public Triangle(Triangle triangle)
{
OrientationPlane = triangle.OrientationPlane;
A = triangle.A;
B = triangle.B;
C = triangle.C;
}
/**
* Categorize or slice @triangle using @plane. Triangles existing completely on the front side of @plane
* are placed in @greaterThan, triangles exisitng completely on the back side of @plane are placed in
* @lessThan. Co-planar triangles go in @greaterThanPlanar if their normals are the same direction as
* @plane's normal, otherwise they go in @lessThanPlanar.
*
* If a triangle spans @plane and has vertices on both sides, it is split into multiple triangles
* along @plane. The resulting triangles are then categorized as specified above (in this case they are either
* in front or back, they could not be co-planar).
*
* Determining the orientation of a given vertex involves taking the dot product of the vertex's position with
* @plane's normal and comparing that to @plane's offset from the origin (plane.D). If the dot-product is greater
* than that offset (plus some small epsilon), it is in front; if it is less than that offset (minus a small epsilon)
* it is in back. Otherwise it is co-planar.
*
*/
public static Orientation SliceTriangle(Triangle triangle, Plane plane,
IList<Triangle> lessThan, IList<Triangle> greaterThan,
IList<Triangle> lessThanPlanar, IList<Triangle> greaterThanPlanar)
{
Orientation[] vertOrientations = new Orientation[3];
Orientation triOrientation = Orientation.CoPlanar;
int orientationsFound = 0;
// loop through each vertex and categorize each based
// on its position relative to @plane.
for(int i =0; i < 3; i++)
{
Orientation currentOrientation = ClassifyVertexOrientation(triangle.GetVertexByIndex(i), plane);
vertOrientations[i] = currentOrientation;
orientationsFound |= (int)currentOrientation;
}
// classify @triangle's orientation relative to @plane based
// on the orientations of all vertices.
if (orientationsFound > (int)Orientation.GreaterThan)
triOrientation = Orientation.Split;
else
triOrientation = (Orientation)orientationsFound;
// place @triangle in the appropriate list based on its orientation, or
// split it if necessary
switch(triOrientation)
{
case Orientation.CoPlanar:
// if @triangle's normal matches @plane's normal, i.e. dot product will be 1,
// then we consider @triangle to be front-facing
float planeTriOrientation = triangle.OrientationPlane.Normal.Dot(plane.Normal);
if(planeTriOrientation > 0)
greaterThanPlanar.Add(triangle);
else
lessThanPlanar.Add(triangle);
break;
case Orientation.LessThan:
lessThan.Add(triangle);
break;
case Orientation.GreaterThan:
greaterThan.Add(triangle);
break;
case Orientation.Split:
List<Vertex> ltSplit = new List<Vertex>();
List<Vertex> gtSplit = new List<Vertex>();
// loop through each edge in @triangle.
// @currentVertex = edge's first vertex.
// @nextVertex = edge's second vertex.
// @currentOrientation = orientation of the edge's first vertex.
// @nextOrientation = orientation of the edge's second vertex.
for(int i=0; i < 3; i++)
{
int currentIndex = i;
int nextIndex = currentIndex < 2 ? currentIndex + 1 : 0;
Orientation currentOrientation = vertOrientations[currentIndex];
Orientation nextOrientation = vertOrientations[nextIndex];
Vertex currentVertex = triangle.GetVertexByIndex(currentIndex);
Vertex nextVertex = triangle.GetVertexByIndex(nextIndex);
Vector3D currentEdge = nextVertex.Position.SubtractedBy(currentVertex.Position);
// vertices are traversed in clock-wise order, which means @triangle's edges
// are also traversed in clock-wise order. this means that as vertices are added to
// @gtSplit and @ltSplit, their respective vertices will also be in clock-wise order.
switch(currentOrientation)
{
case Orientation.CoPlanar:
gtSplit.Add(currentVertex);
ltSplit.Add(currentVertex);
break;
case Orientation.LessThan:
ltSplit.Add(currentVertex);
break;
case Orientation.GreaterThan:
gtSplit.Add(currentVertex);
break;
}
// if we move from a "GreaterThan" orientation to a "LessThan" orientation or vice-versa,
// then we have crossed @plane. this means we need to interpolate between the edge's vertices
// to create a new vertex that is co-planar with @plane.
if(currentOrientation == Orientation.GreaterThan && nextOrientation == Orientation.LessThan ||
currentOrientation == Orientation.LessThan && nextOrientation == Orientation.GreaterThan )
{
float splitPortion = plane.D - plane.Normal.Dot(currentVertex.Position);
float fullPortion = plane.Normal.Dot(currentEdge);
float splitFraction = splitPortion / fullPortion;
Vertex splitVertex = currentVertex.Lerped(nextVertex, splitFraction);
gtSplit.Add(splitVertex);
ltSplit.Add(splitVertex);
}
}
// create 1 new triangle if @ltSplit contains 3 vertices. create 2 triangles if it
// contains 4
if (ltSplit.Count >= 3)
{
lessThan.Add(new Triangle(ltSplit[0], ltSplit[1], ltSplit[2]));
if(ltSplit.Count == 4)
lessThan.Add(new Triangle(ltSplit[0], ltSplit[2], ltSplit[3]));
}
// create 1 new triangle if @gtSplit contains 3 vertices. create 2 triangles if it
// contains 4
if (gtSplit.Count >= 3)
{
greaterThan.Add(new Triangle(gtSplit[0], gtSplit[1], gtSplit[2]));
if(gtSplit.Count == 4)
greaterThan.Add(new Triangle(gtSplit[0], gtSplit[2], gtSplit[3]));
}
break;
}
return triOrientation;
}
