mitigate limits of outline generation

if the outline thickness is going to trigger the noop behaviour of the clipper then scale the inout and output to no trigger that behaviour.

Author: tocsoft

src/ImageSharp.Drawing/Shapes/OutlinePathExtensions.cs

@@ -15,6 +15,31 @@ public static class OutlinePathExtensions
     private const JointStyle DefaultJointStyle = JointStyle.Square;
     private const EndCapStyle DefaultEndCapStyle = EndCapStyle.Butt;
 
+    /// <summary>
+    /// Calculates the scaling matrixes tha tmust be applied to the inout and output paths of for successful clipping.
+    /// </summary>
+    /// <param name="width">the requested width</param>
+    /// <param name="scaleUpMartrix">The matrix to apply to the input path</param>
+    /// <param name="scaleDownMartrix">The matrix to apply to the output path</param>
+    /// <returns>The final width to use internally to outlining</returns>
+    private static float CalculateScalingMatrix(float width, out Matrix3x2 scaleUpMartrix, out Matrix3x2 scaleDownMartrix)
+    {
+        // when the thickness is below a 0.5 threshold we need to scale
+        // the source path (up) and result path (down) by a factor to ensure
+        // the offest is greater than 0.5 to ensure offsetting isn't skipped.
+        scaleUpMartrix = Matrix3x2.Identity;
+        scaleDownMartrix = Matrix3x2.Identity;
+        if (width < 0.5)
+        {
+            float scale = 1 / width;
+            scaleUpMartrix = Matrix3x2.CreateScale(scale);
+            scaleDownMartrix = Matrix3x2.CreateScale(width);
+            width = 1;
+        }
+
+        return width;
+    }
+
     /// <summary>
     /// Generates an outline of the path.
     /// </summary>
@@ -41,10 +66,14 @@ public static IPath GenerateOutline(this IPath path, float width, JointStyle joi
             return Path.Empty;
         }
 
+        width = CalculateScalingMatrix(width, out Matrix3x2 scaleUpMartrix, out Matrix3x2 scaleDownMartrix);
+
         ClipperOffset offset = new(MiterOffsetDelta);
-        offset.AddPath(path, jointStyle, endCapStyle);
 
-        return offset.Execute(width);
+        // transform is noop for Matrix3x2.Identity
+        offset.AddPath(path.Transform(scaleUpMartrix), jointStyle, endCapStyle);
+
+        return offset.Execute(width).Transform(scaleDownMartrix);
     }
 
     /// <summary>
@@ -106,7 +135,9 @@ public static IPath GenerateOutline(this IPath path, float width, ReadOnlySpan<f
             return path.GenerateOutline(width, jointStyle, endCapStyle);
         }
 
-        IEnumerable<ISimplePath> paths = path.Flatten();
+        width = CalculateScalingMatrix(width, out Matrix3x2 scaleUpMartrix, out Matrix3x2 scaleDownMartrix);
+
+        IEnumerable<ISimplePath> paths = path.Transform(scaleUpMartrix).Flatten();
 
         ClipperOffset offset = new(MiterOffsetDelta);
         List<PointF> buffer = new();
@@ -186,6 +217,6 @@ public static IPath GenerateOutline(this IPath path, float width, ReadOnlySpan<f
             }
         }
 
-        return offset.Execute(width);
+        return offset.Execute(width).Transform(scaleDownMartrix);
     }
 }

tests/ImageSharp.Drawing.Tests/Issues/Issue_323.cs

@@ -0,0 +1,54 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Drawing.Processing;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Drawing.Tests.Issues;
+
+public class Issue_323
+{
+    [Theory]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 3f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 1f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 0.3f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 0.7f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 0.003f)]
+    public void DrawPolygonMustDrawoutlineOnly<TPixel>(TestImageProvider<TPixel> provider, float scale)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        Color color = Color.RebeccaPurple;
+        provider.RunValidatingProcessorTest(
+            x => x.DrawPolygon(
+                color,
+                scale,
+                new PointF[] {
+                    new(5, 5),
+                    new(5, 150),
+                    new(190, 150),
+                }),
+            new { scale });
+    }
+
+    [Theory]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 3f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 1f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 0.3f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 0.7f)]
+    [WithSolidFilledImages(300, 300, "White", PixelTypes.Rgba32, 0.003f)]
+    public void DrawPolygonMustDrawoutlineOnly_Pattern<TPixel>(TestImageProvider<TPixel> provider, float scale)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        Color color = Color.RebeccaPurple;
+        var pen = Pens.DashDot(color, scale);
+        provider.RunValidatingProcessorTest(
+                    x => x.DrawPolygon(
+                      pen,
+                      new PointF[] {
+                            new(5, 5),
+                            new(5, 150),
+                            new(190, 150),
+                        }),
+                    new { scale });
+    }
+}