// Cohen-Sutherland line clippign algorithm, adapted to efficiently
// handle polylines rather than just segments
public static GeoPoint[] Polyline(GeoPoint[] points, GeoBoundary bbox)
{
int len = points.Length;
int codeA = BitCode(points[0], bbox);
int lastCode = 0;
List <GeoPoint> ret = new List <GeoPoint>();
List <GeoPoint> part = new List <GeoPoint>();
for (int i = 1; i < len; i++)
{
GeoPoint a = new GeoPoint(points[i - 1]);
GeoPoint b = new GeoPoint(points[i]);
int codeB = lastCode = BitCode(b, bbox);
while (true)
{
if ((codeA | codeB) == 0)
{ // accept
part.Add(a);
if (codeB != lastCode)
{ // segment went outside
part.Add(b);
if (i < len - 1)
{ // start a new line
ret.AddRange(part);
part.Clear();
}
}
else if (i == len - 1)
{
part.Add(b);
}
break;
}
else if ((codeA & codeB) != 0)
{ // trivial reject
break;
}
else if (codeA != 0)
{ // a outside, intersect with clip edge
a = Intersect(a, b, codeA, bbox);
codeA = BitCode(a, bbox);
}
else
{ // b outside
b = Intersect(a, b, codeB, bbox);
codeB = BitCode(b, bbox);
}
}
codeA = lastCode;
}
if (part.Count > 0)
{
ret.AddRange(part);
}
return(ret.ToArray());
}
// intersect a segment against one of the 4 lines that make up the bbox
protected static GeoPoint Intersect(GeoPoint a, GeoPoint b, int edge, GeoBoundary bbox)
{
GeoPoint ret = null;
if ((edge & 8) != 0)
{
ret = new GeoPoint(a.x + (b.x - a.x) * (bbox.north - a.y) / (b.y - a.y), bbox.north);// top
}
else if ((edge & 4) != 0)
{
ret = new GeoPoint(a.x + (b.x - a.x) * (bbox.south - a.y) / (b.y - a.y), bbox.south);// bottom
}
else if ((edge & 2) != 0)
{
ret = new GeoPoint(bbox.east, a.y + (b.y - a.y) * (bbox.east - a.x) / (b.x - a.x));// right
}
else if ((edge & 1) != 0)
{
ret = new GeoPoint(bbox.west, a.y + (b.y - a.y) * (bbox.west - a.x) / (b.x - a.x));// left
}
else
{
ret = null;
}
return(ret);
}
// Sutherland-Hodgeman polygon clipping algorithm
public static GeoPoint[] Polygon(GeoPoint[] points, GeoBoundary bbox)
{
List <GeoPoint> ret = new List <GeoPoint>();
// clip against each side of the clip rectangle
for (int edge = 1; edge <= 8; edge *= 2)
{
ret.Clear();
GeoPoint prev = new GeoPoint(points[points.Length - 1]);
bool prevInside = (BitCode(prev, bbox) & edge) == 0;
for (int i = 0; i < points.Length; i++)
{
GeoPoint p = new GeoPoint(points[i]);
bool inside = (BitCode(p, bbox) & edge) == 0;
// if segment goes through the clip window, add an intersection
if (inside != prevInside)
{
ret.Add(Intersect(prev, p, edge, bbox));
}
if (inside)
{
ret.Add(p); // add a point if it's inside
}
prev = p;
prevInside = inside;
}
points = ret.ToArray();
if (points.Length == 0)
{
break;
}
}
return(ret.ToArray());
}
// bit code reflects the point position relative to the bbox:
// left mid right
// top 1001 1000 1010
// mid 0001 0000 0010
// bottom 0101 0100 0110
public static int BitCode(GeoPoint p, GeoBoundary bbox)
{
int code = 0;
if (p.x < bbox.west)
{
code |= 1; // left
}
else if (p.x > bbox.east)
{
code |= 2; // right
}
if (p.y < bbox.south)
{
code |= 4; // bottom
}
else if (p.y > bbox.north)
{
code |= 8; // top
}
return(code);
}
