add path outlining

samples/DrawShapesWithImageSharp/DrawShapesWithImageSharp.csproj

@@ -15,8 +15,8 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="ImageSharp.Drawing" Version="1.0.0-alpha5-00040" />
-    <PackageReference Include="ImageSharp" Version="1.0.0-alpha5-00046" />
+    <PackageReference Include="ImageSharp" Version="1.0.0-alpha6-00065" />
+    <PackageReference Include="ImageSharp.Drawing" Version="1.0.0-alpha6-00026" />
   </ItemGroup>
 
 </Project>

samples/DrawShapesWithImageSharp/Program.cs

@@ -33,9 +33,28 @@ namespace SixLabors.Shapes.DrawShapesWithImageSharp
             DrawSerializedOPenSansLetterShape_a();
             DrawSerializedOPenSansLetterShape_o();
 
-            DrawFatL();
+            OutlineFatL();
+            OutlineSquare();
         }
 
+        private static void OutlineFatL()
+        {
+            var shape = new Path(new LinearLineSegment(new Vector2(8, 8),
+               new Vector2(64, 8),
+               new Vector2(64, 64),
+               new Vector2(120, 64),
+               new Vector2(120, 120),
+               new Vector2(8, 120))).GenerateOutline(3, new[] { 2f, 1f });
+            shape.SaveImage("Outline", "ClippedCorner.png");
+        }
+		
+        private static void OutlineSquare()
+        {
+            var p = new Rectangle(new Vector2(10, 10), new Size(30, 30)).GenerateOutline(3, new[] { 2f, 1f });
+            p.SaveImage("Outline", "Square.png");
+        }
+		
+
         private static void DrawFatL()
         {
             var shape = new Polygon(new LinearLineSegment(new Vector2(8, 8),
@@ -80,7 +99,7 @@ namespace SixLabors.Shapes.DrawShapesWithImageSharp
                             .ToArray();
                 return new Polygon(new LinearLineSegment(points));
             }).ToArray();
-            var complex = new ComplexPolygon(polys);
+            var complex = new ComplexPolygon(polys).Scale(10).GenerateOutline(6, new float[] { 2, 1 }); ;
             complex.SaveImage("letter", "o.png");
         }
 

src/SixLabors.Shapes/InternalPath.cs

@@ -282,8 +282,11 @@ namespace SixLabors.Shapes
 
                     if (precaclulate[i].RemoveLastIntersectionAndSkip)
                     {
-                        position--;
-                        count++;
+                        if (position > 0)
+                        {
+                            position--;
+                            count++;
+                        }
                         continue;
                     }
                     if (precaclulate[i].DoIntersect)

src/SixLabors.Shapes/Outliner.cs

@@ -0,0 +1,176 @@
+using ClipperLib;
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
+using System.Numerics;
+using System.Text;
+
+namespace SixLabors.Shapes
+{
+    /// <summary>
+    /// Path extensions to generate outlines of paths.
+    /// </summary>
+    public static class Outliner
+    {
+        private const float ScalingFactor = 1000.0f;
+
+        /// <summary>
+        /// Generates a outline of the path with alternating on and off segments based on the pattern.
+        /// </summary>
+        /// <param name="path">the path to outline</param>
+        /// <param name="width">The final width outline</param>
+        /// <param name="pattern">The pattern made of multiples of the width.</param>
+        /// <returns>A new path representing the outline.</returns>
+        public static IPath GenerateOutline(this IPath path, float width, float[] pattern)
+        {
+            return path.GenerateOutline(width, pattern, false);
+        }
+
+        /// <summary>
+        /// Generates a outline of the path with alternating on and off segments based on the pattern.
+        /// </summary>
+        /// <param name="path">the path to outline</param>
+        /// <param name="width">The final width outline</param>
+        /// <param name="pattern">The pattern made of multiples of the width.</param>
+        /// <param name="startOff">Weather the first item in the pattern is on or off.</param>
+        /// <returns>A new path representing the outline.</returns>
+        public static IPath GenerateOutline(this IPath path, float width, float[] pattern, bool startOff)
+        {
+            if (pattern == null || pattern.Length < 2)
+            {
+                return path.GenerateOutline(width);
+            }
+
+            ImmutableArray<ISimplePath> paths = path.Flatten();
+
+            ClipperOffset offset = new ClipperOffset();
+
+            List<IntPoint> buffer = new List<IntPoint>(3);
+            foreach (ISimplePath p in paths)
+            {
+                bool online = !startOff;
+                float targetLength = pattern[0] * width;
+                int patternPos = 0;
+                // create a new list of points representing the new outline
+                int pCount = p.Points.Length;
+                if (!p.IsClosed)
+                {
+                    pCount--;
+                }
+                int i = 0;
+                Vector2 currentPoint = p.Points[0];
+                
+                while (i < pCount)
+                {
+                    int next = (i + 1) % p.Points.Length;
+                    Vector2 targetPoint = p.Points[next];
+                    float distToNext = Vector2.Distance(currentPoint, targetPoint);
+                    if (distToNext > targetLength)
+                    {
+                        // find a point between the 2 
+                        float t = targetLength / distToNext;
+
+                        Vector2 point = (currentPoint * (1 - t)) + (targetPoint * t);
+                        buffer.Add(currentPoint.ToPoint());
+                        buffer.Add(point.ToPoint());
+                        // we now inset a line joining 
+
+                        if (online)
+                        {
+                            offset.AddPath(buffer, JoinType.jtSquare, EndType.etOpenButt);
+                        }
+                        online = !online;
+
+                        buffer.Clear();
+
+                        currentPoint = point;
+                        
+                        // next length 
+                        patternPos = (patternPos + 1) % pattern.Length;
+                        targetLength = pattern[patternPos] * width;
+
+                    }
+                    else if (distToNext <= targetLength)
+                    {
+                        buffer.Add(currentPoint.ToPoint());
+                        currentPoint = targetPoint;
+                        i++;
+                        targetLength -= distToNext;
+                    }
+                }
+                if (buffer.Count > 0)
+                {
+                    if (p.IsClosed)
+                    {
+                        buffer.Add(p.Points.First().ToPoint());
+                    }
+                    else
+                    {
+                        buffer.Add(p.Points.Last().ToPoint());
+                    }
+
+                    if (online)
+                    {
+                        offset.AddPath(buffer, JoinType.jtSquare, EndType.etOpenButt);
+                    }
+                    online = !online;
+
+                    buffer.Clear();
+                    patternPos = (patternPos + 1) % pattern.Length;
+                    targetLength = pattern[patternPos] * width;
+                }
+            }
+
+            return ExecuteOutliner(width, offset);
+        }
+        /// <summary>
+        /// Generates a solid outline of the path.
+        /// </summary>
+        /// <param name="path">the path to outline</param>
+        /// <param name="width">The final width outline</param>
+        /// <returns>A new path representing the outline.</returns>
+        public static IPath GenerateOutline(this IPath path, float width)
+        {
+            ClipperOffset offset = new ClipperLib.ClipperOffset();
+
+            //pattern can be applied to the path by cutting it into segments
+            System.Collections.Immutable.ImmutableArray<ISimplePath> paths = path.Flatten();
+            foreach (ISimplePath p in paths)
+            {
+                System.Collections.Immutable.ImmutableArray<Vector2> vectors = p.Points;
+                List<IntPoint> points = new List<ClipperLib.IntPoint>(vectors.Length);
+                foreach (Vector2 v in vectors)
+                {
+                    points.Add(new IntPoint(v.X * ScalingFactor, v.Y * ScalingFactor));
+                }
+
+                EndType type = p.IsClosed ? EndType.etClosedLine : EndType.etOpenButt;
+
+
+                offset.AddPath(points, JoinType.jtMiter, type);
+            }
+
+            return ExecuteOutliner(width, offset);
+        }
+
+        private static IPath ExecuteOutliner(float width, ClipperOffset offset)
+        {
+            List<List<IntPoint>> tree = new List<List<IntPoint>>();
+            offset.Execute(ref tree, width * ScalingFactor / 2);
+            List<Polygon> polygons = new List<Polygon>();
+            foreach (List<IntPoint> pt in tree)
+            {
+                Vector2[] points = pt.Select(p => new Vector2(p.X / ScalingFactor, p.Y / ScalingFactor)).ToArray();
+                polygons.Add(new Polygon(new LinearLineSegment(points)));
+            }
+
+            return new ComplexPolygon(polygons.ToArray());
+        }
+
+        private static IntPoint ToPoint(this Vector2 vector)
+        {
+            return new IntPoint(vector.X * ScalingFactor, vector.Y * ScalingFactor);
+        }
+    }
+}