## Summary
Bookerly's native TrueType hinting is effectively a no-op at the sizes
used here, causing FreeType to place stems at inconsistent sub-pixel
positions. This results in the 'k' stem (8-bit fringe: 0x38=56) falling
just below the 2-bit quantization threshold while 'l' and 'h' stems
(fringes: 0x4C=76, 0x40=64) land above it --- making 'k' visibly
narrower (2.00px vs 2.33px effective width).
FreeType's auto-hinter snaps all stems to consistent grid positions,
normalizing effective stem width to 2.67px across all glyphs.
Adds --force-autohint flag to fontconvert.py and applies it to Bookerly
only. NotoSans, OpenDyslexic, and Ubuntu fonts are unaffected.
Here is an example of before/after. Take notice of the vertical stems on
characters like `l`, `k`, `n`, `i`, etc. The font is Bookerly 12pt
regular:
**BEFORE**:
**AFTER**:
Claude generated this script to quantitatively determine that this
change makes the vertical stems on a variety of characters more
consistent for Bookerly _only_.
<details>
<summary>Python script</summary>
```python
#!/usr/bin/env python3
"""Compare stem consistency across all font families with and without
auto-hinting.
Run from repo root:
python3 compare_all_fonts.py
"""
import freetype
DPI = 150
CHARS = ["k", "l", "h", "i", "b", "d"]
SIZES = [12, 14, 16, 18]
FONTS = {
"Bookerly":
"lib/EpdFont/builtinFonts/source/Bookerly/Bookerly-Regular.ttf",
"NotoSans":
"lib/EpdFont/builtinFonts/source/NotoSans/NotoSans-Regular.ttf",
"OpenDyslexic":
"lib/EpdFont/builtinFonts/source/OpenDyslexic/OpenDyslexic-Regular.otf",
"Ubuntu": "lib/EpdFont/builtinFonts/source/Ubuntu/Ubuntu-Regular.ttf",
}
MODES = {
"default": freetype.FT_LOAD_RENDER,
"autohint": freetype.FT_LOAD_RENDER | freetype.FT_LOAD_FORCE_AUTOHINT,
}
def q4to2(v):
if v >= 12:
return 3
elif v >= 8:
return 2
elif v >= 4:
return 1
else:
return 0
def get_stem_eff(face, char, flags):
gi = face.get_char_index(ord(char))
if gi == 0:
return None
face.load_glyph(gi, flags)
bm = face.glyph.bitmap
w, h = bm.width, bm.rows
if w == 0 or h == 0:
return None
p2 = []
for y in range(h):
row = []
for x in range(w):
row.append(q4to2(bm.buffer[y * bm.pitch + x] >> 4))
p2.append(row)
# Measure leftmost stem in stable middle rows
mid_start, mid_end = h // 4, h - h // 4
widths = []
for y in range(mid_start, mid_end):
first = next((x for x in range(w) if p2[y][x] > 0), -1)
if first < 0:
continue
last = first
for x in range(first, w):
if p2[y][x] > 0:
last = x
else:
break
eff = sum(p2[y][x] for x in range(first, last + 1)) / 3.0
widths.append(eff)
return round(sum(widths) / len(widths), 2) if widths else None
def main():
for font_name, font_path in FONTS.items():
try:
freetype.Face(font_path)
except Exception:
print(f"\n {font_name}: SKIPPED (file not found)")
continue
print(f"\n{'=' * 80}")
print(f" {font_name}")
print(f"{'=' * 80}")
for size in SIZES:
print(f"\n {size}pt:")
print(f" {'':6s}", end="")
for c in CHARS:
print(f" '{c}' ", end="")
print(" | spread")
for mode_name, flags in MODES.items():
face = freetype.Face(font_path)
face.set_char_size(size << 6, size << 6, DPI, DPI)
vals = []
print(f" {mode_name:6s}", end="")
for c in CHARS:
v = get_stem_eff(face, c, flags)
vals.append(v)
print(f" {v:5.2f}" if v else " N/A", end="")
valid = [v for v in vals if v is not None]
spread = max(valid) - min(valid) if len(valid) >= 2 else 0
marker = " <-- inconsistent" if spread > 0.5 else ""
print(f" | {spread:.2f}{marker}")
if __name__ == "__main__":
main()
```
</details>
Here are the results. The table compares how the font-generation
`autohint` flag affects the range of widths of various characters. Lower
`spread` mean that glyph stroke widths should appear more consistent.
```
Spread = max stem width - min stem width across glyphs (lower = more consistent):
┌──────────────┬──────┬─────────┬──────────┬──────────┐
│ Font │ Size │ Default │ Autohint │ Winner │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ Bookerly │ 12pt │ 1.49 │ 1.12 │ autohint │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 14pt │ 1.39 │ 1.13 │ autohint │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 16pt │ 1.38 │ 1.16 │ autohint │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 18pt │ 1.90 │ 1.58 │ autohint │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ NotoSans │ 12pt │ 1.16 │ 0.94 │ mixed │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 14pt │ 0.83 │ 1.14 │ default │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 16pt │ 1.41 │ 1.51 │ default │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 18pt │ 1.74 │ 1.63 │ mixed │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ OpenDyslexic │ 12pt │ 2.22 │ 1.44 │ autohint │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 14pt │ 2.57 │ 3.29 │ default │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 16pt │ 3.13 │ 2.60 │ autohint │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 18pt │ 3.21 │ 3.23 │ ~tied │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ Ubuntu │ 12pt │ 1.25 │ 1.31 │ default │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 14pt │ 1.41 │ 1.64 │ default │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 16pt │ 2.21 │ 1.71 │ autohint │
├──────────────┼──────┼─────────┼──────────┼──────────┤
│ │ 18pt │ 1.80 │ 1.71 │ autohint │
└──────────────┴──────┴─────────┴──────────┴──────────┘
```
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? I used AI to make sure I'm
not doing something stupid, since I'm not a typography expert. I made
the changes though.
052f497b9e