font-patcher: Warn if sourcefont is a VF

[why]
fontforge is not really able to work with OpenType variable fonts, at
least not with all. Some support is available as MM, for older formats.
But anyhow we do not really create a patched variable font but a fixed
font.

People might ignore all the errors Fontforge throws on opening, so an
explicit message might be in order.

[how]
It is not possible to detect a VF input font with current fontforge
reliably. Instead we search for the existence of one of the tables that
are  needed for a variable font. We can not rely on STAT, because that
might be also used in fixed fonts.

Some fontforges might crash on VFs, so we give a warning before we even
open the font and one after the patched font has been created.

Fixes: #512

Signed-off-by: Fini Jastrow <[email protected]>

font-patcher

@@ -6,7 +6,7 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 # Change the script version when you edit this script:
-script_version = "3.3.1"
+script_version = "3.3.2"
 
 version = "2.3.0-RC"
 projectName = "Nerd Fonts"
@@ -88,8 +88,8 @@ class TableHEADWriter:
         checksum = (checksum + extra) & 0xFFFFFFFF
         return checksum
 
-    def find_head_table(self, idx):
-        """ Search all tables for the HEAD table and store its metadata """
+    def find_table(self, tablenames, idx):
+        """ Search all tables for one of the tables in tablenames and store its metadata """
         # Use font with index idx if this is a font collection file
         self.f.seek(0, 0)
         tag = self.f.read(4)
@@ -117,9 +117,17 @@ class TableHEADWriter:
             self.tab_check = self.getlong()
             self.tab_offset = self.getlong()
             self.tab_length = self.getlong()
-            if tab_name == b'head':
-                return
-        raise Exception('No HEAD table found in font idx {}'.format(idx))
+            if tab_name in tablenames:
+                return True
+        return False
+
+    def find_head_table(self, idx):
+        """ Search all tables for the HEAD table and store its metadata """
+        # Use font with index idx if this is a font collection file
+        found = self.find_table([ b'head' ], idx)
+        if not found:
+            raise Exception('No HEAD table found in font idx {}'.format(idx))
+
 
     def goto(self, where):
         """ Go to a named location in the file or to the specified index """
@@ -376,6 +384,8 @@ class font_patcher:
                     dest_font.close()
                 except:
                     pass
+        if self.args.is_variable:
+            print("Warning: Source font is a variable open type font (VF) and the patch results will most likely not be what you want")
         print(message)
 
         if self.args.postprocess:
@@ -1534,7 +1544,7 @@ def setup_arguments():
         args.windows = False
 
     if args.nonmono and args.single:
-        print("Warniung: Specified contradicting --variable-width-glyphs and --use-single-width-glyph. Ignoring --variable-width-glyphs.")
+        print("Warning: Specified contradicting --variable-width-glyphs and --use-single-width-glyph. Ignoring --variable-width-glyphs.")
         args.nonmono = False
 
     make_sure_path_exists(args.outputdir)
@@ -1543,6 +1553,18 @@ def setup_arguments():
     if not os.access(args.font, os.R_OK):
         sys.exit("{}: Can not open font file for reading: {}".format(projectName, args.font))
     is_ttc = len(fontforge.fontsInFile(args.font)) > 1
+    try:
+        source_font_test = TableHEADWriter(args.font)
+        args.is_variable = source_font_test.find_table([b'avar', b'cvar', b'fvar', b'gvarb', b'HVAR', b'MVAR', b'VVAR'], 0)
+        if args.is_variable:
+            print("  Warning: Source font is a variable open type font (VF), opening might fail...")
+    except:
+        args.is_variable = False
+    finally:
+        try:
+            source_font_test.close()
+        except:
+            pass
 
     if args.extension == "":
         args.extension = os.path.splitext(args.font)[1]