Use one font descriptor for each Type-1 font

The old optimization where the same font file and descriptor could be
shared by multiple differently-encoded fonts is wrong in the presence of
subsetting, unless we also take the exact glyph subset into account.
It is very unlikely for the exact same subset to be used by different
encodings, so just remove the optimization.

Rearrange _embedTeXFont in neater blocks.

Author: jkseppan

lib/matplotlib/backends/backend_pdf.py

@@ -722,8 +722,6 @@ def __init__(self, filename, metadata=None):
         self._internal_font_seq = (Name(f'F{i}') for i in itertools.count(1))
         self._fontNames = {}     # maps filenames to internal font names
         self._dviFontInfo = {}   # maps dvi font names to embedding information
-        # differently encoded Type-1 fonts may share the same descriptor
-        self._type1Descriptors = {}
         self._character_tracker = _backend_pdf_ps.CharacterTracker()
 
         self.alphaStates = {}   # maps alpha values to graphics state objects
@@ -767,8 +765,6 @@ def __init__(self, filename, metadata=None):
 
     fontNames = _api.deprecated("3.11")(property(lambda self: self._fontNames))
     dviFontInfo = _api.deprecated("3.11")(property(lambda self: self._dviFontInfo))
-    type1Descriptors = _api.deprecated("3.11")(
-        property(lambda self: self._type1Descriptors))
 
     def newPage(self, width, height):
         self.endStream()
@@ -1005,48 +1001,42 @@ def _embedTeXFont(self, fontinfo):
         _log.debug('Embedding TeX font %s - fontinfo=%s',
                    fontinfo.dvifont.texname, fontinfo.__dict__)
 
-        # Font dictionary
+        # The font dictionary is the top-level object describing a font
         fontdictObject = self.reserveObject('font dictionary')
         fontdict = {
             'Type':      Name('Font'),
             'Subtype':   Name('Type1'),
         }
 
-        # We have a font file to embed - read it in and apply any effects
+        # Read the font file and apply any encoding changes and effects
         t1font = _type1font.Type1Font(fontinfo.fontfile)
         if fontinfo.encodingfile is not None:
             t1font = t1font.with_encoding(
                 {i: c for i, c in enumerate(dviread._parse_enc(fontinfo.encodingfile))}
             )
-
         if fontinfo.effects:
             t1font = t1font.transform(fontinfo.effects)
+
+        # Reduce the font to only the glyphs used in the document, get the encoding
+        # for that subset, and compute various properties based on the encoding.
         chars = frozenset(self._character_tracker.used[fontinfo.dvifont.fname])
         t1font = t1font.subset(chars, self._get_subset_prefix(chars))
         fontdict['BaseFont'] = Name(t1font.prop['FontName'])
+        # createType1Descriptor writes the font data as a side effect
+        fontdict['FontDescriptor'] = self.createType1Descriptor(t1font)
         encoding = t1font.prop['Encoding']
+        fontdict['Encoding'] = self._generate_encoding(encoding)
         fc = fontdict['FirstChar'] = min(encoding.keys(), default=0)
         lc = fontdict['LastChar'] = max(encoding.keys(), default=255)
-        fontdict['Encoding'] = self._generate_encoding(encoding)
-        # Font descriptors may be shared between differently encoded
-        # Type-1 fonts, so only create a new descriptor if there is no
-        # existing descriptor for this font.
-        effects = (fontinfo.effects.get('slant', 0.0),
-                   fontinfo.effects.get('extend', 1.0))
-        fontdesc = self._type1Descriptors.get((fontinfo.fontfile, effects))
-        if fontdesc is None:
-            fontdesc = self.createType1Descriptor(t1font)
-            self._type1Descriptors[(fontinfo.fontfile, effects)] = fontdesc
-        fontdict['FontDescriptor'] = fontdesc
-
-        # Use TeX Font Metrics file to get glyph widths (TeX uses its 12.20 fixed point
-        # representation and we want 1/1000 text space units)
+
+        # Convert glyph widths from TeX 12.20 fixed point to 1/1000 text space units
         tfm = fontinfo.dvifont._tfm
         widths = [(1000 * metrics.tex_width) >> 20
                   if (metrics := tfm.get_metrics(char)) else 0
                   for char in range(fc, lc + 1)]
         fontdict['Widths'] = widthsObject = self.reserveObject('glyph widths')
         self.writeObject(widthsObject, widths)
+
         self.writeObject(fontdictObject, fontdict)
         return fontdictObject