Add Trapezoid.Rotate() & Translate().

Use them to rotate tree segments appropriately.

Shared/TreeScene.cs

@@ -42,6 +42,20 @@ namespace SemiColinGames {
 
     public struct Trapezoid {
       public Vector2 p1, p2, p3, p4;
+
+      public void Rotate(float angle) {
+        p1 = p1.Rotate(angle);
+        p2 = p2.Rotate(angle);
+        p3 = p3.Rotate(angle);
+        p4 = p4.Rotate(angle);
+      }
+
+      public void Translate(Vector2 position) {
+        p1 = Vector2.Add(p1, position);
+        p2 = Vector2.Add(p2, position);
+        p3 = Vector2.Add(p3, position);
+        p4 = Vector2.Add(p4, position);
+      }
     }
 
     public class TreeNode {
@@ -104,11 +118,25 @@ namespace SemiColinGames {
         queue.RemoveFirst();
         Vector2 outVector = new Vector2(0, parent.Length).Rotate(parent.Orientation);
         Vector2 outPosition = Vector2.Add(parent.Position, outVector);
+
+        // We want a trapezoid with 4 points. A is the in position, B is the out position.
+        // The TreeNode.Length is the distance from A to B.
+        // We come up with the points relative to A being the origin, then rotate the trapezoid
+        // by its orientation, and translate the result to A's actual position.
+        //
+        //     3---B---4     <-- length = outWidth
+        //    /    |    \
+        //   /     |     \
+        //  1------A------2  <-- length = inWidth
+
         Trapezoid t = new Trapezoid();
-        t.p1 = new Vector2(parent.Position.X - parent.InWidth, parent.Position.Y);
-        t.p2 = new Vector2(parent.Position.X + parent.InWidth, parent.Position.Y);
-        t.p3 = new Vector2(outPosition.X - parent.OutWidth, outPosition.Y);
-        t.p4 = new Vector2(outPosition.X + parent.OutWidth, outPosition.Y);
+        t.p1 = new Vector2(-parent.InWidth, 0);
+        t.p2 = new Vector2(parent.InWidth, 0);
+        t.p3 = new Vector2(-parent.OutWidth, parent.Length);
+        t.p4 = new Vector2(parent.OutWidth, parent.Length);
+        t.Rotate(parent.Orientation);
+        t.Translate(parent.Position);
+
         segments.Add(t);
         foreach (TreeNode child in parent.Children) {
           child.Position = outPosition;