Add support for tracking (letter-spacing)

Introduces a Tracking property to TextOptions to allow uniform
adjustment of spacing between characters, measured in em units.

Author: Socolin

src/SixLabors.Fonts/TextLayout.cs

@@ -1195,6 +1195,22 @@ VerticalOrientationType.Rotate or
                     // Work out the scaled metrics for the glyph.
                     GlyphMetrics metric = metrics[i];
 
+                    // Adjust the advance for the last decomposed glyph to add
+                    // tracking if applicable.
+                    if (options.Tracking != 0 && decomposedAdvance > 0 && i == decomposedAdvances.Length - 1)
+                    {
+                        if (isHorizontalLayout || shouldRotate)
+                        {
+                            float scaleAX = pointSize / glyph.ScaleFactor.X;
+                            decomposedAdvance += options.Tracking * metric.FontMetrics.UnitsPerEm * scaleAX;
+                        }
+                        else
+                        {
+                            float scaleAY = pointSize / glyph.ScaleFactor.Y;
+                            decomposedAdvance += options.Tracking * metric.FontMetrics.UnitsPerEm * scaleAY;
+                        }
+                    }
+
                     // Convert design-space units to pixels based on the target point size.
                     // ScaleFactor.Y represents the vertical UPEM scaling factor for this glyph.
                     float scaleY = pointSize / metric.ScaleFactor.Y;

src/SixLabors.Fonts/TextOptions.cs

@@ -170,6 +170,13 @@ public float LineSpacing
     /// </summary>
     public KerningMode KerningMode { get; set; }
 
+    /// <summary>
+    /// Gets or sets the tracking (letter-spacing) value.
+    /// Tracking adjusts the spacing between all characters uniformly and is measured in em.
+    /// Positive values increase spacing, negative values decrease spacing, and zero applies no adjustment.
+    /// </summary>
+    public float Tracking { get; set; }
+
     /// <summary>
     /// Gets or sets the positioning mode used for rendering decorations.
     /// </summary>

tests/SixLabors.Fonts.Tests/TextLayoutTests.cs

@@ -967,6 +967,57 @@ public void TrueTypeHinting_CanHintSmallOpenSans(char c, FontRectangle expected)
         Assert.Equal(expected, actual, Comparer);
     }
 
+    [Theory]
+    [InlineData("aaaa", 0, 134.0)]
+    [InlineData("awwa", 0, 162.1)]
+    [InlineData("aaaa", 0.1, 153.3)]
+    [InlineData("awwa", 0.1, 181.4)]
+    [InlineData("aaaa", 1, 326.1)]
+    [InlineData("awwa", 1, 354.1)]
+    public void FontTracking_SpaceCharacters(string text, float tracking, float width)
+    {
+        Font font = new FontCollection().Add(TestFonts.OpenSansFile).CreateFont(64);
+        TextOptions options = new(font)
+        {
+            Tracking = tracking,
+        };
+
+        FontRectangle actual = TextMeasurer.MeasureSize(text, options);
+        Assert.Equal(new FontRectangle(0, 0, width, 35.4f), actual, Comparer);
+    }
+
+    [Theory]
+    [InlineData("\u1B3C", 1, 83.8)]
+    [InlineData("\u1B3C\u1B3C", 1, 83.8)]
+    public void FontTracking_DoNotAddSpacingAfterCharacterThatDidNotAdvance(string text, float tracking, float width)
+    {
+        Font font = new FontCollection().Add(TestFonts.NotoSansBalineseRegular).CreateFont(64);
+        TextOptions options = new(font)
+        {
+            Tracking = tracking,
+        };
+
+        FontRectangle actual = TextMeasurer.MeasureSize(text, options);
+        Assert.Equal(new FontRectangle(0, 0, width, 103.3f), actual, Comparer);
+    }
+
+    [Theory]
+    [InlineData("\u093f", 1, 48.4)]
+    [InlineData("\u0930\u094D\u0915\u093F", 1, 225.6)]
+    [InlineData("\u0930\u094D\u0915\u093F\u0930\u094D\u0915\u093F", 1, 419)]
+    [InlineData("\u093fa", 1, 145.5f)]
+    public void FontTracking_CorrectlyAddSpacingForComposedCharacter(string text, float tracking, float width)
+    {
+        Font font = new FontCollection().Add(TestFonts.NotoSansDevanagariRegular).CreateFont(64);
+        TextOptions options = new(font)
+        {
+            Tracking = tracking,
+        };
+
+        FontRectangle actual = TextMeasurer.MeasureSize(text, options);
+        Assert.Equal(width, actual.Width, Comparer);
+    }
+
     [Fact]
     public void CanMeasureTextAdvance()
     {