Merge pull request #324 from SixLabors/issue323

Mitigate issue where outline thicknesses less than 0.5 fail out generate solid shapes not outlines

Author: JimBobSquarePants

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 });
+    }
+}