Fallback to using the name table to infer the encoding for TrueType fonts missing such data (issue 15910)

The relevant TrueType font is missing both /ToUnicode *and* /Encoding entires, either of which would have prevented the (current) broken textLayer rendering.
My first idea was that we could use the `post` table in the TrueType font, see https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6post.html, to get the actual glyphNames and amend the fallback ToUnicode-map that way. Unfortunately that didn't work, since the `post` table only contained ".notdef" and "" (i.e. empty string) entries.

Instead we try to use the `name` table in the TrueType font, see https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6name.html, to determine if the platform is Windows and thus fallback to generate a ToUnicode-map from the `WinAnsiEncoding`.

src/core/fonts.js

@@ -45,6 +45,7 @@ import {
   MacRomanEncoding,
   StandardEncoding,
   SymbolSetEncoding,
+  WinAnsiEncoding,
   ZapfDingbatsEncoding,
 } from "./encodings.js";
 import {
@@ -133,7 +134,56 @@ function adjustWidths(properties) {
   properties.defaultWidth *= scale;
 }
 
-function adjustToUnicode(properties, builtInEncoding) {
+function adjustTrueTypeToUnicode(properties, isSymbolicFont, nameRecords) {
+  if (properties.isInternalFont) {
+    return;
+  }
+  if (properties.hasIncludedToUnicodeMap) {
+    return; // The font dictionary has a `ToUnicode` entry.
+  }
+  if (properties.hasEncoding) {
+    return; // The font dictionary has an `Encoding` entry.
+  }
+  if (properties.toUnicode instanceof IdentityToUnicodeMap) {
+    return;
+  }
+  if (!isSymbolicFont) {
+    return; // A non-symbolic font should default to `StandardEncoding`.
+  }
+  if (nameRecords.length === 0) {
+    return;
+  }
+
+  // Try to infer if the fallback encoding should really be `WinAnsiEncoding`.
+  if (properties.defaultEncoding === WinAnsiEncoding) {
+    return;
+  }
+  for (const r of nameRecords) {
+    if (!isWinNameRecord(r)) {
+      return; // Not Windows, hence `WinAnsiEncoding` wouldn't make sense.
+    }
+  }
+  const encoding = WinAnsiEncoding;
+
+  const toUnicode = [],
+    glyphsUnicodeMap = getGlyphsUnicode();
+  for (const charCode in encoding) {
+    const glyphName = encoding[charCode];
+    if (glyphName === "") {
+      continue;
+    }
+    const unicode = glyphsUnicodeMap[glyphName];
+    if (unicode === undefined) {
+      continue;
+    }
+    toUnicode[charCode] = String.fromCharCode(unicode);
+  }
+  if (toUnicode.length > 0) {
+    properties.toUnicode.amend(toUnicode);
+  }
+}
+
+function adjustType1ToUnicode(properties, builtInEncoding) {
   if (properties.isInternalFont) {
     return;
   }
@@ -170,7 +220,7 @@ function adjustToUnicode(properties, builtInEncoding) {
 
 /**
  * NOTE: This function should only be called at the *end* of font-parsing,
- *       after e.g. `adjustToUnicode` has run, to prevent any issues.
+ *       after e.g. `adjustType1ToUnicode` has run, to prevent any issues.
  */
 function amendFallbackToUnicode(properties) {
   if (!properties.fallbackToUnicode) {
@@ -409,6 +459,19 @@ function buildToFontChar(encoding, glyphsUnicodeMap, differences) {
   return toFontChar;
 }
 
+// Please refer to:
+//  - https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6name.html
+function isMacNameRecord(r) {
+  return r.platform === 1 && r.encoding === 0 && r.language === 0;
+}
+
+// Please refer to:
+//  - https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6name.html
+//  - https://learn.microsoft.com/en-us/typography/opentype/spec/name#windows-language-ids
+function isWinNameRecord(r) {
+  return r.platform === 3 && r.encoding === 1 && r.language === 0x409;
+}
+
 function convertCidString(charCode, cid, shouldThrow = false) {
   switch (cid.length) {
     case 1:
@@ -1377,7 +1440,7 @@ class Font {
             'TrueType Collection font must contain a "name" table.'
           );
         }
-        const nameTable = readNameTable(potentialTables.name);
+        const [nameTable] = readNameTable(potentialTables.name);
 
         for (let j = 0, jj = nameTable.length; j < jj; j++) {
           for (let k = 0, kk = nameTable[j].length; k < kk; k++) {
@@ -2186,18 +2249,18 @@ class Font {
       const start = (font.start || 0) + nameTable.offset;
       font.pos = start;
 
-      const names = [[], []];
+      const names = [[], []],
+        records = [];
       const length = nameTable.length,
         end = start + length;
       const format = font.getUint16();
       const FORMAT_0_HEADER_LENGTH = 6;
       if (format !== 0 || length < FORMAT_0_HEADER_LENGTH) {
         // unsupported name table format or table "too" small
-        return names;
+        return [names, records];
       }
       const numRecords = font.getUint16();
       const stringsStart = font.getUint16();
-      const records = [];
       const NAME_RECORD_LENGTH = 12;
       let i, ii;
 
@@ -2211,10 +2274,7 @@ class Font {
           offset: font.getUint16(),
         };
         // using only Macintosh and Windows platform/encoding names
-        if (
-          (r.platform === 1 && r.encoding === 0 && r.language === 0) ||
-          (r.platform === 3 && r.encoding === 1 && r.language === 0x409)
-        ) {
+        if (isMacNameRecord(r) || isWinNameRecord(r)) {
           records.push(r);
         }
       }
@@ -2240,7 +2300,7 @@ class Font {
           names[0][nameIndex] = font.getString(record.length);
         }
       }
-      return names;
+      return [names, records];
     }
 
     // prettier-ignore
@@ -2996,9 +3056,16 @@ class Font {
       };
     } else {
       // ... using existing 'name' table as prototype
-      const namePrototype = readNameTable(tables.name);
+      const [namePrototype, nameRecords] = readNameTable(tables.name);
+
       tables.name.data = createNameTable(name, namePrototype);
       this.psName = namePrototype[0][6] || null;
+
+      if (!properties.composite) {
+        // For TrueType fonts that do not include `ToUnicode` or `Encoding`
+        // data, attempt to use the name-table to improve text selection.
+        adjustTrueTypeToUnicode(properties, this.isSymbolicFont, nameRecords);
+      }
     }
 
     const builder = new OpenTypeFileBuilder(header.version);
@@ -3015,7 +3082,7 @@ class Font {
     if (properties.builtInEncoding) {
       // For Type1 fonts that do not include either `ToUnicode` or `Encoding`
       // data, attempt to use the `builtInEncoding` to improve text selection.
-      adjustToUnicode(properties, properties.builtInEncoding);
+      adjustType1ToUnicode(properties, properties.builtInEncoding);
     }
 
     // Type 1 fonts have a notdef inserted at the beginning, so glyph 0

test/pdfs/.gitignore

@@ -242,6 +242,7 @@
 !bug1108301.pdf
 !issue10301.pdf
 !bug1157493.pdf
+!issue15910.pdf
 !issue4260_reduced.pdf
 !bug1250079.pdf
 !bug1473809.pdf

test/test_manifest.json

@@ -3056,6 +3056,12 @@
        "link": true,
        "type": "eq"
     },
+    {  "id": "issue15910",
+       "file": "pdfs/issue15910.pdf",
+       "md5": "6429d8490e11e226b1cbdf2033c04237",
+       "rounds": 1,
+       "type": "text"
+    },
     {  "id": "issue7872",
        "file": "pdfs/issue7872.pdf",
        "md5": "81781dfecfcb7e9cd9cc7e60f8b747b7",