Record svg glyphs in 1/64 pixel units.

anntzer · anntzer · commit 8ab1ad6e5d43 · 2020-06-18T19:20:35.000+02:00
1/64-pixel is the "natural" unit for TrueType and FreeType ("F26Dot6").
Using it makes glyphs recorded in SVG files shorter, e.g.
```
M 88.796875 4.296875
Q 88.796875 2.09375 87.140625 0.5
Q 85.5 -1.09375 83.203125 -1.09375
```
becomes
```
M 1113 -72
Q 709 -72 476 233
Q 244 538 244 953
```
(the concatenated svg output of test_mathtext.py is ~10% shorter)
and easier to compare with FreeType values when troubleshooting font
embedding.  An additional `transform="scale(0.015625)"` (i.e. 1/64) is
added at the end to scale the glyph, but given that a further scaling
and translation is done to actually render the glyph at the right size
and position in the svg, I doubt that this really affects svg renderer
performance (likely they can compose the two scalings first).
diff --git a/lib/matplotlib/backends/backend_svg.py b/lib/matplotlib/backends/backend_svg.py
@@ -1009,9 +1009,12 @@ def _update_glyph_map_defs(self, glyph_map_new):
             writer.start('defs')
             for char_id, (vertices, codes) in glyph_map_new.items():
                 char_id = self._adjust_char_id(char_id)
+                # x64 to go back to FreeType's internal (integral) units.
                 path_data = self._convert_path(
-                    Path(vertices, codes), simplify=False)
-                writer.element('path', id=char_id, d=path_data)
+                    Path(vertices * 64, codes), simplify=False)
+                writer.element(
+                    'path', id=char_id, d=path_data,
+                    transform=generate_transform([('scale', (1 / 64,))]))
             writer.end('defs')
             self._glyph_map.update(glyph_map_new)
 
diff --git a/lib/matplotlib/tests/test_backend_svg.py b/lib/matplotlib/tests/test_backend_svg.py
@@ -121,7 +121,7 @@ def test_unicode_won():
     won_id = 'SFSS3583-8e'
     tree = xml.etree.ElementTree.fromstring(buf)
     assert len(tree.findall(f'.//{{*}}path[@d][@id="{won_id}"]')) == 1
-    assert won_id in tree.findall(f'.//{{*}}use').attrib.values
+    assert f'#{won_id}' in tree.find('.//{*}use').attrib.values()
 
 
 def test_svgnone_with_data_coordinates():
