[80_5] Improving Fraction Box Typesetting

## What
Improving Fraction Box Typesetting.

## Why
For some OpenType fonts, the heights of the numerator and denominator in
the fraction box environment are not suitable.

## How to test your changes?
Open `devel/80_5.tmu`.

Before (Anasa Math font):

![image](https://github.com/user-attachments/assets/a6f3d3a4-9ecc-491c-958f-ee3c0be8e4b2)

After:

![image](https://github.com/user-attachments/assets/a8bc5314-6d96-44a9-97f9-899b78f53e5b)

devel/80_5.tmu

@@ -0,0 +1,20 @@
+<TMU|<tuple|1.0.3|1.2.9>>
+
+<style|<tuple|generic|chinese>>
+
+<\body>
+  <math|<frac|1+3|2+4> <frac|1+<frac|1+<frac|1+<frac|1|2>|2+<frac|1|2>>|2+<frac|1+<frac|1|2>|2+<frac|1|2>>>|1+<frac|1+<frac|1+<frac|1|2>|2+<frac|1|2>>|2+<frac|1+<frac|1|2>|2+<frac|1|2>>>> >
+
+  <\equation*>
+    <frac|1|2> <frac|1+<frac|1+<frac|1+<frac|1|2>|2+<frac|1|2>>|2+<frac|1+<frac|1|2>|2+<frac|1|2>>>|1+<frac|1+<frac|1+<frac|1|2>|2+<frac|1|2>>|2+<frac|1+<frac|1|2>|2+<frac|1|2>>>>\ 
+  </equation*>
+</body>
+
+<\initial>
+  <\collection>
+    <associate|font|Asana Math>
+    <associate|font-family|rm>
+    <associate|page-medium|paper>
+    <associate|page-screen-margin|false>
+  </collection>
+</initial>

src/Graphics/Fonts/font.cpp

@@ -100,6 +100,15 @@ font_rep::copy_math_pars (font fn) {
   upper_limit_baseline_rise_min= fn->upper_limit_baseline_rise_min;
   lower_limit_gap_min          = fn->lower_limit_gap_min;
   lower_limit_baseline_drop_min= fn->lower_limit_baseline_drop_min;
+  frac_rule_thickness          = fn->frac_rule_thickness;
+  frac_num_shift_up            = fn->frac_num_shift_up;
+  frac_num_disp_shift_up       = fn->frac_num_disp_shift_up;
+  frac_num_gap_min             = fn->frac_num_gap_min;
+  frac_num_disp_gap_min        = fn->frac_num_disp_gap_min;
+  frac_denom_shift_down        = fn->frac_denom_shift_down;
+  frac_denom_disp_shift_down   = fn->frac_denom_disp_shift_down;
+  frac_denom_gap_min           = fn->frac_denom_gap_min;
+  frac_denom_disp_gap_min      = fn->frac_denom_disp_gap_min;
 }
 
 void

src/Graphics/Fonts/font.hpp

@@ -79,6 +79,15 @@ struct font_rep : rep<font> {
   SI upper_limit_baseline_rise_min;
   SI lower_limit_gap_min;
   SI lower_limit_baseline_drop_min;
+  SI frac_rule_thickness;
+  SI frac_num_shift_up;
+  SI frac_num_disp_shift_up;
+  SI frac_num_gap_min;
+  SI frac_num_disp_gap_min;
+  SI frac_denom_shift_down;
+  SI frac_denom_disp_shift_down;
+  SI frac_denom_gap_min;
+  SI frac_denom_disp_gap_min;
 
   SI wpt;   // width of one point in font
   SI hpt;   // height of one point in font (usually wpt)

src/Plugins/Freetype/unicode_font.cpp

@@ -169,8 +169,11 @@ struct unicode_font_rep : font_rep {
   SI           get_rsup_correction (string s);
   SI           get_wide_correction (string s, int mode);
 
-  SI  design_unit_to_metric (int du);
-  int metric_to_design_unit (SI m);
+  double design_unit_to_metric_factor;
+  double metric_to_design_unit_factor;
+  void   init_design_unit_factor ();
+  SI     design_unit_to_metric (int du);
+  int    metric_to_design_unit (SI m);
 };
 
 /******************************************************************************
@@ -444,6 +447,8 @@ unicode_font_rep::unicode_font_rep (string name, string family2, int size2,
       this->math_face = math_face2;
       this->math_table= math_face2->math_table;
       math_type       = MATH_TYPE_OPENTYPE;
+      init_design_unit_factor ();
+      // limit boxes
       upper_limit_gap_min=
           design_unit_to_metric (math_table->constants_table[upperLimitGapMin]);
       upper_limit_baseline_rise_min= design_unit_to_metric (
@@ -452,6 +457,26 @@ unicode_font_rep::unicode_font_rep (string name, string family2, int size2,
           design_unit_to_metric (math_table->constants_table[lowerLimitGapMin]);
       lower_limit_baseline_drop_min= design_unit_to_metric (
           math_table->constants_table[lowerLimitBaselineDropMin]);
+      // frac boxes
+      frac_rule_thickness= design_unit_to_metric (
+          math_table->constants_table[fractionRuleThickness]);
+      frac_num_shift_up= design_unit_to_metric (
+          math_table->constants_table[fractionNumeratorShiftUp]);
+      frac_num_disp_shift_up= design_unit_to_metric (
+          math_table->constants_table[fractionNumeratorDisplayStyleShiftUp]);
+      frac_num_gap_min= design_unit_to_metric (
+          math_table->constants_table[fractionNumeratorGapMin]);
+      frac_num_disp_gap_min= design_unit_to_metric (
+          math_table->constants_table[fractionNumDisplayStyleGapMin]);
+      frac_denom_shift_down= design_unit_to_metric (
+          math_table->constants_table[fractionDenominatorShiftDown]);
+      frac_denom_disp_shift_down= design_unit_to_metric (
+          math_table
+              ->constants_table[fractionDenominatorDisplayStyleShiftDown]);
+      frac_denom_gap_min= design_unit_to_metric (
+          math_table->constants_table[fractionDenominatorGapMin]);
+      frac_denom_disp_gap_min= design_unit_to_metric (
+          math_table->constants_table[fractionDenominatorGapMin]);
     }
   }
 }
@@ -1003,41 +1028,32 @@ unicode_font (string family, int size, int hdpi, int vdpi) {
  * OpenType
  ******************************************************************************/
 
+inline void
+unicode_font_rep::init_design_unit_factor () {
+  int units_of_m= 0;
+  SI  em        = 0;
+  // get the design units of the width of 'm'
+  FT_UInt glyph_index= FT_Get_Char_Index (math_face->ft_face, 'm');
+  FT_Load_Glyph (math_face->ft_face, glyph_index, FT_LOAD_NO_SCALE);
+  units_of_m= math_face->ft_face->glyph->metrics.horiAdvance;
+
+  // get the width of the character 'm'
+  metric ex;
+  get_extents ("m", ex);
+  em= ex->x2 - ex->x1;
+
+  metric_to_design_unit_factor= (double) units_of_m / (double) em;
+  design_unit_to_metric_factor= (double) em / (double) units_of_m;
+}
+
 inline SI
 unicode_font_rep::design_unit_to_metric (int du) {
-  // use 'm' as a reference character
-  static int units_of_m= 0;
-  static SI  em        = 0;
-  if (units_of_m == 0) {
-    // get the design units of the width of 'm'
-    FT_UInt glyph_index= FT_Get_Char_Index (math_face->ft_face, 'm');
-    FT_Load_Glyph (math_face->ft_face, glyph_index, FT_LOAD_NO_SCALE);
-    units_of_m= math_face->ft_face->glyph->metrics.horiAdvance;
-
-    // get the width of the character 'm'
-    metric ex;
-    get_extents ("m", ex);
-    em= ex->x2 - ex->x1;
-  }
-  return (SI) ((du * em) / units_of_m);
+  return (SI) design_unit_to_metric_factor * du;
 }
 
 inline int
 unicode_font_rep::metric_to_design_unit (SI m) {
-  static int units_of_m= 0;
-  static SI  em        = 0;
-  if (units_of_m == 0) {
-    // get the units of the character 'm'
-    FT_UInt glyph_index= FT_Get_Char_Index (math_face->ft_face, 'm');
-    FT_Load_Glyph (math_face->ft_face, glyph_index, FT_LOAD_NO_SCALE);
-    units_of_m= math_face->ft_face->glyph->metrics.horiAdvance;
-
-    // get the width of the character 'm'
-    metric ex;
-    get_extents ("m", ex);
-    em= ex->x2 - ex->x1;
-  }
-  return (int) ((m * units_of_m) / em);
+  return (int) metric_to_design_unit_factor * m;
 }
 
 font

src/Typeset/Boxes/Composite/math_boxes.cpp

@@ -14,6 +14,7 @@
 #include "Boxes/composite.hpp"
 #include "Boxes/construct.hpp"
 #include "analyze.hpp"
+#include "font.hpp"
 
 /******************************************************************************
  * Miscellaneous routines
@@ -62,15 +63,16 @@ italic_correction (box L, box R) {
 struct frac_box_rep : public composite_box_rep {
   font   fn, sfn;
   pencil pen;
-  frac_box_rep (path ip, box b1, box b2, font fn, font sfn, pencil pen);
+  frac_box_rep (path ip, box b1, box b2, font fn, font sfn, pencil pen,
+                bool opentype_disp);
   operator tree () { return tree (TUPLE, "frac", bs[0], bs[1]); }
   box adjust_kerning (int mode, double factor);
   box expand_glyphs (int mode, double factor);
   int find_child (SI x, SI y, SI delta, bool force);
 };
 
 frac_box_rep::frac_box_rep (path ip, box b1, box b2, font fn2, font sfn2,
-                            pencil pen2)
+                            pencil pen2, bool opentype_disp)
     : composite_box_rep (ip), fn (fn2), sfn (sfn2), pen (pen2) {
   // Italic correction does not lead to nicer results,
   // because right correction is not equilibrated w.r.t. left correction
@@ -84,10 +86,34 @@ frac_box_rep::frac_box_rep (path ip, box b1, box b2, font fn2, font sfn2,
   SI d    = sep >> 1;
 
   pencil bar_pen= pen->set_width (bar_w);
-  insert (b1, (w >> 1) - (b1->x2 >> 1), bar_y + sep + (bar_w >> 1) - b1_y);
-  insert (b2, (w >> 1) - (b2->x2 >> 1), bar_y - sep - (bar_w >> 1) - b2_y);
-  insert (line_box (decorate_middle (ip), d, 0, w - d, 0, bar_pen), 0, bar_y);
 
+  bool use_opentype=
+      (fn->math_type == MATH_TYPE_OPENTYPE) && (fn->frac_num_gap_min > 0);
+
+  if (use_opentype) {
+    SI num_gap_min   = fn->frac_num_gap_min;
+    SI den_gap_min   = fn->frac_denom_gap_min;
+    SI num_shift_up  = fn->frac_num_shift_up;
+    SI den_shift_down= fn->frac_denom_shift_down;
+
+    if (opentype_disp) {
+      num_gap_min   = fn->frac_num_disp_gap_min;
+      den_gap_min   = fn->frac_denom_disp_gap_min;
+      num_shift_up  = fn->frac_num_disp_shift_up;
+      den_shift_down= fn->frac_denom_disp_shift_down;
+    }
+
+    insert (b1, (w >> 1) - (b1->x2 >> 1),
+            max (num_shift_up, bar_y + num_gap_min + (bar_w >> 1) - b1->y1));
+    insert (b2, (w >> 1) - (b2->x2 >> 1),
+            min (-den_shift_down, bar_y - den_gap_min - (bar_w >> 1) - b2->y2));
+    insert (line_box (decorate_middle (ip), d, 0, w - d, 0, bar_pen), 0, bar_y);
+  }
+  else {
+    insert (b1, (w >> 1) - (b1->x2 >> 1), bar_y + sep + (bar_w >> 1) - b1_y);
+    insert (b2, (w >> 1) - (b2->x2 >> 1), bar_y - sep - (bar_w >> 1) - b2_y);
+    insert (line_box (decorate_middle (ip), d, 0, w - d, 0, bar_pen), 0, bar_y);
+  }
   italic_correct (b1);
   italic_correct (b2);
   position ();
@@ -698,8 +724,9 @@ wide_box_rep::get_bracket_extents (SI& lo, SI& hi) {
  ******************************************************************************/
 
 box
-frac_box (path ip, box b1, box b2, font fn, font sfn, pencil pen) {
-  return tm_new<frac_box_rep> (ip, b1, b2, fn, sfn, pen);
+frac_box (path ip, box b1, box b2, font fn, font sfn, pencil pen,
+          bool opentype_disp) {
+  return tm_new<frac_box_rep> (ip, b1, b2, fn, sfn, pen, opentype_disp);
 }
 
 box

src/Typeset/Boxes/construct.hpp

@@ -145,7 +145,8 @@ box highlight_box (path ip, box b, box xb, ornament_parameters ps);
 box highlight_box (path ip, box b, SI w, brush col, brush sunc, brush shad);
 box art_box (path ip, box b, art_box_parameters ps);
 
-box frac_box (path ip, box b1, box b2, font fn, font sfn, pencil pen);
+box frac_box (path ip, box b1, box b2, font fn, font sfn, pencil pen,
+              bool opentype_disp= false);
 box sqrt_box (path ip, box b1, box b2, box sqrtb, font fn, pencil pen);
 box neg_box (path ip, box b, font fn, pencil pen);
 box tree_box (path ip, array<box> bs, font fn, pencil pen);

src/Typeset/Concat/concat_math.cpp

@@ -393,7 +393,7 @@ concater_rep::typeset_frac (tree t, path ip) {
   if (disp) env->local_end (MATH_DISPLAY, old);
   else env->local_end_script (old);
   if (num->w () <= env->frac_max && den->w () <= env->frac_max)
-    print (frac_box (ip, num, den, env->fn, sfn, env->pen));
+    print (frac_box (ip, num, den, env->fn, sfn, env->pen, disp));
   else typeset_wide_frac (t, ip);
 }