Added respect for the "useTypoMetrics" flag

Signed-off-by: Vsevolod Volkov <st.lyn4@gmail.com>

Author: Vsevolod Volkov

README.md

@@ -83,6 +83,7 @@ The following properties describe the general metrics of the font. See [here](ht
 * `underlinePosition` - the offset from the normal underline position that should be used
 * `underlineThickness` - the weight of the underline that should be used
 * `italicAngle` - if this is an italic font, the angle the cursor should be drawn at to match the font design
+* `lineHeight` - is the vertical space between adjacent lines (their baselines) of text, also known as leading. See [here](https://en.wikipedia.org/wiki/Leading) for more details.
 * `capHeight` - the height of capital letters above the baseline. See [here](http://en.wikipedia.org/wiki/Cap_height) for more details.
 * `xHeight`- the height of lower case letters. See [here](http://en.wikipedia.org/wiki/X-height) for more details.
 * `bbox` - the font’s bounding box, i.e. the box that encloses all glyphs in the font

src/TTFFont.js

@@ -91,6 +91,44 @@ export default class TTFFont {
     return result;
   }
 
+  _getMetrics() {
+    if (this._metrics) { return this._metrics; }
+
+    let { 'OS/2': os2, hhea } = this;
+    let useTypoMetrics = os2.fsSelection.useTypoMetrics;
+    let ascent, descent, lineGap, lineHeight;
+
+    // Use the same approach as FreeType
+    // https://gitlab.freedesktop.org/freetype/freetype/-/blob/4d8db130ea4342317581bab65fc96365ce806b77/src/sfnt/sfobjs.c#L1310
+
+    if (useTypoMetrics) {
+      ascent = os2.typoAscender;
+      descent = os2.typoDescender;
+      lineGap = os2.typoLineGap;
+      lineHeight = ascent - descent + lineGap;
+    } else {
+      ascent = hhea.ascent;
+      descent = hhea.descent;
+      lineGap = hhea.lineGap;
+      lineHeight = ascent - descent + lineGap;
+    }
+
+    if (!ascent || !descent) {
+      if (os2.typoAscender || os2.typoDescender) {
+        ascent = os2.typoAscender;
+        descent = os2.typoDescender;
+        lineGap = os2.typoLineGap;
+        lineHeight = ascent - descent + lineGap;
+      } else {
+        ascent = os2.winAscent;
+        descent = -os2.winDescent;
+        lineHeight = ascent - descent;
+      }
+    }
+
+    return this._metrics = {ascent, descent, lineGap, lineHeight};
+  }
+
   /**
    * Gets a string from the font's `name` table
    * `lang` is a BCP-47 language code.
@@ -166,23 +204,23 @@ export default class TTFFont {
    * @type {number}
    */
   get ascent() {
-    return this.hhea.ascent;
+    return this._getMetrics().ascent;
   }
 
   /**
    * The font’s [descender](https://en.wikipedia.org/wiki/Descender)
    * @type {number}
    */
   get descent() {
-    return this.hhea.descent;
+    return this._getMetrics().descent;
   }
 
   /**
    * The amount of space that should be included between lines
    * @type {number}
    */
   get lineGap() {
-    return this.hhea.lineGap;
+    return this._getMetrics().lineGap;
   }
 
   /**
@@ -209,6 +247,15 @@ export default class TTFFont {
     return this.post.italicAngle;
   }
 
+  /**
+   * The vertical space between adjacent lines (their baselines) of text.
+   * See [here](https://en.wikipedia.org/wiki/Leading) for more details.
+   * @type {number}
+   */
+  get lineHeight() {
+    return this._getMetrics().lineHeight;
+  }
+
   /**
    * The height of capital letters above the baseline.
    * See [here](https://en.wikipedia.org/wiki/Cap_height) for more details.

src/glyph/Glyph.js

@@ -64,23 +64,12 @@ export default class Glyph {
 
     let {advance:advanceWidth, bearing:leftBearing} = this._getTableMetrics(this._font.hmtx);
 
-    // For vertical metrics, use vmtx if available, or fall back to global data from OS/2 or hhea
+    // For vertical metrics, use vmtx if available, or fall back to global data
     if (this._font.vmtx) {
       var {advance:advanceHeight, bearing:topBearing} = this._getTableMetrics(this._font.vmtx);
-
     } else {
-      let os2;
-      if (typeof cbox === 'undefined' || cbox === null) { ({ cbox } = this); }
-
-      if ((os2 = this._font['OS/2']) && os2.version > 0) {
-        var advanceHeight = Math.abs(os2.typoAscender - os2.typoDescender);
-        var topBearing = os2.typoAscender - cbox.maxY;
-
-      } else {
-        let { hhea } = this._font;
-        var advanceHeight = Math.abs(hhea.ascent - hhea.descent);
-        var topBearing = hhea.ascent - cbox.maxY;
-      }
+      var advanceHeight = Math.abs(this._font.ascent - this._font.descent);
+      var topBearing = this._font.ascent - cbox.maxY;
     }
 
     if (this._font._variationProcessor && this._font.HVAR) {