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LineClip.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+
+namespace LineClip
+{
+    /// <summary>
+    /// A very fast C# library for clipping polylines and polygons by a bounding box.
+    /// 移植 https://github.com/mapbox/lineclip/blob/master/index.js，Latest commit 10a422f on 19 Jul 2018 to csharp版
+    /// source license : https://github.com/mapbox/lineclip/blob/master/LICENSE
+    /// </summary>
+    class LineClip
+    {
+        // 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();
+        }
+
+        // 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();
+        }
+
+        // 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;
+        }
+
+        // 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;
+        }
+
+    }
+
+    /// <summary>
+    /// 幾何點
+    /// </summary>
+    class GeoPoint
+    {
+        public double x, y;
+        public GeoPoint(double a_x, double a_y)
+        {
+            x = a_x;
+            y = a_y;
+        }
+
+        public GeoPoint(GeoPoint a)
+        {
+            x = a.x;
+            y = a.y;
+        }
+
+
+    }
+
+    /// <summary>
+    /// 幾何邊界，西南東北
+    /// </summary>
+    class GeoBoundary
+    {
+        public double west, south, east, north;
+
+        public GeoBoundary(double a_west, double a_south, double a_east, double a_north)
+        {
+            west = a_west;
+            south = a_south;
+            east = a_east;
+            north = a_north;
+        }
+    }
+
+}