Add LineMetrics and per-line measurement API

Author: JimBobSquarePants

src/SixLabors.Fonts/LineMetrics.cs

@@ -0,0 +1,79 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.Fonts;
+
+/// <summary>
+/// Encapsulates measured metrics for a single laid-out text line.
+/// </summary>
+/// <remarks>
+/// <para>This type is layout-mode agnostic:</para>
+/// <list type="bullet">
+/// <item><description>Horizontal layouts: <see cref="Start"/> is the X start position and <see cref="Extent"/> is the width.</description></item>
+/// <item><description>Vertical layouts: <see cref="Start"/> is the Y start position and <see cref="Extent"/> is the height.</description></item>
+/// </list>
+/// </remarks>
+public readonly struct LineMetrics
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="LineMetrics"/> struct.
+    /// </summary>
+    /// <param name="ascender">Distance from the baseline to the ascender line.</param>
+    /// <param name="baseline">Baseline position within the line box.</param>
+    /// <param name="descender">Descender line position within the line box.</param>
+    /// <param name="lineHeight">Total line-box size (includes effective line spacing).</param>
+    /// <param name="start">Line start position in the primary layout flow direction after alignment.</param>
+    /// <param name="extent">Line extent in the primary layout flow direction.</param>
+    public LineMetrics(
+        float ascender,
+        float baseline,
+        float descender,
+        float lineHeight,
+        float start,
+        float extent)
+    {
+        this.Ascender = ascender;
+        this.Baseline = baseline;
+        this.Descender = descender;
+        this.LineHeight = lineHeight;
+        this.Start = start;
+        this.Extent = extent;
+    }
+
+    /// <summary>
+    /// Gets the distance from the baseline to the ascender line.
+    /// </summary>
+    public float Ascender { get; }
+
+    /// <summary>
+    /// Gets the baseline position within the line box.
+    /// </summary>
+    /// <remarks>
+    /// Use this value as the guide-line position for drawing a baseline relative to the current line origin.
+    /// </remarks>
+    public float Baseline { get; }
+
+    /// <summary>
+    /// Gets the descender line position within the line box.
+    /// </summary>
+    /// <remarks>
+    /// This is a position value (not a baseline-relative distance).
+    /// Use this value to draw the descender guide line relative to the current line origin.
+    /// </remarks>
+    public float Descender { get; }
+
+    /// <summary>
+    /// Gets the total line-box size for this line.
+    /// </summary>
+    public float LineHeight { get; }
+
+    /// <summary>
+    /// Gets the line start position in the primary layout flow direction.
+    /// </summary>
+    public float Start { get; }
+
+    /// <summary>
+    /// Gets the line extent in the primary layout flow direction.
+    /// </summary>
+    public float Extent { get; }
+}