/// <summary>
/// Find the intersection points of lines that cross a polygon
/// </summary>
/// <param name="lineFeatureSet">The line features</param>
/// <param name="polygon">The polygon feature</param>
/// <param name="intersectsPerLineSeg">Output array of intersection counts</param>
/// <param name="intersectionPts">Output array of intersection points</param>
/// <param name="polyIntersectLocs">Output array of intersection locations in reference to polygon vertex indices</param>
/// <returns></returns>
public static int FindIntersections(
IFeatureSet lineFeatureSet,
IFeature polygon,
out int[] intersectsPerLineSeg,
out Point[][] intersectionPts,
out int[][] polyIntersectLocs)
{
int numSignChanges = 0; // tracks number of determinant sign changes
int numLines = lineFeatureSet.Features.Count;
int numVerticies = polygon.NumPoints;
int[][] detSigns = new int[numLines][];
bool[][] signChanges = new bool[numLines][]; // keeps track of where sign changes occur
int[][] changeLocations = new int[numLines][];
int[] intersectsPerLine = new int[numLines];
Point[][] intersectPts = new Point[numLines][];
IList<Coordinate> coorPoly = polygon.Coordinates;
// ICoordinate[] secCoor = new ICoordinate[2];
for (int lineNo = 0; lineNo <= numLines - 1; lineNo++)
{
IFeature line = lineFeatureSet.Features[lineNo];
IList<Coordinate> coorLine = line.Coordinates;
int numChangesPerLine = 0;
detSigns[lineNo] = new int[numVerticies];
signChanges[lineNo] = new bool[numVerticies];
intersectPts[lineNo] = new Point[numVerticies];
changeLocations[lineNo] = new int[numVerticies];
for (int vertNo = 0; vertNo <= numVerticies - 1; vertNo++)
{
intersectPts[lineNo][vertNo] = new Point();
Point intersectPt = new Point();
// Calculate the determinant (3x3 square matrix)
double si = TurboDeterm(
coorPoly[vertNo].X,
coorLine[0].X,
coorLine[1].X,
coorPoly[vertNo].Y,
coorLine[0].Y,
coorLine[1].Y);
// Check the determinant result
switch (vertNo)
{
case 0:
if (Math.Abs(si) <= FindIntersectionTolerance)
{
detSigns[lineNo][vertNo] = 0; // we have hit a vertex
}
else if (si > 0)
{
detSigns[lineNo][vertNo] = 1; // +'ve
}
else if (si < 0)
{
detSigns[lineNo][vertNo] = -1; // -'ve
}
signChanges[lineNo][0] = false; // First element will NEVER be a sign change
break;
default:
if (Math.Abs(si) <= FindIntersectionTolerance)
{
detSigns[lineNo][vertNo] = 0;
}
else if (si > 0)
{
detSigns[lineNo][vertNo] = 1;
}
else if (si < 0)
{
detSigns[lineNo][vertNo] = -1;
}
// Check for sign change
if (detSigns[lineNo][vertNo - 1] != detSigns[lineNo][vertNo])
{
IList<Coordinate> secCoor = new List<Coordinate>
{
coorPoly[vertNo - 1], coorPoly[vertNo]
};
// calculate the actual intercept point
LineString polyTestLine1 = new LineString(secCoor);
secCoor = new List<Coordinate> { coorLine[0], coorLine[1] };
LineString polyTestLine2 = new LineString(secCoor);
bool validIntersect = polyTestLine1.Intersects(polyTestLine2);
IGeometry inPt = polyTestLine1.Intersection(polyTestLine2);
if (inPt.Coordinates.Count == 1)
{
intersectPt = new Point(inPt.Coordinate);
}
if (validIntersect)
{
signChanges[lineNo][vertNo] = true;
numSignChanges += 1;
numChangesPerLine += 1;
intersectsPerLine[lineNo] = numChangesPerLine;
// we want to store the valid intersect pts at the
// beginning of the array so we don't have to search for them
intersectPts[lineNo][numChangesPerLine - 1] = intersectPt;
// keep track of where the intersect occurs in reference to polygon
changeLocations[lineNo][numChangesPerLine - 1] = vertNo;
// intersect pt occurs between vertNo-1 and vertNo
}
else
{
signChanges[lineNo][vertNo] = false;
}
}
else
{
signChanges[lineNo][vertNo] = false;
}
break;
}
// end of switch
}
}
polyIntersectLocs = changeLocations;
intersectionPts = intersectPts;
intersectsPerLineSeg = intersectsPerLine;
return numSignChanges;
}
