Horcrux

A diary for the reMarkable 2 that drinks your ink, writes back,
remembers what you told it, and writes its own account of every day.

---

Write a question on the page. A few seconds after the pen leaves the paper, the diary consumes the handwriting from left to right. The page goes still. Then another hand begins to write.

Horcrux turns a reMarkable 2 into an enchanted, conversational diary. It captures real pen strokes, sends the page to a vision-capable language model, and renders the answer as animated handwriting directly on the e-ink display. It remembers earlier pages, keeps a dated journal, and returns cleanly to the stock reMarkable interface when closed.

The diary does not merely remember the day. It writes the day down itself.

➜ For the full tour, see Features.

The enchantment

The page drinks your ink

Pen input is rendered immediately with pressure-sensitive strokes and fast e-ink updates. After a configurable pause (3.8 seconds by default), the page dissolves the writer's ink in writing order, as though the paper were absorbing it.

An unseen hand answers

The written page is rasterized and sent to an OpenAI-compatible vision model. The complete reply is set in the largest script size that keeps every wrapped line visible, then its actual ink bounds are centered on the screen before the first stroke appears. It is reduced to single-stroke paths and replayed from left to right rather than displayed as a block of text.

The diary remembers

Every completed exchange is stored locally. Recent pages return to the model as conversational memory, allowing the diary to remember names, promises, questions, and earlier answers across sessions.

At day's end, the diary writes its own entry

When the diary is closed after a day in which at least one page was written, it gathers the entire retained conversation from that calendar day—every question and every answer—and composes a short first-person account in Tom's voice.

That daily summary then writes itself onto a clean page before the diary closes. Both its words and its handwriting strokes are saved under that date. Only one entry is created per day. A second five-finger tap during composition skips the ritual and closes immediately.

This is more than a chat history: the diary becomes an author of its own memory.

Old pages can be conjured

Ask naturally:

• “Show me yesterday's entry.”
• “What did we write on the 20th?”
• “Let me see the entry from 4 July.”

The stored handwriting is replayed quickly in faded ink. If an entry is too large to conjure safely, the diary recalls its opening sentences instead.

It leaves no visible machinery behind

A five-finger tap closes Horcrux, stops its display server, and restores the normal reMarkable UI. With the optional AppLoad integration, Tom's Diary appears as an on-tablet application and returns automatically after a reboot. No laptop or USB cable is needed after installation.

Feature list

• Live pressure-sensitive pen input from the reMarkable digitizer
• Fast partial e-ink refreshes through rm2fb
• Left-to-right disappearing-ink animation
• Vision-model reading of natural handwriting
• Streaming replies that begin writing before the model has finished
• Script-font rasterization, thinning, stroke tracing, and animated replay
• Persistent conversational memory with configurable context depth
• Automatic once-per-day summary of the full retained daily conversation
• Dated journal storage as both readable text and replayable handwriting
• Natural-language recall of earlier journal dates
• Eraser support
• Five-finger exit and reliable restoration of the stock UI
• Optional persistent AppLoad launcher for cable-free use
• Configurable model endpoint, model, font, memory, journal, and timezone
• No cloud database: memory and journal files remain on the tablet

Compatibility

Horcrux currently targets:

• Device: reMarkable 2
• Architecture: 32-bit ARMv7 hard-float
• Tested OS: reMarkable OS 3.22.4.2
• Display backend: rm2display/rm2fb from timower/rM2-stuff

The reMarkable 1, Paper Pro, and Paper Pro Move are not supported.

Caution

rm2display and xovi are tied to particular reMarkable OS releases. Confirm compatibility before updating the tablet. An unsupported OS update may remove or break the display shim and launcher.

Requirements

• A reMarkable 2 with SSH access
• A compatible rm2display installation providing /opt/lib/librm2fb_client.so.1
• Wi-Fi on the tablet
• An OpenAI-compatible endpoint with a vision-capable model
• An API key for that endpoint
• For the optional launcher: xovi and AppLoad builds compatible with the installed reMarkable OS

Install a release

Download the latest reMarkable 2 archive and its checksum from GitHub Releases, verify it, and extract it:

sha256sum -c horcrux-v*-remarkable2-armv7.tar.gz.sha256
tar -xzf horcrux-v*-remarkable2-armv7.tar.gz
cd horcrux-v*-remarkable2-armv7
scripts/deploy.sh

The default device address is root@10.11.99.1 over USB. A Wi-Fi address can be passed explicitly:

scripts/deploy.sh root@192.168.20.45

Deployment copies the binary, launchers, fonts, and systemd unit to /home/root/horcrux/. It never overwrites an existing private configuration.

Build from source

The one-command cross-build downloads a pinned Zig toolchain locally and targets the glibc version used by supported reMarkable OS releases:

scripts/build.sh
scripts/deploy.sh

See BUILD.md for the compiler, ABI, and dynamic-linking details.

Configure the oracle

On the tablet:

cp /home/root/horcrux/horcrux.env.example /home/root/horcrux/horcrux.env
chmod 600 /home/root/horcrux/horcrux.env
$EDITOR /home/root/horcrux/horcrux.env

Set at least:

HORCRUX_OPENAI_KEY=CHANGEME

The configuration file is excluded from Git and should always remain mode 600.

Run

Start from SSH:

systemctl start horcrux.service

Or install the on-tablet launcher after compatible xovi and AppLoad builds are already working:

scripts/install-appload.sh

Reboot once, then open AppLoad → Tom's Diary. The diary unit intentionally remains disabled at boot; the stock UI starts normally and the launcher opens the diary only when selected.

Gestures

Gesture	Effect
Write, then lift the pen	Submit after the configured idle pause
Use the eraser	Remove nearby ink
Double tap	Dismiss a completed reply sooner
Five-finger tap	Write today's entry, then exit
Five-finger tap again while journaling	Skip the entry and exit immediately

Configuration

Variable	Default	Meaning
HORCRUX_OPENAI_KEY	—	Required. API key for the model endpoint
HORCRUX_OPENAI_BASE	https://api.openai.com/v1	OpenAI-compatible base URL
HORCRUX_OPENAI_MODEL	gpt-4o-mini	Vision-capable model
HORCRUX_OPENAI_MAX_TOKENS	2000	Reply length cap
HORCRUX_OPENAI_TEMPERATURE	0.9	Sampling temperature
HORCRUX_REASONING_SPLIT	auto	Separate reasoning for compatible models
HORCRUX_FONT	embedded Dancing Script	Optional .ttf reply font
HORCRUX_IDLE_MS	3800	Quiet time before submitting a written page
HORCRUX_DRINK_STAGE_MS	140	Delay between disappearing-ink stages
HORCRUX_WRITE_TICK_MS	16	Delay between handwriting batches
HORCRUX_WRITE_POINTS_PER_TICK	12	Points written per animation batch
HORCRUX_FADE_STAGE_MS	140	Delay between reply-fade stages
HORCRUX_MEMORY	on	off disables persistent memory
HORCRUX_MEMORY_DIR	/home/root/horcrux-data/memories	Memory storage
HORCRUX_MEMORY_TURNS	6	Recent turns supplied as conversational context
HORCRUX_JOURNAL	on	off disables daily entries and recall
HORCRUX_JOURNAL_DIR	beside memory, under journal/	Daily text and stroke storage
HORCRUX_LIBRARY_DIR	reMarkable xochitl library	Destination for the readable diary EPUB
HORCRUX_TZ_NAME	Australia/Perth	Writer timezone name supplied to the oracle
HORCRUX_TZ_OFFSET	8	Whole hours added to UTC for the writer's local clock

Animation values are deliberately bounded so an accidental zero or extreme value cannot lock the display loop. Invalid values are ignored and the default is used. See horcrux.env.example for ranges, examples, and detailed descriptions of every variable.

On-tablet data

/home/root/horcrux-data/
├── memories/
│   ├── index.tsv
│   └── <timestamp>.strokes
└── journal/
    ├── Horcrux Diary.md
    ├── YYYY-MM-DD.txt
    └── YYYY-MM-DD.strokes

Memory is capped at the 400 most recent conversational pages. The daily entry uses every page from the selected day that remains in that retained history. At every session end, the current day's summary is regenerated so it includes the latest pages, and the cumulative Markdown diary is rebuilt. Because the stock reMarkable interface does not open Markdown, Horcrux also refreshes a matching Horcrux Diary EPUB in My Files. Journal entries do not feed back into conversation memory.

How the spell is made

Horcrux is a small Rust application with a single-threaded display/input state machine:

Listening → Drinking → Thinking → Replying → Lingering → Fading
                           ↘ Journaling
                           ↘ Conjuring

• pen.rs reads raw evdev pen and touch input.
• surface.rs writes RGB565 pixels through the rm2fb shared framebuffer.
• ink.rs records strokes, builds the model image, and plans dissolves.
• oracle.rs streams an OpenAI-compatible vision response.
• script.rs rasterizes, thins, traces, and orders reply strokes.
• memory.rs stores conversation pages locally.
• journal.rs composes, stores, dates, and recalls daily entries.
• main.rs conducts the state machine and animations.

Privacy and security

• The page image and configured conversational context are sent to the chosen model endpoint.
• API credentials remain in the on-tablet horcrux.env.
• Conversation memory and journal entries are plain local files.
• No telemetry, analytics, account system, or hosted Horcrux service exists.
• Before sharing logs, remove personal writing, endpoint details, and any credentials.

Please report security problems privately as described in SECURITY.md.

Development

cargo fmt --all -- --check
cargo test --locked --all-targets
cargo clippy --locked --all-targets -- -D warnings
cargo run --example render_sample

The project follows Semantic Versioning and keeps a human-readable changelog. Release maintainers should follow the release guide.

Contributions are welcome. Read CONTRIBUTING.md and the Code of Conduct before opening a pull request.

Recovery

Making xovi boot-persistent is convenient, but it removes part of xovi's tethered recovery safety. If an extension ever causes a boot loop, connect over USB and run:

systemctl disable xovi.service
reboot

Horcrux itself restores xochitl through an exit trap, including after most application failures. Logs are available from:

journalctl -u horcrux.service
cat /tmp/horcrux.log

Credits

• Inspired by riddle by Maxime Rivest (MIT), reimplemented in Rust for the reMarkable 2.
• Dancing Script by Pablo Impallari and Cedarville Cursive by Kimberly Geswein, distributed under the SIL Open Font License 1.1. License texts are included in fonts/.
• Display definitions were cross-checked against FBInk and rmkit.

See THIRD_PARTY_NOTICES.md for details.

Disclaimer

Horcrux is an unofficial fan project. It is not affiliated with or endorsed by reMarkable AS, J.K. Rowling, Warner Bros., or their affiliates. Harry Potter and related characters and elements are trademarks of their respective owners. Running custom software on a tablet is at your own risk.

License

The Horcrux source code and original project assets are available under the MIT License. Bundled fonts remain under the SIL Open Font License.