prevent a subpixel being able to reset dirty flag

Author: tocsoft

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

@@ -45,7 +45,7 @@ private static void ScanCurrentSubpixelLineInto(this ref PolygonScanner scanner,
                     float subpixelWidth = (startX + 1 - scanStart) / scanner.SubpixelDistance;
 
                     scanline[startX] += subpixelWidth * scanner.SubpixelArea;
-                    scanlineDirty = subpixelWidth > 0;
+                    scanlineDirty |= subpixelWidth > 0;
                 }
 
                 if (endX >= 0 && endX < scanline.Length)
@@ -54,7 +54,7 @@ private static void ScanCurrentSubpixelLineInto(this ref PolygonScanner scanner,
                     float subpixelWidth = (scanEnd - endX) / scanner.SubpixelDistance;
 
                     scanline[endX] += subpixelWidth * scanner.SubpixelArea;
-                    scanlineDirty = subpixelWidth > 0;
+                    scanlineDirty |= subpixelWidth > 0;
                 }
 
                 int nextX = startX + 1;

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

@@ -0,0 +1,51 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Collections.Generic;
+using System.Text;
+using SixLabors.Fonts;
+using SixLabors.ImageSharp.Drawing.Processing;
+using SixLabors.ImageSharp.Drawing.Shapes.Rasterization;
+using SixLabors.ImageSharp.Drawing.Tests.TestUtilities.ImageComparison;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Drawing.Tests.Shapes.Scan
+{
+    public class RasterizerExtensionsTests
+    {
+        [Fact]
+        public void DoesNotOverwriteIsDirtyFlagWhenOnlyFillingSubpixels()
+        {
+            var scanner = PolygonScanner.Create(new RectangularPolygon(0.3f, 0.2f, 0.7f, 1.423f), 0, 20, 1, IntersectionRule.OddEven, MemoryAllocator.Default);
+
+            float[] buffer = new float[12];
+
+            scanner.MoveToNextPixelLine(); // offset
+
+            bool isDirty = scanner.ScanCurrentPixelLineInto(0, 0, buffer.AsSpan());
+
+            Assert.True(isDirty);
+        }
+
+        [Theory]
+        [WithSolidFilledImages(400, 75, "White", PixelTypes.Rgba32)]
+        public void AntialiasingIsAntialiased<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            Font font36 = TestFontUtilities.GetFont(TestFonts.OpenSans, 20);
+            var textOpt = new TextOptions(font36)
+            {
+                Dpi = 96,
+                Origin = new PointF(0, 0)
+            };
+
+            provider.RunValidatingProcessorTest(x => x
+                 .SetGraphicsOptions(o => o.Antialias = false)
+                 .DrawText(textOpt, "Hello, World!", Color.Black));
+        }
+    }
+}