Use empty anchor as fallback when parsing mark anchors

src/SixLabors.Fonts/Tables/AdvancedTypographic/GPos/AnchorTable.cs

@@ -11,6 +11,8 @@ namespace SixLabors.Fonts.Tables.AdvancedTypographic.GPos;
 [DebuggerDisplay("X: {XCoordinate}, Y: {YCoordinate}")]
 internal abstract class AnchorTable
 {
+    private static readonly AnchorTable Empty = new EmptyAnchor();
+
     /// <summary>
     /// Initializes a new instance of the <see cref="AnchorTable"/> class.
     /// </summary>
@@ -50,7 +52,10 @@ public static AnchorTable Load(BigEndianBinaryReader reader, long offset)
             1 => AnchorFormat1.Load(reader),
             2 => AnchorFormat2.Load(reader),
             3 => AnchorFormat3.Load(reader),
-            _ => throw new InvalidFontFileException($"anchorFormat identifier {anchorFormat} is invalid. Should be '1', '2' or '3'.")
+
+            // Harfbuzz (Anchor.hh) and FontKit appear to treat this as a default anchor and do not throw.
+            // NotoSans Regular can trigger this. See https://github.com/SixLabors/Fonts/issues/417
+            _ => Empty,
         };
     }
 
@@ -119,7 +124,6 @@ public override AnchorXY GetAnchor(FontMetrics fontMetrics, GlyphShapingData dat
                 {
                     foreach (GlyphMetrics metric in metrics)
                     {
-                        // TODO: What does HarfBuzz do here?
                         if (metric is not TrueTypeGlyphMetrics ttmetric)
                         {
                             break;
@@ -185,4 +189,18 @@ public static AnchorFormat3 Load(BigEndianBinaryReader reader)
         public override AnchorXY GetAnchor(FontMetrics fontMetrics, GlyphShapingData data, GlyphPositioningCollection collection)
             => new(this.XCoordinate, this.YCoordinate);
     }
+
+    internal sealed class EmptyAnchor : AnchorTable
+    {
+        public EmptyAnchor()
+            : base(0, 0)
+        {
+        }
+
+        public override AnchorXY GetAnchor(
+            FontMetrics fontMetrics,
+            GlyphShapingData data,
+            GlyphPositioningCollection collection)
+            => new(0, 0);
+    }
 }

tests/SixLabors.Fonts.Tests/Issues/Issues_417.cs

@@ -0,0 +1,38 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.Fonts.Tests.Issues;
+
+public class Issues_417
+{
+    [Fact]
+    public void DoesNotThrow_InvalidAnchor()
+    {
+        FontFamily family = new FontCollection().Add(TestFonts.NotoSansRegular);
+        family.TryGetMetrics(FontStyle.Regular, out FontMetrics metrics);
+
+        Font font = family.CreateFont(metrics?.UnitsPerEm ?? 1000);
+
+        TextOptions options = new(font);
+
+        // References values generated using.
+        // https://www.corvelsoftware.co.uk/crowbar/
+        TextMeasurer.TryMeasureCharacterAdvances("Text", options, out ReadOnlySpan<GlyphBounds> advances);
+
+        Assert.Equal(4, advances.Length);
+        Assert.Equal(486, advances[0].Bounds.Width);
+        Assert.Equal(544, advances[1].Bounds.Width);
+        Assert.Equal(529, advances[2].Bounds.Width);
+        Assert.Equal(361, advances[3].Bounds.Width);
+
+        GlyphRenderer renderer = new();
+        TextRenderer.RenderTextTo(renderer, "Text", new TextOptions(font));
+
+        int[] expectedGlyphIndices = { 55, 72, 91, 87 };
+        Assert.Equal(expectedGlyphIndices.Length, renderer.GlyphKeys.Count);
+        for (int i = 0; i < expectedGlyphIndices.Length; i++)
+        {
+            Assert.Equal(expectedGlyphIndices[i], renderer.GlyphKeys[i].GlyphId);
+        }
+    }
+}

tests/SixLabors.Fonts.Tests/TestFonts.cs

@@ -257,6 +257,8 @@
 
     public static string KellySlabFile => GetFullPath("KellySlab-Regular.ttf");
 
+    public static string NotoSansRegular => GetFullPath("NotoSans-Regular.ttf");
+
     public static Stream TwemojiMozillaData() => OpenStream(TwemojiMozillaFile);
 
     public static Stream SegoeuiEmojiData() => OpenStream(SegoeuiEmojiFile);
@@ -308,7 +310,7 @@
 
     private static string GetFullPath(string path)
     {
         string root = Path.GetDirectoryName(new Uri(typeof(TestFonts).GetTypeInfo().Assembly.CodeBase).LocalPath);
 
         string[] paths = new[]
         {