private static void ComputeInitialTetrahedron(RawList <Vector3> points, RawList <int> outsidePointCandidates, RawList <int> triangleIndices, out Vector3 centroid)
{
//Find four points on the hull.
//We'll start with using the x axis to identify two points on the hull.
int a, b, c, d;
Vector3 direction;
//Find the extreme points along the x axis.
float minimumX = float.MaxValue, maximumX = -float.MaxValue;
int minimumXIndex = 0, maximumXIndex = 0;
for (int i = 0; i < points.Count; ++i)
{
var v = points.Elements[i];
if (v.X > maximumX)
{
maximumX = v.X;
maximumXIndex = i;
}
else if (v.X < minimumX)
{
minimumX = v.X;
minimumXIndex = i;
}
}
a = minimumXIndex;
b = maximumXIndex;
//Check for redundancies..
if (a == b)
{
throw new ArgumentException("Point set is degenerate; convex hulls must have volume.");
}
//Now, use a second axis perpendicular to the two points we found.
Vector3 ab;
Vector3.Subtract(ref points.Elements[b], ref points.Elements[a], out ab);
Vector3.Cross(ref ab, ref Toolbox.UpVector, out direction);
if (direction.LengthSquared() < Toolbox.Epsilon)
{
Vector3.Cross(ref ab, ref Toolbox.RightVector, out direction);
}
float minimumDot, maximumDot;
int minimumIndex, maximumIndex;
GetExtremePoints(ref direction, points, out maximumDot, out minimumDot, out maximumIndex, out minimumIndex);
//Compare the location of the extreme points to the location of the axis.
float dot;
Vector3.Dot(ref direction, ref points.Elements[a], out dot);
//Use the point further from the axis.
if (Math.Abs(dot - minimumDot) > Math.Abs(dot - maximumDot))
{
//In this case, we should use the minimum index.
c = minimumIndex;
}
else
{
//In this case, we should use the maximum index.
c = maximumIndex;
}
//Check for redundancies..
if (a == c || b == c)
{
throw new ArgumentException("Point set is degenerate; convex hulls must have volume.");
}
//Use a third axis perpendicular to the plane defined by the three unique points a, b, and c.
Vector3 ac;
Vector3.Subtract(ref points.Elements[c], ref points.Elements[a], out ac);
Vector3.Cross(ref ab, ref ac, out direction);
GetExtremePoints(ref direction, points, out maximumDot, out minimumDot, out maximumIndex, out minimumIndex);
//Compare the location of the extreme points to the location of the plane.
Vector3.Dot(ref direction, ref points.Elements[a], out dot);
//Use the point further from the plane.
if (Math.Abs(dot - minimumDot) > Math.Abs(dot - maximumDot))
{
//In this case, we should use the minimum index.
d = minimumIndex;
}
else
{
//In this case, we should use the maximum index.
d = maximumIndex;
}
//Check for redundancies..
if (a == d || b == d || c == d)
{
throw new ArgumentException("Point set is degenerate; convex hulls must have volume.");
}
//Add the triangles.
triangleIndices.Add(a);
triangleIndices.Add(b);
triangleIndices.Add(c);
triangleIndices.Add(a);
triangleIndices.Add(b);
triangleIndices.Add(d);
triangleIndices.Add(a);
triangleIndices.Add(c);
triangleIndices.Add(d);
triangleIndices.Add(b);
triangleIndices.Add(c);
triangleIndices.Add(d);
//The centroid is guaranteed to be within the convex hull. It will be used to verify the windings of triangles throughout the hull process.
Vector3.Add(ref points.Elements[a], ref points.Elements[b], out centroid);
Vector3.Add(ref centroid, ref points.Elements[c], out centroid);
Vector3.Add(ref centroid, ref points.Elements[d], out centroid);
Vector3.Multiply(ref centroid, 0.25f, out centroid);
for (int i = 0; i < triangleIndices.Count; i += 3)
{
var vA = points.Elements[triangleIndices.Elements[i]];
var vB = points.Elements[triangleIndices.Elements[i + 1]];
var vC = points.Elements[triangleIndices.Elements[i + 2]];
//Check the signed volume of a parallelepiped with the edges of this triangle and the centroid.
Vector3 cross;
Vector3.Subtract(ref vB, ref vA, out ab);
Vector3.Subtract(ref vC, ref vA, out ac);
Vector3.Cross(ref ac, ref ab, out cross);
Vector3 offset;
Vector3.Subtract(ref vA, ref centroid, out offset);
float volume;
Vector3.Dot(ref offset, ref cross, out volume);
//This volume/cross product could also be used to check for degeneracy, but we already tested for that.
if (Math.Abs(volume) < Toolbox.BigEpsilon)
{
throw new ArgumentException("Point set is degenerate; convex hulls must have volume.");
}
if (volume < 0)
{
//If the signed volume is negative, that means the triangle's winding is opposite of what we want.
//Flip it around!
var temp = triangleIndices.Elements[i];
triangleIndices.Elements[i] = triangleIndices.Elements[i + 1];
triangleIndices.Elements[i + 1] = temp;
}
}
//Points which belong to the tetrahedra are guaranteed to be 'in' the convex hull. Do not allow them to be considered.
var tetrahedronIndices = CommonResources.GetIntList();
tetrahedronIndices.Add(a);
tetrahedronIndices.Add(b);
tetrahedronIndices.Add(c);
tetrahedronIndices.Add(d);
//Sort the indices to allow a linear time loop.
Array.Sort(tetrahedronIndices.Elements, 0, 4);
int tetrahedronIndex = 0;
for (int i = 0; i < points.Count; ++i)
{
if (tetrahedronIndex < 4 && i == tetrahedronIndices[tetrahedronIndex])
{
//Don't add a tetrahedron index. Now that we've found this index, though, move on to the next one.
++tetrahedronIndex;
}
else
{
outsidePointCandidates.Add(i);
}
}
CommonResources.GiveBack(tetrahedronIndices);
}
/// <summary>
/// Identifies the indices of points in a set which are on the outer convex hull of the set.
/// </summary>
/// <param name="points">List of points in the set.</param>
/// <param name="outputTriangleIndices">List of indices into the input point set composing the triangulated surface of the convex hull.
/// Each group of 3 indices represents a triangle on the surface of the hull.</param>
public static void GetConvexHull(RawList <Vector3> points, RawList <int> outputTriangleIndices)
{
if (points.Count == 0)
{
throw new ArgumentException("Point set must have volume.");
}
RawList <int> outsidePoints = CommonResources.GetIntList();
if (outsidePoints.Capacity < points.Count - 4)
{
outsidePoints.Capacity = points.Count - 4;
}
//Build the initial tetrahedron.
//It will also give us the location of a point which is guaranteed to be within the
//final convex hull. We can use this point to calibrate the winding of triangles.
//A set of outside point candidates (all points other than those composing the tetrahedron) will be returned in the outsidePoints list.
//That list will then be further pruned by the RemoveInsidePoints call.
Vector3 insidePoint;
ComputeInitialTetrahedron(points, outsidePoints, outputTriangleIndices, out insidePoint);
//Compute outside points.
RemoveInsidePoints(points, outputTriangleIndices, outsidePoints);
var edges = CommonResources.GetIntList();
var toRemove = CommonResources.GetIntList();
var newTriangles = CommonResources.GetIntList();
//We're now ready to begin the main loop.
while (outsidePoints.Count > 0)
{
//While the convex hull is incomplete...
for (int k = 0; k < outputTriangleIndices.Count; k += 3)
{
//Find the normal of the triangle
Vector3 normal;
FindNormal(outputTriangleIndices, points, k, out normal);
//Get the furthest point in the direction of the normal.
int maxIndexInOutsideList = GetExtremePoint(ref normal, points, outsidePoints);
int maxIndex = outsidePoints.Elements[maxIndexInOutsideList];
Vector3 maximum = points.Elements[maxIndex];
//If the point is beyond the current triangle, continue.
Vector3 offset;
Vector3.Subtract(ref maximum, ref points.Elements[outputTriangleIndices.Elements[k]], out offset);
float dot;
Vector3.Dot(ref normal, ref offset, out dot);
if (dot > 0)
{
//It's been picked! Remove the maximum point from the outside.
outsidePoints.FastRemoveAt(maxIndexInOutsideList);
//Remove any triangles that can see the point, including itself!
edges.Clear();
toRemove.Clear();
for (int n = outputTriangleIndices.Count - 3; n >= 0; n -= 3)
{
//Go through each triangle, if it can be seen, delete it and use maintainEdge on its edges.
if (IsTriangleVisibleFromPoint(outputTriangleIndices, points, n, ref maximum))
{
//This triangle can see it!
//TODO: CONSIDER CONSISTENT WINDING HAPPYTIMES
MaintainEdge(outputTriangleIndices[n], outputTriangleIndices[n + 1], edges);
MaintainEdge(outputTriangleIndices[n], outputTriangleIndices[n + 2], edges);
MaintainEdge(outputTriangleIndices[n + 1], outputTriangleIndices[n + 2], edges);
//Because fast removals are being used, the order is very important.
//It's pulling indices in from the end of the list in order, and also ensuring
//that we never issue a removal order beyond the end of the list.
outputTriangleIndices.FastRemoveAt(n + 2);
outputTriangleIndices.FastRemoveAt(n + 1);
outputTriangleIndices.FastRemoveAt(n);
}
}
//Create new triangles.
for (int n = 0; n < edges.Count; n += 2)
{
//For each edge, create a triangle with the extreme point.
newTriangles.Add(edges[n]);
newTriangles.Add(edges[n + 1]);
newTriangles.Add(maxIndex);
}
//Only verify the windings of the new triangles.
VerifyWindings(newTriangles, points, ref insidePoint);
outputTriangleIndices.AddRange(newTriangles);
newTriangles.Clear();
//Remove all points from the outsidePoints if they are inside the polyhedron
RemoveInsidePoints(points, outputTriangleIndices, outsidePoints);
//The list has been significantly messed with, so restart the loop.
break;
}
}
}
CommonResources.GiveBack(outsidePoints);
CommonResources.GiveBack(edges);
CommonResources.GiveBack(toRemove);
CommonResources.GiveBack(newTriangles);
}
