matplotlib · tfpf · Jul 17, 2022
diff --git a/lib/matplotlib/_mathtext.py b/lib/matplotlib/_mathtext.py
@@ -249,16 +249,11 @@ def _get_info(self, fontname, font_class, sym, fontsize, dpi):
             )
 
     def get_xheight(self, fontname, fontsize, dpi):
-        font = self._get_font(fontname)
-        font.set_size(fontsize, dpi)
-        pclt = font.get_sfnt_table('pclt')
-        if pclt is None:
-            # Some fonts don't store the xHeight, so we do a poor man's xHeight
-            metrics = self.get_metrics(
-                fontname, mpl.rcParams['mathtext.default'], 'x', fontsize, dpi)
-            return metrics.iceberg
-        xHeight = (pclt['xHeight'] / 64.0) * (fontsize / 12.0) * (dpi / 100.0)
-        return xHeight
+        # Some fonts report the wrong x-height, while some don't store it, so
+        # we do a poor man's x-height.
+        metrics = self.get_metrics(
+            fontname, mpl.rcParams['mathtext.default'], 'x', fontsize, dpi)
+        return metrics.iceberg
 
     def get_underline_thickness(self, font, fontsize, dpi):
         # This function used to grab underline thickness from the font
@@ -759,16 +754,44 @@ class FontConstantsBase:
     # integrals
     delta_integral = 0.1
 
+    # Percentage of x-height the numerator is shifted up in display style.
+    num1 = 1.4
+
+    # Percentage of x-height the numerator is shifted up in text, script and
+    # scriptscript styles if there is a fraction line.
+    num2 = 1.5
+
+    # Percentage of x-height the numerator is shifted up in text, script and
+    # scriptscript styles if there is no fraction line.
+    num3 = 1.3
+
+    # Percentage of x-height the denominator is shifted down in display style.
+    denom1 = 1.3
+
+    # Percentage of x-height the denominator is shifted down in text, script
+    # and scriptscript styles.
+    denom2 = 1.1
+
 
 class ComputerModernFontConstants(FontConstantsBase):
-    script_space = 0.075
-    subdrop = 0.2
-    sup1 = 0.45
-    sub1 = 0.2
-    sub2 = 0.3
-    delta = 0.075
+    # Previously, the x-height of Computer Modern was obtained from the font
+    # table. However, that x-height was greater than the the actual (rendered)
+    # x-height by a factor of 1.771484375 (at font size 12, DPI 100 and hinting
+    # type 32). Now that we're using the rendered x-height, some font constants
+    # have been increased by the same factor to compensate.
+    script_space = 0.132861328125
+    subdrop = 0.354296875
+    sup1 = 0.79716796875
+    sub1 = 0.354296875
+    sub2 = 0.5314453125
+    delta = 0.132861328125
     delta_slanted = 0.3
     delta_integral = 0.3
+    num1 = 1.5
+    num2 = 1.5
+    num3 = 1.5
+    denom1 = 1.6
+    denom2 = 1.2
 
 
 class STIXFontConstants(FontConstantsBase):
@@ -778,17 +801,27 @@ class STIXFontConstants(FontConstantsBase):
     delta = 0.05
     delta_slanted = 0.3
     delta_integral = 0.3
+    num1 = 1.6
+    num2 = 1.6
+    num3 = 1.6
+    denom1 = 1.6
 
 
 class STIXSansFontConstants(FontConstantsBase):
     script_space = 0.05
     sup1 = 0.8
     delta_slanted = 0.6
     delta_integral = 0.3
+    num1 = 1.5
+    num3 = 1.5
+    denom1 = 1.5
 
 
 class DejaVuSerifFontConstants(FontConstantsBase):
-    pass
+    num1 = 1.5
+    num2 = 1.6
+    num3 = 1.4
+    denom1 = 1.4
 
 
 class DejaVuSansFontConstants(FontConstantsBase):
@@ -1832,6 +1865,7 @@ def set_names_and_parse_actions():
             | p.underset
             | p.sqrt
             | p.overline
+            | p.auto_delim
         )
 
         p.simple        <<= (
@@ -2156,6 +2190,19 @@ def is_slanted(self, nucleus):
     def is_between_brackets(self, s, loc):
         return False
 
+    def is_char_node(self, node):
+        # TeX defines a `char_node` as one which represents a single character,
+        # but also states that a `char_node` will never appear in a `Vlist`
+        # (node134). Further, nuclei made of one `Char` and nuclei made of
+        # multiple `Char`s have their superscripts and subscripts shifted by
+        # the same amount. In order to make Mathtext behave similarly, just
+        # check whether this node is a `Vlist` or has any `Vlist` descendants.
+        if isinstance(node, Vlist):
+            return False
+        if not isinstance(node, Hlist):
+            return True
+        return all(map(self.is_char_node, node.children))
+
     def subsuper(self, s, loc, toks):
         nucleus = toks.get("nucleus", Hbox(0))
         subsuper = toks.get("subsuper", [])
@@ -2269,36 +2316,56 @@ def subsuper(self, s, loc, toks):
             else:
                 subkern = 0
 
+        # Set the minimum shifts for the superscript and subscript (node756).
+        if self.is_char_node(nucleus):
+            shift_up = 0
+            shift_down = 0
+        else:
+            drop_amount = constants.subdrop * xHeight * SHRINK_FACTOR
+            shift_up = nucleus.height - drop_amount
+            shift_down = nucleus.depth + drop_amount
+
         if super is None:
-            # node757
+            # Align subscript without superscript (node757).
             x = Hlist([Kern(subkern), sub])
             x.shrink()
             if self.is_dropsub(last_char):
                 shift_down = lc_baseline + constants.subdrop * xHeight
             else:
-                shift_down = constants.sub1 * xHeight
+                shift_down = max(shift_down,
+                                 constants.sub1 * xHeight,
+                                 x.height - xHeight * 4 / 5)
             x.shift_amount = shift_down
         else:
+            # Align superscript (node758).
             x = Hlist([Kern(superkern), super])
             x.shrink()
             if self.is_dropsub(last_char):
                 shift_up = lc_height - constants.subdrop * xHeight
             else:
-                shift_up = constants.sup1 * xHeight
+                shift_up = max(shift_up,
+                               constants.sup1 * xHeight,
+                               x.depth + xHeight / 4)
             if sub is None:
                 x.shift_amount = -shift_up
-            else:  # Both sub and superscript
+            else:
+                # Align subscript with superscript (node759).
                 y = Hlist([Kern(subkern), sub])
                 y.shrink()
                 if self.is_dropsub(last_char):
                     shift_down = lc_baseline + constants.subdrop * xHeight
                 else:
-                    shift_down = constants.sub2 * xHeight
-                # If sub and superscript collide, move super up
-                clr = (2.0 * rule_thickness -
+                    shift_down = max(shift_down, constants.sub2 * xHeight)
+                # If the subscript and superscript are too close to each other,
+                # move the subscript down.
+                clr = (4 * rule_thickness -
                        ((shift_up - x.depth) - (y.height - shift_down)))
                 if clr > 0.:
-                    shift_up += clr
+                    shift_down += clr
+                    clr = xHeight * 4 / 5 - shift_up + x.depth
+                    if clr > 0:
+                        shift_up += clr
+                        shift_down -= clr
                 x = Vlist([
                     x,
                     Kern((shift_up - x.depth) - (y.height - shift_down)),
@@ -2322,32 +2389,73 @@ def _genfrac(self, ldelim, rdelim, rule, style, num, den):
         state = self.get_state()
         thickness = state.get_current_underline_thickness()
 
+        # The fraction line (if present) must be aligned with the minus sign.
+        # Therefore, the height of the latter from the baseline is the axis
+        # height.
+        metrics = state.font_output.get_metrics(
+            state.font, mpl.rcParams['mathtext.default'],
+            '\u2212', state.fontsize, state.dpi)
+        axis_height = (metrics.ymax + metrics.ymin) / 2
+        constants = _get_font_constant_set(state)
+        xHeight = state.font_output.get_xheight(
+            state.font, state.fontsize, state.dpi)
+
         for _ in range(style.value):
+            xHeight *= SHRINK_FACTOR
             num.shrink()
             den.shrink()
         cnum = HCentered([num])
         cden = HCentered([den])
         width = max(num.width, den.width)
         cnum.hpack(width, 'exactly')
         cden.hpack(width, 'exactly')
-        vlist = Vlist([cnum,                      # numerator
-                       Vbox(0, thickness * 2.0),  # space
-                       Hrule(state, rule),        # rule
-                       Vbox(0, thickness * 2.0),  # space
-                       cden                       # denominator
-                       ])
-
-        # Shift so the fraction line sits in the middle of the
-        # equals sign
-        metrics = state.font_output.get_metrics(
-            state.font, mpl.rcParams['mathtext.default'],
-            '=', state.fontsize, state.dpi)
-        shift = (cden.height -
-                 ((metrics.ymax + metrics.ymin) / 2 -
-                  thickness * 3.0))
-        vlist.shift_amount = shift
 
-        result = [Hlist([vlist, Hbox(thickness * 2.)])]
+        # Align the fraction with a fraction line (node743, node744 and
+        # node746).
+        if rule:
+            if style is self._MathStyle.DISPLAYSTYLE:
+                num_shift_up = constants.num1 * xHeight
+                den_shift_down = constants.denom1 * xHeight
+                min_clr = 3 * rule
+            else:
+                num_shift_up = constants.num2 * xHeight
+                den_shift_down = constants.denom2 * xHeight
+                min_clr = rule
+            num_clr = max(
+                num_shift_up - cnum.depth - axis_height - rule / 2, min_clr)
+            den_clr = max(
+                axis_height - rule / 2 + den_shift_down - cden.height, min_clr)
+            # Possible bug in fraction rendering. See GitHub PR 22852 comments.
+            vlist = Vlist([cnum,                     # numerator
+                           Vbox(0, num_clr - rule),  # space
+                           Hrule(state, rule),       # rule
+                           Vbox(0, den_clr + rule),  # space
+                           cden                      # denominator
+                           ])
+            vlist.shift_amount = cden.height + den_clr + rule / 2 - axis_height
+
+        # Align the fraction without a fraction line (node743, node744 and
+        # node745).
+        else:
+            if style is self._MathStyle.DISPLAYSTYLE:
+                num_shift_up = constants.num1 * xHeight
+                den_shift_down = constants.denom1 * xHeight
+                min_clr = 7 * thickness
+            else:
+                num_shift_up = constants.num3 * xHeight
+                den_shift_down = constants.denom2 * xHeight
+                min_clr = 3 * thickness
+            def_clr = num_shift_up - cnum.depth + den_shift_down - cden.height
+            clr = max(def_clr, min_clr)
+            vlist = Vlist([cnum,          # numerator
+                           Vbox(0, clr),  # space
+                           cden           # denominator
+                           ])
+            vlist.shift_amount = den_shift_down
+            if def_clr < min_clr:
+                vlist.shift_amount += (min_clr - def_clr) / 2
+
+        result = [Hlist([Hbox(thickness), vlist, Hbox(thickness)])]
         if ldelim or rdelim:
             if ldelim == '':
                 ldelim = '.'

diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_01.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_01.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_02.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_02.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_03.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_03.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_04.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_04.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_05.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext0_cm_05.svg