// This crops a polygon with a plane and keeps only a certain part of the polygon
protected void CropPoly(ref WallPolygon poly, Plane p, bool keepfront)
{
const float NEAR_ZERO = 0.01f;
float sideswitch = keepfront ? 1 : -1;
WallPolygon newp = new WallPolygon(poly.Count);
poly.CopyProperties(newp);
if (poly.Count > 0)
{
// First split lines that cross the plane so that we have vertices on the plane where the lines cross
Vector3D v1 = poly[poly.Count - 1];
float side1 = p.Distance(v1) * sideswitch;
for (int i = 0; i < poly.Count; i++)
{
// Fetch vertex and determine side
Vector3D v2 = poly[i];
float side2 = p.Distance(v2) * sideswitch;
// Front?
if (side2 > NEAR_ZERO)
{
if (side1 < -NEAR_ZERO)
{
// Split line with plane and insert the vertex
float u = 0.0f;
p.GetIntersection(v1, v2, ref u);
Vector3D v3 = v1 + (v2 - v1) * u;
newp.Add(v3);
}
newp.Add(v2);
}
// Back?
else if (side2 < -NEAR_ZERO)
{
if (side1 > NEAR_ZERO)
{
// Split line with plane and insert the vertex
float u = 0.0f;
p.GetIntersection(v1, v2, ref u);
Vector3D v3 = v1 + (v2 - v1) * u;
newp.Add(v3);
}
}
else
{
// On the plane
newp.Add(v2);
}
// Next
v1 = v2;
side1 = side2;
}
}
poly = newp;
}
// This splits a polygon with a plane and returns the other part as a new polygon
// The polygon is expected to be convex and clockwise
protected WallPolygon SplitPoly(ref WallPolygon poly, Plane p, bool keepfront)
{
const float NEAR_ZERO = 0.01f;
WallPolygon front = new WallPolygon(poly.Count);
WallPolygon back = new WallPolygon(poly.Count);
poly.CopyProperties(front);
poly.CopyProperties(back);
if (poly.Count > 0)
{
// Go for all vertices to see which side they have to be on
Vector3D v1 = poly[poly.Count - 1];
float side1 = p.Distance(v1);
for (int i = 0; i < poly.Count; i++)
{
// Fetch vertex and determine side
Vector3D v2 = poly[i];
float side2 = p.Distance(v2);
// Front?
if (side2 > NEAR_ZERO)
{
if (side1 < -NEAR_ZERO)
{
// Split line with plane and insert the vertex
float u = 0.0f;
p.GetIntersection(v1, v2, ref u);
Vector3D v3 = v1 + (v2 - v1) * u;
front.Add(v3);
back.Add(v3);
}
front.Add(v2);
}
// Back?
else if (side2 < -NEAR_ZERO)
{
if (side1 > NEAR_ZERO)
{
// Split line with plane and insert the vertex
float u = 0.0f;
p.GetIntersection(v1, v2, ref u);
Vector3D v3 = v1 + (v2 - v1) * u;
front.Add(v3);
back.Add(v3);
}
back.Add(v2);
}
else
{
// On the plane, add to both polygons
front.Add(v2);
back.Add(v2);
}
// Next
v1 = v2;
side1 = side2;
}
}
if (keepfront)
{
poly = front;
return(back);
}
else
{
poly = back;
return(front);
}
}