// Returns the centroid of the shape multiplied by the measure of the shape,
// which is defined as follows:
//
// - For dimension 0 shapes, the measure is shape.num_edges().
// - For dimension 1 shapes, the measure is GetLength(shape).
// - For dimension 2 shapes, the measure is GetArea(shape).
//
// Note that the result is not unit length, so you may need to call
// Normalize() before passing it to other S2 functions.
//
// The result is scaled by the measure defined above for two reasons: (1) it
// is cheaper to compute this way, and (2) this makes it easier to compute the
// centroid of a collection of shapes. (This requires simply summing the
// centroids of all shapes in the collection whose dimension is maximal.)
public static S2Point GetCentroid(S2Shape shape)
{
var centroid = S2Point.Empty;
S2Point[] vertices;
int dimension = shape.Dimension();
int num_chains = shape.NumChains();
for (int chain_id = 0; chain_id < num_chains; ++chain_id)
{
switch (dimension)
{
case 0:
centroid += shape.GetEdge(chain_id).V0;
break;
case 1:
GetChainVertices(shape, chain_id, out vertices);
centroid += S2PolylineMeasures.GetCentroid(vertices);
break;
default:
GetChainVertices(shape, chain_id, out vertices);
centroid += S2.GetCentroid(vertices.ToList());
break;
}
}
return(centroid);
}
// Converts a 1-dimensional S2Shape into an S2Polyline. Converts the first
// chain in the shape to a vector of S2Point vertices and uses that to construct
// the S2Polyline. Note that the input 1-dimensional S2Shape must contain at
// most 1 chain, and that this method does not accept an empty shape (i.e. a
// shape with no vertices).
public static S2Polyline ShapeToS2Polyline(S2Shape line)
{
System.Diagnostics.Debug.Assert(line.Dimension() == 1);
System.Diagnostics.Debug.Assert(line.NumChains() == 1);
S2Point[] vertices;
S2.GetChainVertices(line, 0, out vertices);
return new S2Polyline(vertices);
}
// Converts a 0-dimensional S2Shape into a list of S2Points.
// This method does allow an empty shape (i.e. a shape with no vertices).
public static List<S2Point> ShapeToS2Points(S2Shape multipoint)
{
System.Diagnostics.Debug.Assert(multipoint.Dimension() == 0);
List<S2Point> points = new();
points.Capacity = multipoint.NumEdges();
for (int i = 0; i < multipoint.NumEdges(); ++i)
{
points.Add(multipoint.GetEdge(i).V0);
}
return points;
}
// For shapes of dimension 2, returns the area of the shape on the unit
// sphere. The result is between 0 and 4*Pi steradians. Otherwise returns
// zero. This method has good relative accuracy for both very large and very
// small regions.
//
// All edges are modeled as spherical geodesics. The result can be converted
// to an area on the Earth's surface (with a worst-case error of 0.900% near
// the poles) using the functions in s2earth.h.
public static double GetArea(S2Shape shape)
{
if (shape.Dimension() != 2)
{
return(0.0);
}
// Since S2Shape uses the convention that the interior of the shape is to
// the left of all edges, in theory we could compute the area of the polygon
// by simply adding up all the loop areas modulo 4*Pi. The problem with
// this approach is that polygons holes typically have areas near 4*Pi,
// which can create large cancellation errors when computing the area of
// small polygons with holes. For example, a shell with an area of 4 square
// meters (1e-13 steradians) surrounding a hole with an area of 3 square
// meters (7.5e-14 sterians) would lose almost all of its accuracy if the
// area of the hole was computed as 12.566370614359098.
//
// So instead we use S2.GetSignedArea() to ensure that all loops have areas
// in the range [-2*Pi, 2*Pi].
//
// TODO(ericv): Rarely, this function returns the area of the complementary
// region (4*Pi - area). This can only happen when the true area is very
// close to zero or 4*Pi and the polygon has multiple loops. To make this
// function completely robust requires checking whether the signed area sum is
// ambiguous, and if so, determining the loop nesting structure. This allows
// the sum to be evaluated in a way that is guaranteed to have the correct
// sign.
double area = 0;
double max_error = 0;
int num_chains = shape.NumChains();
for (int chain_id = 0; chain_id < num_chains; ++chain_id)
{
GetChainVertices(shape, chain_id, out var vertices);
area += S2.GetSignedArea(vertices.ToList());
#if DEBUG
max_error += S2.GetCurvatureMaxError(new(vertices));
#endif
}
// Note that S2.GetSignedArea() guarantees that the full loop (containing
// all points on the sphere) has a very small negative area.
System.Diagnostics.Debug.Assert(Math.Abs(area) <= S2.M_4_PI + max_error);
if (area < 0.0)
{
area += S2.M_4_PI;
}
return(area);
}
// For shapes of dimension 2, returns the sum of all loop perimeters on the
// unit sphere. Otherwise returns zero. (See GetLength for shapes of
// dimension 1.)
//
// All edges are modeled as spherical geodesics. The result can be converted
// to a distance on the Earth's surface (with a worst-case error of 0.562%
// near the equator) using the functions in s2earth.h.
public static S1Angle GetPerimeter(S2Shape shape)
{
if (shape.Dimension() != 2)
{
return(S1Angle.Zero);
}
var perimeter = S1Angle.Zero;
int num_chains = shape.NumChains();
for (int chain_id = 0; chain_id < num_chains; ++chain_id)
{
GetChainVertices(shape, chain_id, out var vertices);
perimeter += S2.GetPerimeter(vertices);
}
return(perimeter);
}
// For shapes of dimension 1, returns the sum of all polyline lengths on the
// unit sphere. Otherwise returns zero. (See GetPerimeter for shapes of
// dimension 2.)
//
// All edges are modeled as spherical geodesics. The result can be converted
// to a distance on the Earth's surface (with a worst-case error of 0.562%
// near the equator) using the functions in s2earth.h.
public static S1Angle GetLength(S2Shape shape)
{
if (shape.Dimension() != 1)
{
return(S1Angle.Zero);
}
var length = S1Angle.Zero;
int num_chains = shape.NumChains();
for (int chain_id = 0; chain_id < num_chains; ++chain_id)
{
GetChainVertices(shape, chain_id, out var vertices);
length += S2PolylineMeasures.GetLength(vertices);
}
return(length);
}
// Converts a 2-dimensional S2Shape into an S2Polygon. Each chain is converted
// to an S2Loop and the vector of loops is used to construct the S2Polygon.
public static S2Polygon ShapeToS2Polygon(S2Shape poly)
{
if (poly.IsFull())
{
return new S2Polygon(S2Loop.kFull);
}
System.Diagnostics.Debug.Assert(poly.Dimension() == 2);
List<S2Loop> loops = new();
for (int i = 0; i < poly.NumChains(); ++i)
{
S2.GetChainVertices(poly, i, out var vertices);
loops.Add(new S2Loop(vertices));
}
var output_poly = new S2Polygon(loops, initOriented: true);
return output_poly;
}
// Overwrites "vertices" with the vertices of the given edge chain of "shape".
// If dimension == 1, the chain will have (chain.length + 1) vertices, and
// otherwise it will have (chain.length) vertices.
//
// This is a low-level helper method used in the implementations of some of
// the methods above.
public static void GetChainVertices(S2Shape shape, int chain_id, out S2Point[] vertices)
{
S2Shape.Chain chain = shape.GetChain(chain_id);
int num_vertices = chain.Length + (shape.Dimension() == 1 ? 1 : 0);
int e = 0;
var verts = new List <S2Point>();
if ((num_vertices & 1) != 0)
{
verts.Add(shape.ChainEdge(chain_id, e++).V0);
}
for (; e < num_vertices; e += 2)
{
var edge = shape.ChainEdge(chain_id, e);
verts.Add(edge.V0);
verts.Add(edge.V1);
}
vertices = verts.ToArray();
}
/// <summary>
/// Verifies that all methods of the two S2Shapes return identical results,
/// except for id() and type_tag().
/// </summary>
public static void ExpectEqual(S2Shape a, S2Shape b)
{
Assert.True(a.NumEdges() == b.NumEdges());
for (int i = 0; i < a.NumEdges(); ++i)
{
Assert.Equal(a.GetEdge(i), b.GetEdge(i));
Assert.True(a.GetChainPosition(i) == b.GetChainPosition(i));
}
Assert.True(a.Dimension() == b.Dimension());
Assert.True(a.GetReferencePoint() == b.GetReferencePoint());
Assert.True(a.NumChains() == b.NumChains());
for (int i = 0; i < a.NumChains(); ++i)
{
Assert.True(a.GetChain(i) == b.GetChain(i));
int chain_length = a.GetChain(i).Length;
for (int j = 0; j < chain_length; ++j)
{
Assert.True(a.ChainEdge(i, j) == b.ChainEdge(i, j));
}
}
}
// Like GetArea(), except that this method is faster and has more error. The
// additional error is at most 2.22e-15 steradians per vertex, which works out
// to about 0.09 square meters per vertex on the Earth's surface. For
// example, a loop with 100 vertices has a maximum error of about 9 square
// meters. (The actual error is typically much smaller than this.)
public static double GetApproxArea(S2Shape shape)
{
if (shape.Dimension() != 2)
{
return(0.0);
}
double area = 0;
int num_chains = shape.NumChains();
for (int chain_id = 0; chain_id < num_chains; ++chain_id)
{
GetChainVertices(shape, chain_id, out var vertices);
area += S2.GetApproxArea(vertices.ToList()); //todo
}
// Special case to ensure that full polygons are handled correctly.
if (area <= S2.M_4_PI)
{
return(area);
}
return(area % S2.M_4_PI);
}
// Returns true if the given shape contains the given point. Most clients
// should not use this method, since its running time is linear in the number
// of shape edges. Instead clients should create an S2ShapeIndex and use
// S2ContainsPointQuery, since this strategy is much more efficient when many
// points need to be tested.
//
// Polygon boundaries are treated as being semi-open (see S2ContainsPointQuery
// and S2VertexModel for other options).
//
// CAVEAT: Typically this method is only used internally. Its running time is
// linear in the number of shape edges.
public static bool ContainsBruteForce(this S2Shape shape, S2Point point)
{
if (shape.Dimension() < 2)
{
return(false);
}
var ref_point = shape.GetReferencePoint();
if (ref_point.Point == point)
{
return(ref_point.Contained);
}
S2CopyingEdgeCrosser crosser = new(ref_point.Point, point);
bool inside = ref_point.Contained;
for (int e = 0; e < shape.NumEdges(); ++e)
{
var edge = shape.GetEdge(e);
inside ^= crosser.EdgeOrVertexCrossing(edge.V0, edge.V1);
}
return(inside);
}
// This is a helper function for implementing S2Shape.GetReferencePoint().
//
// Given a shape consisting of closed polygonal loops, the interior of the
// shape is defined as the region to the left of all edges (which must be
// oriented consistently). This function then chooses an arbitrary point and
// returns true if that point is contained by the shape.
//
// Unlike S2Loop and S2Polygon, this method allows duplicate vertices and
// edges, which requires some extra care with definitions. The rule that we
// apply is that an edge and its reverse edge "cancel" each other: the result
// is the same as if that edge pair were not present. Therefore shapes that
// consist only of degenerate loop(s) are either empty or full; by convention,
// the shape is considered full if and only if it contains an empty loop (see
// S2LaxPolygonShape for details).
//
// Determining whether a loop on the sphere contains a point is harder than
// the corresponding problem in 2D plane geometry. It cannot be implemented
// just by counting edge crossings because there is no such thing as a "point
// at infinity" that is guaranteed to be outside the loop.
public static S2Shape.ReferencePoint GetReferencePoint(this S2Shape shape)
{
System.Diagnostics.Debug.Assert(shape.Dimension() == 2);
if (shape.NumEdges() == 0)
{
// A shape with no edges is defined to be full if and only if it
// contains at least one chain.
return(S2Shape.ReferencePoint.FromContained(shape.NumChains() > 0));
}
// Define a "matched" edge as one that can be paired with a corresponding
// reversed edge. Define a vertex as "balanced" if all of its edges are
// matched. In order to determine containment, we must find an unbalanced
// vertex. Often every vertex is unbalanced, so we start by trying an
// arbitrary vertex.
var edge = shape.GetEdge(0);
if (GetReferencePointAtVertex(shape, edge.V0, out var result))
{
return(result);
}
// That didn't work, so now we do some extra work to find an unbalanced
// vertex (if any). Essentially we gather a list of edges and a list of
// reversed edges, and then sort them. The first edge that appears in one
// list but not the other is guaranteed to be unmatched.
int n = shape.NumEdges();
var edges = new S2Shape.Edge[n];
var rev_edges = new S2Shape.Edge[n];
for (int i = 0; i < n; ++i)
{
var edge2 = shape.GetEdge(i);
edges[i] = edge2;
rev_edges[i] = new S2Shape.Edge(edge2.V1, edge2.V0);
}
Array.Sort(edges);
Array.Sort(rev_edges);
for (int i = 0; i < n; ++i)
{
if (edges[i] < rev_edges[i])
{ // edges[i] is unmatched
System.Diagnostics.Debug.Assert(GetReferencePointAtVertex(shape, edges[i].V0, out result));
return(result);
}
if (rev_edges[i] < edges[i])
{ // rev_edges[i] is unmatched
System.Diagnostics.Debug.Assert(GetReferencePointAtVertex(shape, rev_edges[i].V0, out result));
return(result);
}
}
// All vertices are balanced, so this polygon is either empty or full except
// for degeneracies. By convention it is defined to be full if it contains
// any chain with no edges.
for (int i = 0; i < shape.NumChains(); ++i)
{
if (shape.GetChain(i).Length == 0)
{
return(S2Shape.ReferencePoint.FromContained(true));
}
}
return(S2Shape.ReferencePoint.FromContained(false));
}