Special-case usetex minus to zero depth.

anntzer · anntzer · commit 4c6d9afb2be3 · 2020-01-29T12:07:04.000+01:00
This avoids vertical misalignment between e.g. "$1$" and "$-1$":
```
for x in [-1, 1]: figtext(.5, .5, f"${x}$", usetex=True, ha="right")
```
diff --git a/lib/matplotlib/dviread.py b/lib/matplotlib/dviread.py
@@ -566,6 +566,12 @@ def _height_depth_of(self, char):
                 result.append(0)
             else:
                 result.append(_mul2012(value, self._scale))
+        # cmsy10 glyph 0 ("minus") has a nonzero descent so that TeX aligns
+        # equations properly (https://tex.stackexchange.com/questions/526103/),
+        # but we actually care about the rasterization depth to align the
+        # dvipng-generated images.
+        if self.texname == b"cmsy10" and char == 0:
+            result[-1] = 0
         return result
 
 
diff --git a/lib/matplotlib/tests/test_usetex.py b/lib/matplotlib/tests/test_usetex.py
@@ -1,6 +1,8 @@
-import pytest
 import platform
 
+import numpy as np
+import pytest
+
 import matplotlib as mpl
 from matplotlib.testing.decorators import check_figures_equal, image_comparison
 import matplotlib.pyplot as plt
@@ -43,3 +45,21 @@ def test_mathdefault():
     # problems when later switching usetex on.
     mpl.rcParams['text.usetex'] = True
     fig.canvas.draw()
+
+
+def test_minus_no_descent():
+    # Test special-casing of minus descent in DviFont._height_depth_of, by
+    # checking that overdrawing a 1 and a -1 results in an overall height
+    # equivalent to drawing either of them separately.
+    mpl.style.use("mpl20")
+    heights = {}
+    fig = plt.figure()
+    for vals in [(1,), (-1,), (-1, 1)]:
+        fig.clf()
+        for x in vals:
+            fig.text(.5, .5, f"${x}$", usetex=True)
+        fig.canvas.draw()
+        # The following counts the number of non-fully-blank pixel rows.
+        heights[vals] = ((np.array(fig.canvas.buffer_rgba())[..., 0] != 255)
+                         .any(axis=1).sum())
+    assert len({*heights.values()}) == 1
