Fix signed fractional CFF coordinates

src/SixLabors.Fonts/Tables/Cff/SimpleBinaryReader.cs

@@ -78,9 +78,9 @@ public float ReadFloatFixed1616()
         byte b0 = this.buffer[this.Position];
         this.Position += 4;
 
-        // This number is interpreted as a Fixed; that is, a signed number with 16 bits of fraction
+        // The integer word is signed, while the fractional word is an unsigned 16-bit value.
         float number = (short)((b0 << 8) | b1);
-        float fraction = (short)((b2 << 8) | b3) / 65536F;
+        float fraction = (ushort)((b2 << 8) | b3) / 65536F;
         return number + fraction;
     }
 }

tests/SixLabors.Fonts.Tests/Tables/Cff/CffEvaluationEngineTests.cs

@@ -0,0 +1,69 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Numerics;
+using SixLabors.Fonts.Rendering;
+using SixLabors.Fonts.Tables.Cff;
+
+namespace SixLabors.Fonts.Tests.Tables.Cff;
+
+public class CffEvaluationEngineTests
+{
+    // This is the raw Type 2 prefix that reproduces the fixed-point coordinates from
+    // glyph 54 of the Igrunok repro font. The font itself is not redistributed.
+    private static readonly byte[] IgrunokGlyph54Prefix =
+    [
+        0xF8, 0x66,
+        0xFF, 0x01, 0xBE, 0x88, 0xC3,
+        0xFF, 0x00, 0x4E, 0xD0, 0x92,
+        0x15,
+        0xFF, 0xFF, 0xE7, 0x82, 0x98,
+        0xFF, 0x00, 0x10, 0xD0, 0x9F,
+        0xFF, 0xFF, 0xC9, 0x66, 0x95,
+        0xFF, 0x00, 0x3D, 0x98, 0xAE,
+        0xFF, 0xFF, 0xE4, 0xB0, 0x7E,
+        0xFF, 0x00, 0x23, 0x01, 0xE4,
+        0x08,
+        0xFF, 0x00, 0x56, 0x1B, 0xAE,
+        0xFF, 0x00, 0x22, 0x49, 0x2B,
+        0xFF, 0x00, 0x14, 0xFD, 0xC7,
+        0xFF, 0x00, 0x62, 0x00, 0xD2,
+        0xFF, 0xFF, 0xED, 0xC9, 0x21,
+        0xFF, 0x00, 0x4B, 0x9C, 0xCB,
+        0x08,
+        0x0E
+    ];
+
+    [Fact]
+    public void Fixed1616_UsesUnsignedFraction()
+    {
+        SimpleBinaryReader reader = new([0x01, 0xBE, 0x88, 0xC3]);
+
+        Assert.Equal(446.534225F, reader.ReadFloatFixed1616(), 4);
+    }
+
+    [Fact]
+    public void RawIgrunokGlyph54Prefix_PreservesCurrentPointAfterEachCurve()
+    {
+        using CffEvaluationEngine engine = new(
+            IgrunokGlyph54Prefix,
+            Array.Empty<byte[]>(),
+            Array.Empty<byte[]>(),
+            0,
+            1);
+        GlyphRenderer renderer = new();
+
+        engine.RenderTo(renderer, Vector2.Zero, Vector2.One, Vector2.Zero, Matrix3x2.Identity);
+
+        Assert.Equal(7, renderer.ControlPoints.Count);
+        AssertPoint(renderer.ControlPoints[0], 446.534225F, -78.814728F);
+        AssertPoint(renderer.ControlPoints[3], 340.13449F, -192.23344F);
+        AssertPoint(renderer.ControlPoints[6], 429.01959F, -400.13495F);
+    }
+
+    private static void AssertPoint(Vector2 actual, float expectedX, float expectedY)
+    {
+        Assert.Equal(expectedX, actual.X, 4);
+        Assert.Equal(expectedY, actual.Y, 4);
+    }
+}