This PR adds the ability to provide custom vertical font metrics when layouting text.

The background here is that Skia report different font metrics depending on platform. Windows uses usWinAscent, usWinDescent, and this lovely formula for calculating leading: MAX(0, (hhea.ascender - hhea.descender + hhea.lineGap) - (usWinAscent + usWinDescent)). If I remember correctly Linux uses the sTypo* metrics.

For cross-platform .NET applications that want to have similar rendering on both Windows and Linux (in particular vertical positioning) there needs to be a way to override the metrics reported by Skia.

toptensoftware#67

Author: xq

Topten.RichTextKit/FontMetricsMapper.cs

@@ -0,0 +1,58 @@
+// RichTextKit
+// Copyright © 2019-2020 Topten Software. All Rights Reserved.
+// 
+// Licensed under the Apache License, Version 2.0 (the "License"); you may 
+// not use this product except in compliance with the License. You may obtain 
+// a copy of the License at
+// 
+// http://www.apache.org/licenses/LICENSE-2.0
+// 
+// Unless required by applicable law or agreed to in writing, software 
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT 
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the 
+// License for the specific language governing permissions and limitations 
+// under the License.
+
+using SkiaSharp;
+
+namespace Topten.RichTextKit
+{
+    public class VerticalFontMetrics
+    {
+        public float Ascent { get; }
+        public float Descent { get; }
+        public float Leading { get; }
+
+        public VerticalFontMetrics(float ascent, float descent, float leading)
+        {
+            Ascent = ascent;
+            Descent = descent;
+            Leading = leading;
+        }
+    }
+
+    /// <summary>
+    /// The FontMetricsMapper class is responsible for mapping a font to a set of metrics
+    /// which affect its layouting. The default instance can be replaced in cases where different
+    /// layouting is desired.
+    /// </summary>
+    public class FontMetricsMapper
+    {
+        /// <summary>
+        /// The default metrics mapper instance.
+        /// </summary>
+        public static FontMetricsMapper Default = new FontMetricsMapper();
+
+        /// <summary>
+        /// Maps a given typeface and font size to its vertical metrics.
+        /// </summary>
+        /// <param name="typeface">The typeface.</param>
+        /// <param name="fontSize">The font size in pixels.</param>
+        /// <returns>The vertical font metrics.</returns>
+        public virtual VerticalFontMetrics GetVerticalMetrics(SKTypeface typeface, float fontSize)
+        {
+            var fontMetrics = typeface.ToFont(fontSize).Metrics;
+            return new VerticalFontMetrics(fontMetrics.Ascent, fontMetrics.Descent, fontMetrics.Descent);
+        }
+    }
+}

Topten.RichTextKit/TextShaping/TextShaper.cs

@@ -79,7 +79,9 @@ private TextShaper(SKTypeface typeface)
             {
                 font.Typeface = typeface;
                 font.Size = overScale;
-                _fontMetrics = font.Metrics;
+                //_fontMetrics = font.Metrics;
+                _fontXMin = font.Metrics.XMin;
+                _verticalMetrics = FontMetricsMapper.Default.GetVerticalMetrics(typeface, overScale);
 
                 // This is a temporary hack until SkiaSharp exposes
                 // a way to check if a font is fixed pitch.  For now
@@ -114,10 +116,20 @@ public void Dispose()
         /// </summary>
         SKTypeface _typeface;
 
+        ///// <summary>
+        ///// Font metrics for the font
+        ///// </summary>
+        //SKFontMetrics _fontMetrics;
+
+        /// <summary>
+        /// The minimum bounding box x value for the font.
+        /// </summary>
+        float _fontXMin;
+
         /// <summary>
-        /// Font metrics for the font
+        /// The vertical font metrics as provided by the metrics mapper.
         /// </summary>
-        SKFontMetrics _fontMetrics;
+        readonly VerticalFontMetrics _verticalMetrics;
 
         /// <summary>
         /// True if this font face is fixed pitch
@@ -478,10 +490,14 @@ public Result Shape(ResultBufferSet bufferSet, Slice<int> codePoints, IStyle sty
         private void ApplyFontMetrics(ref Result result, float fontSize)
         {
             // And some other useful metrics
-            result.Ascent = _fontMetrics.Ascent * fontSize / overScale;
-            result.Descent = _fontMetrics.Descent * fontSize / overScale;
-            result.Leading = _fontMetrics.Leading * fontSize / overScale;
-            result.XMin = _fontMetrics.XMin * fontSize / overScale;
+            //result.Ascent = _fontMetrics.Ascent * fontSize / overScale;
+            //result.Descent = _fontMetrics.Descent * fontSize / overScale;
+            //result.Leading = _fontMetrics.Leading * fontSize / overScale;
+            //result.XMin = _fontMetrics.XMin * fontSize / overScale;
+            result.Ascent = _verticalMetrics.Ascent * fontSize / overScale;
+            result.Descent = _verticalMetrics.Descent * fontSize / overScale;
+            result.Leading = _verticalMetrics.Leading * fontSize / overScale;
+            result.XMin = _fontXMin * fontSize / overScale;
         }
 
         private static Blob GetHarfBuzzBlob(SKStreamAsset asset)