select fonts via postscript name on Linux

greatly improves font matching accuracy

Fixes Automattic#1572

CHANGELOG.md

@@ -14,6 +14,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 * Add missing include for `toupper`.
 * Throw an error instead of crashing the process if `getImageData` or `putImageData` is called on a PDF or SVG canvas (#1853)
 * Compatibility with Typescript 4.6
+* Near-perfect font matching on Linux (#1572)
 
 2.9.0
 ==================

src/register_font.cc

@@ -1,5 +1,6 @@
 #include "register_font.h"
 
+#include <string>
 #include <pango/pangocairo.h>
 #include <pango/pango-fontmap.h>
 #include <pango/pango.h>
@@ -10,6 +11,7 @@
 #include <windows.h>
 #else
 #include <fontconfig/fontconfig.h>
+#include <pango/pangofc-fontmap.h>
 #endif
 
 #include <ft2build.h>
@@ -32,11 +34,29 @@
 #define PREFERRED_ENCODING_ID TT_MS_ID_UNICODE_CS
 #endif
 
+// With PangoFcFontMaps (the pango font module on Linux) we're able to add a
+// hook that lets us get perfect matching. Tie the conditions for enabling that
+// feature to one variable
+#if !defined(__APPLE__) && !defined(_WIN32) && PANGO_VERSION_CHECK(1, 47, 0)
+#define PERFECT_MATCHES_ENABLED
+#endif
+
 #define IS_PREFERRED_ENC(X) \
   X.platform_id == PREFERRED_PLATFORM_ID && X.encoding_id == PREFERRED_ENCODING_ID
 
+#ifdef PERFECT_MATCHES_ENABLED
+// On Linux-like OSes using FontConfig, the PostScript name ranks higher than
+// preferred family and family name since we'll use it to get perfect font
+// matching (see fc_font_map_substitute_hook)
+#define GET_NAME_RANK(X) \
+  ((IS_PREFERRED_ENC(X) ? 1 : 0) << 2) | \
+  ((X.name_id == TT_NAME_ID_PS_NAME ? 1 : 0) << 1) | \
+  (X.name_id == TT_NAME_ID_PREFERRED_FAMILY ? 1 : 0)
+#else
 #define GET_NAME_RANK(X) \
-  (IS_PREFERRED_ENC(X) ? 1 : 0) + (X.name_id == TT_NAME_ID_PREFERRED_FAMILY ? 1 : 0)
+  ((IS_PREFERRED_ENC(X) ? 1 : 0) << 1) | \
+  (X.name_id == TT_NAME_ID_PREFERRED_FAMILY ? 1 : 0)
+#endif
 
 /*
  * Return a UTF-8 encoded string given a TrueType name buf+len
@@ -104,15 +124,31 @@ get_family_name(FT_Face face) {
   for (unsigned i = 0; i < FT_Get_Sfnt_Name_Count(face); ++i) {
     FT_Get_Sfnt_Name(face, i, &name);
 
-    if (name.name_id == TT_NAME_ID_FONT_FAMILY || name.name_id == TT_NAME_ID_PREFERRED_FAMILY) {
-      char *buf = to_utf8(name.string, name.string_len, name.platform_id, name.encoding_id);
+    if (
+      name.name_id == TT_NAME_ID_FONT_FAMILY ||
+#ifdef PERFECT_MATCHES_ENABLED
+      name.name_id == TT_NAME_ID_PS_NAME ||
+#endif
+      name.name_id == TT_NAME_ID_PREFERRED_FAMILY
+    ) {
+      int rank = GET_NAME_RANK(name);
 
-      if (buf) {
-        int rank = GET_NAME_RANK(name);
-        if (rank > best_rank) {
+      if (rank > best_rank) {
+        char *buf = to_utf8(name.string, name.string_len, name.platform_id, name.encoding_id);
+        if (buf) {
           best_rank = rank;
           if (best_buf) free(best_buf);
           best_buf = buf;
+
+#ifdef PERFECT_MATCHES_ENABLED
+          // Prepend an '@' to the postscript name
+          if (name.name_id == TT_NAME_ID_PS_NAME) {
+            std::string best_buf_modified = "@";
+            best_buf_modified += best_buf;
+            free(best_buf);
+            best_buf = strdup(best_buf_modified.c_str());
+          }
+#endif
         } else {
           free(buf);
         }
@@ -209,6 +245,21 @@ get_pango_font_description(unsigned char* filepath) {
   return NULL;
 }
 
+#ifdef PERFECT_MATCHES_ENABLED
+static void
+fc_font_map_substitute_hook(FcPattern *pat, gpointer data) {
+  FcChar8 *family;
+
+  for (int i = 0; FcPatternGetString(pat, FC_FAMILY, i, &family) == FcResultMatch; i++) {
+    if (family[0] == '@') {
+      FcPatternAddString(pat, FC_POSTSCRIPT_NAME, (FcChar8 *)family + 1);
+      FcPatternRemove(pat, FC_FAMILY, i);
+      i -= 1;
+    }
+  }
+}
+#endif
+
 /*
  * Register font with the OS
  */
@@ -233,6 +284,12 @@ register_font(unsigned char *filepath) {
   // font families.
   pango_cairo_font_map_set_default(NULL);
 
+#ifdef PERFECT_MATCHES_ENABLED
+  PangoFontMap* map = pango_cairo_font_map_get_default();
+  PangoFcFontMap* fc_map = PANGO_FC_FONT_MAP(map);
+  pango_fc_font_map_set_default_substitute(fc_map, fc_font_map_substitute_hook, NULL, NULL);
+#endif
+
   return true;
 }