052f497b9e
## 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.
CrossPoint Reader
Firmware for the Xteink X4 e-paper display reader (unaffiliated with Xteink). Built using PlatformIO and targeting the ESP32-C3 microcontroller.
CrossPoint Reader is a purpose-built firmware designed to be a drop-in, fully open-source replacement for the official Xteink firmware. It aims to match or improve upon the standard EPUB reading experience.
Motivation
E-paper devices are fantastic for reading, but most commercially available readers are closed systems with limited customisation. The Xteink X4 is an affordable, e-paper device, however the official firmware remains closed. CrossPoint exists partly as a fun side-project and partly to open up the ecosystem and truely unlock the device's potential.
CrossPoint Reader aims to:
- Provide a fully open-source alternative to the official firmware.
- Offer a document reader capable of handling EPUB content on constrained hardware.
- Support customisable font, layout, and display options.
- Run purely on the Xteink X4 hardware.
This project is not affiliated with Xteink; it's built as a community project.
Features & Usage
- EPUB parsing and rendering (EPUB 2 and EPUB 3)
- Image support within EPUB
- Saved reading position
- File explorer with file picker
- Basic EPUB picker from root directory
- Support nested folders
- EPUB picker with cover art
- Custom sleep screen
- Cover sleep screen
- Wifi book upload
- Wifi OTA updates
- Configurable font, layout, and display options
- User provided fonts
- Full UTF support
- Screen rotation
Multi-language support: Read EPUBs in various languages, including English, Spanish, French, German, Italian, Portuguese, Russian, Ukrainian, Polish, Swedish, Norwegian, and more.
See the user guide for instructions on operating CrossPoint.
For more details about the scope of the project, see the SCOPE.md document.
Installing
Web (latest firmware)
- Connect your Xteink X4 to your computer via USB-C and wake/unlock the device
- Go to https://xteink.dve.al/ and click "Flash CrossPoint firmware"
To revert back to the official firmware, you can flash the latest official firmware from https://xteink.dve.al/, or swap back to the other partition using the "Swap boot partition" button here https://xteink.dve.al/debug.
Web (specific firmware version)
- Connect your Xteink X4 to your computer via USB-C
- Download the
firmware.binfile from the release of your choice via the releases page - Go to https://xteink.dve.al/ and flash the firmware file using the "OTA fast flash controls" section
To revert back to the official firmware, you can flash the latest official firmware from https://xteink.dve.al/, or swap back to the other partition using the "Swap boot partition" button here https://xteink.dve.al/debug.
Manual
See Development below.
Development
Prerequisites
- PlatformIO Core (
pio) or VS Code + PlatformIO IDE - Python 3.8+
- USB-C cable for flashing the ESP32-C3
- Xteink X4
Checking out the code
CrossPoint uses PlatformIO for building and flashing the firmware. To get started, clone the repository:
git clone --recursive https://github.com/crosspoint-reader/crosspoint-reader
# Or, if you've already cloned without --recursive:
git submodule update --init --recursive
Flashing your device
Connect your Xteink X4 to your computer via USB-C and run the following command.
pio run --target upload
Debugging
After flashing the new features, it’s recommended to capture detailed logs from the serial port.
First, make sure all required Python packages are installed:
python3 -m pip install pyserial colorama matplotlib
after that run the script:
# For Linux
# This was tested on Debian and should work on most Linux systems.
python3 scripts/debugging_monitor.py
# For macOS
python3 scripts/debugging_monitor.py /dev/cu.usbmodem2101
Minor adjustments may be required for Windows.
Internals
CrossPoint Reader is pretty aggressive about caching data down to the SD card to minimise RAM usage. The ESP32-C3 only has ~380KB of usable RAM, so we have to be careful. A lot of the decisions made in the design of the firmware were based on this constraint.
Data caching
The first time chapters of a book are loaded, they are cached to the SD card. Subsequent loads are served from the
cache. This cache directory exists at .crosspoint on the SD card. The structure is as follows:
.crosspoint/
├── epub_12471232/ # Each EPUB is cached to a subdirectory named `epub_<hash>`
│ ├── progress.bin # Stores reading progress (chapter, page, etc.)
│ ├── cover.bmp # Book cover image (once generated)
│ ├── book.bin # Book metadata (title, author, spine, table of contents, etc.)
│ └── sections/ # All chapter data is stored in the sections subdirectory
│ ├── 0.bin # Chapter data (screen count, all text layout info, etc.)
│ ├── 1.bin # files are named by their index in the spine
│ └── ...
│
└── epub_189013891/
Deleting the .crosspoint directory will clear the entire cache.
Due the way it's currently implemented, the cache is not automatically cleared when a book is deleted and moving a book file will use a new cache directory, resetting the reading progress.
For more details on the internal file structures, see the file formats document.
Contributing
Contributions are very welcome!
If you are new to the codebase, start with the contributing docs.
If you're looking for a way to help out, take a look at the ideas discussion board. If there's something there you'd like to work on, leave a comment so that we can avoid duplicated effort.
Everyone here is a volunteer, so please be respectful and patient. For more details on our goverance and community principles, please see GOVERNANCE.md.
To submit a contribution:
- Fork the repo
- Create a branch (
feature/dithering-improvement) - Make changes
- Submit a PR
CrossPoint Reader is not affiliated with Xteink or any manufacturer of the X4 hardware.
Huge shoutout to diy-esp32-epub-reader by atomic14, which was a project I took a lot of inspiration from as I was making CrossPoint.