Improve text rendering accuracy

src/ImageSharp.Drawing/Processing/Processors/Text/DrawTextProcessor.cs

@@ -165,9 +165,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
 
         private class CachingGlyphRenderer : IGlyphRenderer, IDisposable
         {
-            // just enough accuracy to allow for half pixel differences which
-            // later are componded into full pixel offsets while rendering.
-            private const float AccuracyMultiple = 2;
+            // just enough accuracy to allow for 1/8 pixel differences which
+            // later are accumulated while rendering, but do not grow into full pixel offsets
+            // The value 8 is benchmarked to:
+            // - Provide a good accuracy (smaller than 0.2% image difference compared to the non-caching variant)
+            // - Cache hit ratio above 60%
+            private const float AccuracyMultiple = 8;
 
             private readonly PathBuilder builder;
 
@@ -222,14 +225,15 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
             public bool BeginGlyph(RectangleF bounds, GlyphRendererParameters parameters)
             {
                 this.currentRenderPosition = Point.Truncate(bounds.Location);
-                PointF dif = bounds.Location - this.currentRenderPosition;
+                PointF subPixelOffset = bounds.Location - this.currentRenderPosition;
 
-                dif.X = ((int)(dif.X * AccuracyMultiple)) / AccuracyMultiple;
-                dif.Y = ((int)(dif.Y * AccuracyMultiple)) / AccuracyMultiple;
+                subPixelOffset.X = MathF.Round(subPixelOffset.X * AccuracyMultiple) / AccuracyMultiple;
+                subPixelOffset.Y = MathF.Round(subPixelOffset.Y * AccuracyMultiple) / AccuracyMultiple;
 
                 // we have offset our rendering origion a little bit down to prevent edge cropping, move the draw origin up to compensate
                 this.currentRenderPosition = new Point(this.currentRenderPosition.X - this.offset, this.currentRenderPosition.Y - this.offset);
-                this.currentGlyphRenderParams = (parameters, dif);
+                this.currentGlyphRenderParams = (parameters, subPixelOffset);
+
                 if (this.glyphData.ContainsKey(this.currentGlyphRenderParams))
                 {
                     // we have already drawn the glyph vectors skip trying again

tests/ImageSharp.Tests/Drawing/Text/DrawTextOnImageTests.cs

@@ -11,6 +11,8 @@ using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 using SixLabors.Primitives;
 
 using Xunit;
+using Xunit.Abstractions;
+
 // ReSharper disable InconsistentNaming
 
 namespace SixLabors.ImageSharp.Tests.Drawing.Text
@@ -25,6 +27,13 @@ namespace SixLabors.ImageSharp.Tests.Drawing.Text
         public static ImageComparer TextDrawingComparer = ImageComparer.TolerantPercentage(1e-5f);
         public static ImageComparer OutlinedTextDrawingComparer = ImageComparer.TolerantPercentage(1e-4f);
 
+        public DrawTextOnImageTests(ITestOutputHelper output)
+        {
+            this.Output = output;
+        }
+
+        private ITestOutputHelper Output { get; }
+
         [Theory]
         [WithSolidFilledImages(200, 100, "White", PixelTypes.Rgba32, 50, 0, 0, "SixLaborsSampleAB.woff", AB)]
         [WithSolidFilledImages(900, 100, "White", PixelTypes.Rgba32, 50, 0, 0, "OpenSans-Regular.ttf", TestText)]
@@ -87,8 +96,12 @@ namespace SixLabors.ImageSharp.Tests.Drawing.Text
 
             TPixel color = NamedColors<TPixel>.Black;
 
+            // Based on the reported 0.0270% difference with AccuracyMultiple = 8
+            // We should avoid quality regressions leading to higher difference!
+            var comparer = ImageComparer.TolerantPercentage(0.03f);
+
             provider.VerifyOperation(
-                TextDrawingComparer,
+                comparer,
                 img =>
                     {
                         img.Mutate(c => c.DrawText(textOptions, sb.ToString(), font, color, new PointF(10, 5)));
@@ -152,8 +165,9 @@ namespace SixLabors.ImageSharp.Tests.Drawing.Text
         }
 
         [Theory]
-        [WithSolidFilledImages(1000, 1500, "White", PixelTypes.Rgba32)]
-        public static void TextPositioningIsRobust<TPixel>(TestImageProvider<TPixel> provider)
+        [WithSolidFilledImages(1000, 1500, "White", PixelTypes.Rgba32, "Baskerville Old Face")]
+        [WithSolidFilledImages(1000, 1500, "White", PixelTypes.Rgba32, "Arial")]
+        public void TextPositioningIsRobust<TPixel>(TestImageProvider<TPixel> provider, string fontName)
             where TPixel : struct, IPixel<TPixel>
         {
             if (!TestEnvironment.IsWindows)
@@ -162,14 +176,24 @@ namespace SixLabors.ImageSharp.Tests.Drawing.Text
                 return;
             }
             
-            Font font = SystemFonts.CreateFont("Baskerville Old Face", 30, FontStyle.Regular);
+            Font font = SystemFonts.CreateFont(fontName, 30, FontStyle.Regular);
 
             string text = Repeat("Beware the Jabberwock, my son!  The jaws that bite, the claws that catch!  Beware the Jubjub bird, and shun The frumious Bandersnatch!\n",
                 20);
             var textOptions = new TextGraphicsOptions(true) { WrapTextWidth = 1000 };
 
+            string details = fontName.Replace(" ", "");
+
+            // Based on the reported 0.1755% difference with AccuracyMultiple = 8
+            // We should avoid quality regressions leading to higher difference!
+            var comparer = ImageComparer.TolerantPercentage(0.2f);
+
             provider.RunValidatingProcessorTest(
-                x => x.DrawText(textOptions, text, font, NamedColors<TPixel>.Black, new PointF(10, 50)), appendPixelTypeToFileName: false, appendSourceFileOrDescription: false);
+                x => x.DrawText(textOptions, text, font, NamedColors<TPixel>.Black, new PointF(10, 50)),
+                details,
+                comparer,
+                appendPixelTypeToFileName: false,
+                appendSourceFileOrDescription: false);
         }
 
         private static string Repeat(string str, int times) => string.Concat(Enumerable.Repeat(str, times));

tests/Images/External

@@ -1 +1 @@
-Subproject commit 5634b8b2affa1158cec31ccbb67f9583e9137234
+Subproject commit b7a7eacc0446a1acc266f30ddadbda40fccde231