Max 10k pairs per kern subtable (Fixes #1)

- Fixes script
- Adds kern diff script to compare fonts to helpers
- Moved ttfconv to helpers folder
- Updated README

README.md

@@ -2,9 +2,6 @@
 
 ## Overview
 
-> [!WARNING]
-> Currently, the `kern` table workaround does not appear to work correctly and requires further investigation. This script is still under development.
-
 `kobofix.py` is a Python script designed to process TTF fonts for Kobo e-readers. 
 
 It generates a renamed font, fixes PANOSE information based on the filename, adjusts the baseline with the `font-line` utility, and adds a legacy `kern` table which allows the `kepub` engine for improved rendering of kerned pairs.
@@ -64,9 +61,11 @@ To process fonts from my [ebook-fonts](https://github.com/nicoverbruggen/ebook-f
 To process all fonts with the "Kobo Fix" preset, simply run:
 
 ```bash
-./kobofix.py --prefix KF --remove-prefix="NV" --line-percent 20 *.ttf
+./kobofix.py --prefix KF --remove-prefix="NV" --line-percent 0 *.ttf
 ```
 
+(In this case, we'll set --line-percent to 0 so the line height changes aren't made, because the fonts in the NV Collection should already have those changes applied.)
+
 ### Generating NV fonts
 
 Tight spacing, with a custom font family name:

helpers/kerndiff.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python3
+
+import sys
+from fontTools.ttLib import TTFont
+
+def analyze_kern_table(font_path):
+    """
+    Loads a font and analyzes its 'kern' table, if present.
+    """
+    try:
+        font = TTFont(font_path)
+    except Exception as e:
+        print(f"Error: Could not open font file at {font_path}. Reason: {e}")
+        return
+
+    print(f"--- Analyzing 'kern' table in: {font_path} ---")
+
+    if 'kern' not in font:
+        print("No 'kern' table found.")
+        return
+
+    kern_table = font['kern']
+    print(f"  > Table version: {kern_table.version}")
+    
+    if not hasattr(kern_table, 'kernTables') or not kern_table.kernTables:
+        print("  > No subtables found.")
+        return
+
+    print(f"  > Number of subtables: {len(kern_table.kernTables)}")
+    
+    for i, subtable in enumerate(kern_table.kernTables):
+        print(f"\n  --- Subtable {i+1} ---")
+        if hasattr(subtable, 'coverage'):
+            print(f"  > Coverage flags (as integer): {subtable.coverage}")
+            # A more detailed breakdown of flags
+            coverage_int = int(subtable.coverage)
+            print(f"  > Coverage flags breakdown:")
+            print(f"    - Horizontal Kerning: {'Yes' if coverage_int & 1 else 'No'} (bit 0)")
+            print(f"    - Minimum Values: {'Yes' if coverage_int & 2 else 'No'} (bit 1)")
+            print(f"    - Cross-stream Kerning: {'Yes' if coverage_int & 4 else 'No'} (bit 2)")
+            print(f"    - Variation Kerning: {'Yes' if coverage_int & 8 else 'No'} (bit 3)")
+        
+        if hasattr(subtable, 'kernTable'):
+            print(f"  > Found {len(subtable.kernTable)} kerning pairs.")
+            # Print the first 20 kerning pairs and values for inspection
+            print("  > Sample of kerning pairs (glyph1, glyph2) -> value:")
+            for pair, value in list(subtable.kernTable.items())[:20]:
+                print(f" - ({pair[0]}, {pair[1]}) -> {value}")
+        else:
+            print("  > Subtable has no 'kernTable' attribute.")
+
+def main():
+    """
+    Main function to compare two fonts.
+    """
+    # Replace these with the actual paths to your font files
+    working_font = "./KC_Garamond-Regular.ttf"
+    broken_font = "./KF_Garamond-Regular.ttf"
+
+    analyze_kern_table(working_font)
+    print("\n" + "="*50 + "\n")
+    analyze_kern_table(broken_font)
+
+if __name__ == "__main__":
+    main()

kobofix.py

@@ -341,30 +341,32 @@ class FontProcessor:
     def add_legacy_kern(font: TTFont, kern_pairs: Dict[Tuple[str, str], int]) -> int:
         """
         Create or replace a legacy 'kern' table with the supplied pairs.
-        Older devices like some Kobo models only recognize the 'kern' table.
-        This function creates a new `kern` table from the extracted GPOS pairs.
+        Splits into multiple subtables if there are more than 10,000 pairs.
         """
         if not kern_pairs:
             return 0
-        
+
         kern_table = newTable("kern")
         kern_table.version = 0
         kern_table.kernTables = []
-        
-        subtable = KernTable_format_0()
-        subtable.version = 0
-        subtable.length = None
-        subtable.coverage = 1
-        subtable.kernTable = {
-            tuple(k): int(v) 
-            for k, v in kern_pairs.items() 
-            if v
-        }
-        kern_table.kernTables.append(subtable)
+
+        # Max pairs per subtable
+        MAX_PAIRS = 10000
+        items = [(tuple(k), int(v)) for k, v in kern_pairs.items() if v]
+
+        for i in range(0, len(items), MAX_PAIRS):
+            chunk = dict(items[i:i + MAX_PAIRS])
+            subtable = KernTable_format_0()
+            subtable.version = 0
+            subtable.length = None
+            subtable.coverage = 1
+            subtable.kernTable = chunk
+            kern_table.kernTables.append(subtable)
+
         font["kern"] = kern_table
-        
-        return len(subtable.kernTable)
-    
+
+        return len(items)
+
     # ============================================================
     # Name table methods
     # ============================================================