I saw a headline about moving files with QR codes and no Wi-Fi and assumed it was a gimmick. Then I opened decimen.app, pointed one phone at another laptop screen, and watched a PDF rebuild itself from flashing squares. No cable. No Bluetooth pairing. No shared network. Just light.
The project is called Decimen, built by Evan Crawley (Bash Alarmist). Some articles spell it "Desimon," including a recent piece on Geeky Gadgets. Same idea. The real repo lives at github.com/bashalarmistalt/decimen-optical-transfer, where it has picked up thousands of stars for a reason: it solves a narrow problem very cleanly.
The One-Minute Version
Optical file transfer works like this:
- Device A encodes a file into a stream of animated QR codes on its screen.
- Device B points a camera at that screen and decodes each frame.
- After enough distinct frames arrive, the receiver reconstructs the original file.
- Nothing travels over Wi-Fi, cellular, USB, or Bluetooth between the two devices.
That last point is the whole pitch. You are not "offline syncing through the cloud." You are literally reading pixels. For air-gapped machines, embedded boards with only a camera, or situations where you do not trust any radio stack, that matters.
Why Fountain Coding Changes Everything
Early QR transfer demos often chopped a file into numbered chunks. Frame 1, frame 2, frame 3. Miss frame 2 and you are stuck waiting for the loop to come back around. In bad lighting, with a cheap camera, or when the screen refresh rate and the camera shutter do not line up, dropped frames are normal.
Decimen uses fountain coding, specifically Luby transform codes (Crawley, 2026; Luby, 2002). Instead of shipping unique sequential slices, the sender emits mathematically mixed frames. The receiver needs roughly K × 1.15 distinct frames, in any order, to peel the file back out. A missed frame slows you down. It does not break correctness.
That design fits the channel. A screen-to-camera link has no back-channel. The sender cannot ask "did you get packet 47?" It can only keep flashing codes and hope the camera catches enough of them. Fountain codes were built for exactly that kind of one-way, lossy pipe.
The same fountain-code idea showed up earlier in Ivan Daniluk's txqr project (Divan, 2018), which is worth reading if you want the theory without the browser polish.
What Decimen Actually Ships
Decimen is not a research slide deck. It is a working web app plus open-source codebase (Crawley, 2026):
- No install required. Open decimen.app on two devices. It can cache for offline use and install as a PWA on iOS and Android.
- Files up to 64 MB, or pasted text, with filename and MIME type preserved.
- SHA-256 verification before download is offered, so you know the rebuild matched the sender.
- Optional gzip when compression helps.
- Export as looping animation (APNG or PNG sequence), so a recorded video of the QR stream can be decoded later by any camera pointed at playback.
- Twelve UI languages, with RTL layout where needed.
Important caveat straight from the README: neither mode is encrypted. Anyone with line of sight to the screen can read what is flashing. Decimen gives you isolation from networks, not secrecy from nearby eyes. Treat it like shouting a file across a room, not like end-to-end encryption.
How Fast Is It, Really?
When Geeky Gadgets covered the project, they cited roughly 128 KB/s as a practical ceiling (Horsey, 2026). That matched early impressions. The project has moved since then. Crawley publishes benchmark "receipts" in the repo, and the current records (August 2026, v0.4.0) look like this:
| Device pair | Sustained | Peak | 1 MB transfer |
|---|---|---|---|
| Desktop monitor to iPhone | 418.5 KB/s | 601.5 KB/s | 2.5 s |
| Phone to phone | 199.2 KB/s | 340.8 KB/s | 5.1 s |
Those numbers are goodput for the whole transfer, not a cherry-picked burst. Still not USB territory. A 10 MB config bundle might take half a minute phone-to-phone. But for logs, keys, firmware patches, or a short video clip into an air-gapped box, that is often fine.
What moves the needle:
- Screen size and brightness. A large, bright panel (the record run used a 49-inch Odyssey G9) gives the camera more pixels per module.
- Camera quality. Better optics resolve denser QR modules, which means more bytes per frame.
- Distance and alignment. Keep the camera square to the screen and close enough that modules stay sharp.
- Refresh vs shutter. LCD ghosting and frame-rate mismatch still cause drops. Fountain coding absorbs the drops, but you pay in time.
Decimen vs LiveDrop and the Rest
Decimen is open source and browser-native. LiveDrop, covered by PetaPixel (Kritsonis, 2025), is a Dutch commercial product chasing the same optical channel with a mobile app and SDK. Both flash dynamic QR-like codes. Both work without Wi-Fi or Bluetooth between devices. The differences are mostly product shape and tuning.
| Project | Access | Reported speed | Notes |
|---|---|---|---|
| Decimen | Free web app + AGPL source | Up to ~419 KB/s sustained (published benchmarks) | Fountain codes, SHA-256 check, 64 MB cap, no account |
| LiveDrop | iOS/Android app, paid Windows tier, SDK | ~700 kbps in 2025; testing 2.6 Mbps | Long-range optics (hundreds of meters with telephoto lenses), optional encryption keys |
| txqr | Open source (Go) | Varies by setup | Early animated QR + fountain code reference implementation |
| libcimbar | Open source | Higher density than QR | Custom color barcode instead of standard QR modules |
LiveDrop's team talks openly about optics as the bottleneck (Kritsonis, 2025). Phone cameras with wide apertures work well at short range. A 600 mm lens on a proper camera body can stretch the link to hundreds of meters. They are also experimenting with grey/blue tones and infrared for night transfers. Decimen stays pragmatic: standard QR modules, browser decoder built on ZXing, and reproducible benchmark JSON in the repo so skeptics can verify claims.
Crawley notes similar ideas arrived independently elsewhere, including airgapped-qr-code-transfer, which uses sequential chunking instead of fountain frames. Convergent evolution, not copying.
Where This Actually Gets Used
Most people will never need optical transfer. For the cases that do, it is oddly elegant:
- Air-gapped systems. Push a patch or config onto a machine that never touches a network. The data path is visible light only.
- Embedded and industrial gear. A laptop screen facing a controller with a camera module, no USB port exposed.
- Remote or disconnected sites. Two laptops on a worksite with no router. Still faster than retyping hex.
- Cross-platform friction. No Apple/Google ecosystem negotiation. If it has a browser and a camera, it can receive.
- Pre-recorded drops. Export the QR animation, play it on a monitor in a lab video, and let receivers decode from the recording.
Geeky Gadgets frames the security angle as keeping data off external networks (Horsey, 2026). That is fair, with one correction: optical transfer is not automatically "secure." It is network-isolated. Shoulder surfers, cameras in the room, and recorded screens are all in scope.
Practical Tips If You Try It Today
I am not going to pretend setup is zero-friction. It is simple, not magic.
- Open decimen.app/send/ on the sending device and load your file.
- Open the receive page on the other device and grant camera access.
- Maximize brightness, kill night mode, and reduce glare if you can.
- Hold the receiver steady. Let the progress bar climb. Fountain coding means stutters are fine; giving up early is not.
- Wait for the SHA-256 check before saving.
Developers can clone the repo, run npm install and npm run dev, and hit the local HTTPS server from a phone on the same LAN for testing (Crawley, 2026). There is also a npm run benchmark path if you want to publish your own receipt.
Limits Worth Knowing Up Front
Optical transfer is a specialty tool, not a replacement for AirDrop or a cable:
- Speed caps. Even hundreds of KB/s is tiny next to gigabit Ethernet.
- No encryption by default. Decimen is explicit about this. LiveDrop can layer access keys, but the channel is still visible.
- Hardware tuning. Ghosting on LCD panels, rolling shutter, and autofocus hunting all eat frames.
- Line of sight. Obstructions and glare are failure modes, not edge cases.
- Human patience. Large media works in theory (64 MB). In practice you will choose smaller files unless you are committed.
That is the honest trade. You gain a data diode made of photons. You lose bandwidth and confidentiality unless you add other layers.
Decimen turns a screen and a camera into a one-way file pipe with no network stack in the middle. Fountain coding is the reason it survives real-world frame drops. LiveDrop pushes the same idea toward commercial speed and long-range optics. Both sit in a growing family of optical exfiltration and sideload tools that sound like spy fiction until you need them on a factory floor.
If you have ten minutes and two devices, try the live demo. Watching a file materialize from blinking QR codes is the fastest way to decide whether this belongs in your toolkit or stays a neat party trick. For me, it crossed from gimmick to "oh, I would use this on an air-gapped laptop" somewhere around the first successful SHA-256 match.
Bibilography
- Crawley, E. (2026). Decimen optical transfer: Fountain-coded QR file transfer [Computer software]. GitHub. https://github.com/bashalarmistalt/decimen-optical-transfer
- Divan. (2018). txqr [Computer software]. GitHub. https://github.com/divan/txqr
- Horsey, J. (2026, August 12). Transfer files without internet using optical QR codes. Geeky Gadgets. https://www.geeky-gadgets.com/transfer-files-without-internet/
- Kritsonis, T. (2025, March 9). LiveDrop wirelessly moves data using dynamic QR codes and cameras. PetaPixel. https://petapixel.com/2025/03/09/livedrop-wirelessly-moves-data-using-dynamic-qr-codes-and-cameras/
- Luby, M. (2002). LT codes. In Proceedings of the 43rd Annual IEEE Symposium on Foundations of Computer Science (pp. 271-280). IEEE.