Upscaling guest photos with a local model instead of an API

I run Knipsmig , a QR-code photo sharing service for weddings and parties. Guests scan a code and upload straight from the phone, no app. Most of those uploads are 12 MP and print fine. A meaningful slice are not: photobooth captures at 1080x810, WhatsApp forwards at 1600x1200, screenshots, old scans someone re-uploaded. Those end up in the printed photo book looking soft. So I added an "Improve resolution" option to the editor. It adds up to 4x the pixels, and the whole thing runs on my own server. No API, no vendor, nothing leaves the box. This post is about why I went local and what it took to make that work inside a Rails app. Why not just call an image API I already have Gemini and OpenAI keys configured in the app for other things, so the lazy path was obvious. I did not take it, for three reasons. The generative models redraw the image. They don't upscale, they regenerate. Faces drift. These are guests' faces at someone's wedding, and "your aunt looks slightly different now" is not a feature. A super-resolution network stays faithful to the input: it only adds pixels consistent with the ones already there. Privacy paperwork. Every third-party processor I add has to go into the DPA. Guests' photos leaving the server to be fetched by a vendor is a real change, not a footnote. Running locally means the data processing agreement doesn't change and the existing opt-out for third-party AI stays about third parties. Cost. Per-image API pricing on a bulk action over hundreds of photos adds up fast. CPU time on a job lane I already pay for is free at the margin. The model I went with realesr-general-x4v3 from the Real-ESRGAN project (BSD-3-Clause). It's the compact SRVGGNet variant: about 1.2M parameters, roughly 5 MB as an ONNX file, and around 10x faster on CPU than the full RRDBNet x4plus. Quality is more than fine for event snapshots. Getting it into a usable shape was a one-off: export the release weights with the repo's pytorch2onnx.py script using dynamic H/W a