Merge pull request #171 from SixLabors/af/fix-SkipEdgesBeforeMinY

Fix SkipEdgesBeforeMinY algorithm

Author: JimBobSquarePants

src/ImageSharp.Drawing/Shapes/Rasterization/PolygonScanner.cs

@@ -139,24 +139,37 @@ private void SkipEdgesBeforeMinY()
             int i0 = 1;
             int i1 = 0;
 
-            // Do fake scans for the lines belonging to edge start and endpoints before minY
+            // Do fake scans of the lines that start before minY.
+            // Instead of fake scanning at every possible subpixel Y location,
+            // only "scan" at start edge Y positions (defined by values in sorted0) and end Y positions (defined by values in sorted1).
+            // Walk the two lists simultaneously following mergesort logic.
             while (this.SubPixelY < this.minY)
             {
                 this.EnterEdges();
                 this.LeaveEdges();
                 this.activeEdges.RemoveLeavingEdges();
 
-                float y0 = this.edges[this.sorted0[i0]].Y0;
-                float y1 = this.edges[this.sorted1[i1]].Y1;
+                bool hasMore0 = i0 < this.sorted0.Length;
+                bool hasMore1 = i1 < this.sorted1.Length;
+
+                if (!hasMore0 && !hasMore1)
+                {
+                    // The entire polygon is outside the scan region, we skipped all edges,
+                    // scanning will not find any intersections.
+                    break;
+                }
+
+                float y0 = hasMore0 ? this.edges[this.sorted0[i0]].Y0 : float.MaxValue;
+                float y1 = hasMore1 ? this.edges[this.sorted1[i1]].Y1 : float.MaxValue;
 
                 if (y0 < y1)
                 {
-                    this.SubPixelY = y1;
+                    this.SubPixelY = y0;
                     i0++;
                 }
                 else
                 {
-                    this.SubPixelY = y0;
+                    this.SubPixelY = y1;
                     i1++;
                 }
             }

tests/ImageSharp.Drawing.Tests/Drawing/DrawPathTests.cs

@@ -5,6 +5,7 @@
 using System.Numerics;
 using SixLabors.ImageSharp.Drawing.Processing;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
 using Xunit;
 
 namespace SixLabors.ImageSharp.Drawing.Tests.Drawing
@@ -74,5 +75,25 @@ public void PathExtendingOffEdgeOfImageShouldNotBeCropped<TPixel>(TestImageProvi
                 appendPixelTypeToFileName: false,
                 appendSourceFileOrDescription: false);
         }
+
+        [Theory]
+        [WithSolidFilledImages(40, 40, "White", PixelTypes.Rgba32)]
+        public void DrawPathClippedOnTop<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            PointF[] points =
+            {
+                new PointF(10f, -10f),
+                new PointF(20f, 20f),
+                new PointF(30f, -30f)
+            };
+
+            IPath path = new PathBuilder().AddLines(points).Build();
+
+            provider.VerifyOperation(
+                image => image.Mutate(x => x.Draw(Color.Black, 1, path)),
+                appendSourceFileOrDescription: false,
+                appendPixelTypeToFileName: false);
+        }
     }
 }

tests/ImageSharp.Drawing.Tests/Shapes/Scan/PolygonScannerTests.cs

@@ -427,7 +427,7 @@ public void NegativeOrientation01(IntersectionRule intersectionRule)
         }
 
         [Fact]
-        public void OutOfBounds()
+        public void OutOfBounds1()
         {
             IPath poly = PolygonFactory.CreatePolygon((1, -5), (5, -5), (5, -3), (10, -1), (10, 2), (12, 4), (1, 4));
 
@@ -445,6 +445,34 @@ public void OutOfBounds()
             this.TestScan(poly, 0, 3, 2, expected);
         }
 
+        [Fact]
+        public void OutOfBounds2()
+        {
+            IPath poly = PolygonFactory.CreatePolygon((3, -3),  (3, 1), (1, 1), (1, -1), (2, -1.5f), (2, 0.5f), (3, -3));
+            FuzzyFloat[][] expected =
+            {
+                new FuzzyFloat[] { 1, 2, 2.14285707, 3 },
+                new FuzzyFloat[] { 1, 2, 2, 3 },
+                new FuzzyFloat[] { 1, 3 }
+            };
+
+            this.TestScan(poly, 0, 1, 2, expected);
+        }
+
+        [Fact]
+        public void AllOutOfBounds()
+        {
+            IPath poly = PolygonFactory.CreatePolygon((1, -3), (3, -3), (2, -1));
+            FuzzyFloat[][] expected =
+            {
+                Array.Empty<FuzzyFloat>(),
+                Array.Empty<FuzzyFloat>(),
+                Array.Empty<FuzzyFloat>(),
+            };
+
+            this.TestScan(poly, 0, 1, 2, expected);
+        }
+
         private static (float Y, FuzzyFloat[] X) Empty(float y) => (y, Array.Empty<FuzzyFloat>());
 
         private static FuzzyFloat F(float x, float eps) => new(x, eps);