Add metric override onumZeroHeightRatio for height of old-style numbers (#784).

changes/4.2.0.md

@@ -4,4 +4,5 @@
    - Bitcoin Symbol (`U+20BF`).
    - Projection (`U+2305`) and Perspective(`U+2306`, #780).
  * Add `raised-turn-comma` variant for ASCII grave (#778).
- * Add `straight-neck` variant for digit TWO (`2`, #779).
+ * Add `straight-neck` variant for digit TWO (`2`, #779).
+ * Add metric override `onumZeroHeightRatio` for height of old-style numbers (#784).

font-src/glyphs/number/index.ptl

@@ -8,7 +8,7 @@ glyph-module
 
 glyph-block Digits-Shared : begin
 	glyph-block-export OnumHeight
-	define OnumHeight : mix XH CAP 0.375
+	define OnumHeight : XH * para.onumZeroHeightRatio
 
 	glyph-block-export ShiftDown
 	define [ShiftDown] : glyph-proc

font-src/support/parameters.js

@@ -132,6 +132,7 @@ const metricOverrideHandlers = {
 	powerlineScaleX: numericFieldHandler,
 	powerlineShiftY: numericFieldHandler,
 	powerlineShiftX: numericFieldHandler,
+	onumZeroHeightRatio: numericFieldHandler,
 	multiplies: subObjectHandler,
 	adds: subObjectHandler
 };

params/parameters.toml

@@ -64,6 +64,9 @@ excludedCharRanges = []
 #  - 2+ : Full     (3.x)
 slab = 0
 
+# Old style number height
+onumZeroHeightRatio = 1.145
+
 [verbose]
 verbose = true