Add skew and italic metrics for ttfs

Summary:
Add the ability to base the metrics for a TTF character on the metrics
from one of the TFM characters.

Test Plan:
 - Make sure the tests still work
 - Make sure huxley tests didn't change

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13217

metrics/extract_ttfs.py

@@ -4,25 +4,32 @@ import fontforge
 import sys
 import json
 
+# map of characters to extract
 metrics_to_extract = {
-    "Main-Regular": [
-        u"\u2260",  # \neq
-        u"\u2245",  # \cong
-        u"\u0020",  # space
-        u"\u00a0",  # nbsp
-        u"\u2026",  # \ldots
-        u"\u22ef",  # \cdots
-        u"\u22f1",  # \ddots
-        u"\u22ee",  # \vdots
-    ],
-    "Size1-Regular": [
-        u"\u222c",  # \iint
-        u"\u222d",  # \iiint
-    ],
-    "Size2-Regular": [
-        u"\u222c",  # \iint
-        u"\u222d",  # \iiint
-    ]
+    # Font name
+    "Main-Regular": {
+        # Skew and italic metrics can't be easily parsed from the TTF. Instead,
+        # we map each character to a "base character", which is a character
+        # from the same font with correct italic and skew metrics. A character
+        # maps to None if it doesn't have a base.
+
+        u"\u2260": None,  # \neq
+        u"\u2245": None,  # \cong
+        u"\u0020": None,  # space
+        u"\u00a0": None,  # nbsp
+        u"\u2026": None,  # \ldots
+        u"\u22ef": None,  # \cdots
+        u"\u22f1": None,  # \ddots
+        u"\u22ee": None,  # \vdots
+    },
+    "Size1-Regular": {
+        u"\u222c": u"\u222b",  # \iint, based on \int
+        u"\u222d": u"\u222b",  # \iiint, based on \int
+    },
+    "Size2-Regular": {
+        u"\u222c": u"\u222b",  # \iint, based on \int
+        u"\u222d": u"\u222b",  # \iiint, based on \int
+    },
 }
 
 
@@ -43,15 +50,22 @@ def main():
 
                 depth = -depth
 
-                # TODO(emily): Figure out a real way to calculate this
-                italic = 0
-                skew = 0
+                base_char = chars[char]
+                if base_char:
+                    base_char_str = str(ord(base_char))
+                    base_metrics = start_json[font][base_char_str]
+
+                    italic = base_metrics["italic"]
+                    skew = base_metrics["skew"]
+                else:
+                    italic = 0
+                    skew = 0
 
                 start_json[font][ord(char)] = {
                     "height": height / fontInfo.em,
                     "depth": depth / fontInfo.em,
-                    "italic": italic / fontInfo.em,
-                    "skew": skew / fontInfo.em,
+                    "italic": italic,
+                    "skew": skew,
                 }
 
     sys.stdout.write(