Compounding Self-Hosting

A small e-ink photo frame for our home. It pulls from a self-hosted Immich¹ library, checks a self-hosted Home Assistant² instance to see whether anyone is home, and displays a photo on the PhotoPainter³ for everyone to enjoy.

Why

Most digital frames offer a choice between two bad options: preprocess the photos and copy them to an SD card yourself, or hand your library to a cloud service like Google Photos. Realistically, the SD card gets updated once a year, and I’d rather not give a cloud provider access to my most cherished memories or let it hold them hostage.

Yet, coming home to a photo from earlier that day, or one from five years ago, feels different when it’s in a real picture frame instead of buried on your phone. So I decided to spend a weekend building a personal version with Claude Code. I experimented with dithering and post-processing, then fine-tuned the photo-picking algorithm, and ended up with this project.

A weekend was enough only because so much came for free. Immich had already detected faces in every photo and reverse-geocoded the place names shown on the overlay, and it knew which photos we’d marked as favourites. Home Assistant already knew who was home. That’s the compounding return of self-hosting: every service exposes an easy-to-integrate API, and each new project gets to stand on all of them. I’m now looking for more ways to leverage the emergence of self hosted services.

How it works

The frame is driven by a Raspberry Pi Zero 2 W, which is fast enough that I could write plain Python and mostly ignore performance. That was a luxury after counting individual bytes on an ATtiny85⁴. Cron runs a script every 15 minutes. Each run:

1. Quits if it’s between midnight and 7 am. The e-ink display holds its image at zero power, so the last photo of the evening simply stays up overnight.
2. Asks Home Assistant whether anyone in HA_PRESENCE is home. If not, it quits; there’s no point wasting power and straining the display.
3. Picks a random photo from Immich, drawn from four overlapping pools: “on this day” memories, favourites, the last 30 days, and the whole library. The weights work out to roughly a 25% chance for a memory (10% if only the ±3-day fallback has matches), 15% for favourites, and 30% for each of the other two. A 7-day rolling history prevents repeats. Photos matching the frame’s orientation get 4× the weight, while any candidate whose crop would clip a head is rejected (more on that below).
4. Crops around any detected faces, boosts contrast and saturation (e-ink is short on both), dithers down to the six-colour palette, and pushes the result to the panel. The photo’s age and EXIF location are painted into the bottom corners as white text with a black stroke, so the dither can’t smear the edges.

Image pipeline

Two choices matter most for how the result looks: the face-aware cropping and dithering.

Cropping

The frame stands in one orientation, but I didn’t want to limit it to photos that happen to match. A face-aware algorithm therefore resizes and crops each image to fill the panel, biasing the crop window towards faces Immich has already detected. A landscape shot with some room around the subject will often crop cleanly to portrait this way. If the crop would cut into a head, the photo is rejected and the picker moves to the next candidate.

The examples below show how the head bounding boxes steer the final crop, and which candidates get rejected.

Dithering

The panel shows exactly six colours: black, white, red, yellow, blue, and green. Unlike an LCD, it has no intensity control, so every pixel must use one of those six colours. Anything photo-like has to be dithered, and the available algorithms differ wildly in output quality and running time. Here’s how a few of them compare:

I settled on Atkinson dithering⁵, which preserved the most contrast without too many visible artefacts. Unlike the alternatives, it diffuses only 6/8 of each pixel’s quantisation error, so highlights stay bright instead of being dragged grey by their neighbours. Dithering was also the first part of the pipeline where pure Python became unusably slow on the Pi Zero. The inner loop, including perceptually weighted nearest-colour matching, is therefore JIT-compiled with Numba⁶. Once the cache is warm, it runs roughly two orders of magnitude faster.

Learnings

The Pi Zero 2 W is overkill. It drains the battery far too quickly if you try to run it cordless and spends most of its life idle, waiting for the next cron tick. An external RTC-controlled power circuit could have fixed that, but I couldn’t be bothered to hack one in. For a battery-powered build, I’d use an ESP32 with deep sleep instead. With 15 minutes between refreshes, even a slow chip has plenty of time for image processing and dithering.

A few reliability quirks are worth knowing. In my setup, the Zero 2 W’s Wi-Fi connection drops when power saving kicks in, so a separate cron job runs wifi-check.sh every 5 minutes to reconnect. I also masked swap and set journald to volatile because SD-card wear is the only thing likely to slowly kill this build.

I’d also like to try an Inky Impression⁷ in a custom-made frame, with a larger display and perhaps integrated lighting. E-ink looks a little muddled in evening light, so a dedicated light source would be the biggest improvement by far.

All in all, the frame already does its job. It stands in the living room as a pleasant speck of curiosity, subtle enough that most visitors never notice it until I inevitably point it out. Once you know it’s there, catching the panel mid-refresh brings a small rush of excitement: you’re about to be pulled into a memory, distant or recent.

1. Immich: https://immich.app/↩
2. Home Assistant: https://www.home-assistant.io/↩
3. PhotoPainter: https://www.waveshare.com/wiki/PhotoPainter↩
4. counting individual bytes on an ATtiny85: https://schmelczer.dev/articles/ad-astra-attiny85-game-engine/↩
5. Atkinson dithering: https://en.wikipedia.org/wiki/Atkinson_dithering↩
6. Numba: https://numba.pydata.org/↩
7. Inky Impression: https://shop.pimoroni.com/products/inky-impression?variant=55186435244411↩