
Vector3 vAvB;
Vector3 vAvC;
Vector3.Subtract(ref vB, ref vA, out vAvB);
Vector3.Subtract(ref vC, ref vA, out vAvC);
Vector3 cross;
Vector3.Cross(ref vAvB, ref vAvC, out cross);
float weight = cross.Length();
totalWeight += weight;
shapeInformation.Center += weight * (vA + vB + vC) / 3;
}
shapeInformation.Center /= totalWeight;
shapeInformation.Volume = 0;
data.worldTransform.Translation -= shapeInformation.Center;
shapeInformation.VolumeDistribution = new Matrix3x3();
for (int i = 0; i < data.indices.Length; i += 3)
{ //Configure the inertia tensor to be local.
Vector3 vA, vB, vC;
data.GetTriangle(i, out vA, out vB, out vC);
Vector3 vAvB;
Vector3 vAvC;
Vector3.Subtract(ref vB, ref vA, out vAvB);
Vector3.Subtract(ref vC, ref vA, out vAvC);
Vector3 cross;
Vector3.Cross(ref vAvB, ref vAvC, out cross);
float weight = cross.Length();
totalWeight += weight;
Matrix3x3 innerProduct;
Matrix3x3.CreateScale(vA.LengthSquared(), out innerProduct);
Matrix3x3 outerProduct;
Matrix3x3.CreateOuterProduct(ref vA, ref vA, out outerProduct);
Matrix3x3 contribution;
Matrix3x3.Subtract(ref innerProduct, ref outerProduct, out contribution);
Matrix3x3.Multiply(ref contribution, weight, out contribution);
Matrix3x3.Add(ref shapeInformation.VolumeDistribution, ref contribution, out shapeInformation.VolumeDistribution);
Matrix3x3.CreateScale(vB.LengthSquared(), out innerProduct);
Matrix3x3.CreateOuterProduct(ref vB, ref vB, out outerProduct);
Matrix3x3.Subtract(ref innerProduct, ref outerProduct, out outerProduct);
Matrix3x3.Multiply(ref contribution, weight, out contribution);
Matrix3x3.Add(ref shapeInformation.VolumeDistribution, ref contribution, out shapeInformation.VolumeDistribution);
Matrix3x3.CreateScale(vC.LengthSquared(), out innerProduct);
Matrix3x3.CreateOuterProduct(ref vC, ref vC, out outerProduct);
Matrix3x3.Subtract(ref innerProduct, ref outerProduct, out contribution);
Matrix3x3.Multiply(ref contribution, weight, out contribution);
Matrix3x3.Add(ref shapeInformation.VolumeDistribution, ref contribution, out shapeInformation.VolumeDistribution);
}
Matrix3x3.Multiply(ref shapeInformation.VolumeDistribution, 1 / (6 * totalWeight), out shapeInformation.VolumeDistribution);
}
////Configure the inertia tensor to be local.
//Vector3 finalOffset = shapeInformation.Center;
//Matrix3X3 finalInnerProduct;
//Matrix3X3.CreateScale(finalOffset.LengthSquared(), out finalInnerProduct);
//Matrix3X3 finalOuterProduct;
//Matrix3X3.CreateOuterProduct(ref finalOffset, ref finalOffset, out finalOuterProduct);
//Matrix3X3 finalContribution;
//Matrix3X3.Subtract(ref finalInnerProduct, ref finalOuterProduct, out finalContribution);
//Matrix3X3.Subtract(ref shapeInformation.VolumeDistribution, ref finalContribution, out shapeInformation.VolumeDistribution);
}
