Teach svglib how to approximate "ex" and "ch" as em/2.

W3 states:
* In the cases where it is impossible or impractical to determine the
  x-height, a value of 0.5em must be assumed.
* In the cases where it is impossible or impractical to determine the
  measure of the “0” glyph, it must be assumed to be 0.5em wide by 1em
  tall.
See https://www.w3.org/TR/css-values-3/#font-relative-lengths

Both ImageMagick and CairoSVG already implement em/2 as the value for ex.

Author: thisismiller

svglib/svglib.py

@@ -325,10 +325,14 @@ def convertLength(self, svgAttr, em_base=DEFAULT_FONT_SIZE, attr_name=None, defa
             return float(text[:-2]) * em_base
         elif text.endswith("px"):
             return float(text[:-2])
-
-        if "ex" in text:
-            logger.warning("Ignoring unit ex")
-            text = text.replace("ex", '')
+        elif text.endswith("ex"):
+            # The x-height of the text must be assumed to be 0.5em tall when the
+            # text cannot be measured.
+            return float(text[:-2]) * em_base / 2
+        elif text.endswith("ch"):
+            # The advance measure of the "0" glyph must be assumed to be 0.5em
+            # wide when the text cannot be measured.
+            return float(text[:-2]) * em_base / 2
 
         text = text.strip()
         length = toLength(text)  # this does the default measurements such as mm and cm

tests/samples/others/em_unit.svg

@@ -439,10 +439,17 @@ def test_external_svg_in_svg(self):
         assert isinstance(img_group.contents[1].contents[0], Rect)
         assert img_group.contents[1].transform, (1, 0, 0, 1, 100.0, 200.0)
 
-    def test_em_unit_svg(self):
-        path = join(TEST_ROOT, "samples", "others", "em_unit.svg")
+    def test_units_svg(self):
+        path = join(TEST_ROOT, "samples", "others", "units.svg")
         drawing = svglib.svg2rlg(path)
-        assert drawing.contents[0].transform[5] == svglib.DEFAULT_FONT_SIZE
+        unit_widths = [line.getBounds()[0] for line in drawing.contents[0].contents]
+        assert unit_widths == sorted(unit_widths)
+        unit_names = ["px", "pt", "mm", "ex", "ch", "em", "pc", "cm"]
+        lengths_by_name = dict(zip(unit_names, unit_widths))
+        assert lengths_by_name["px"] == 1 # 1px == 1px
+        assert lengths_by_name["em"] == svglib.DEFAULT_FONT_SIZE # 1 em == font size
+        assert lengths_by_name["ex"] == lengths_by_name["em"] / 2
+        assert lengths_by_name["ch"] == lengths_by_name["ex"]
 
     def test_empty_style(self):
         path = join(TEST_ROOT, "samples", "others", "empty_style.svg")