Show HN: Infinity Whiteboard, Designed for Teachers
I've created a whiteboard which I use every lesson when teaching maths, though it can be used for anything. It currently has a few hundred teachers using it daily. It's designed for use with touch-screen interactive whiteboards in classrooms, and stays in sync with your phone/tablet/whatever without signup/login.

You can also find me on Twitter where I post updates etc: https://twitter.com/jakegmaths

Some features and cool things:

* Sync devices without signup - offline by default, just hit 'sync devices' and use the same code on multiple devices to sync

* Touch-first - 1 finger draws; 2 finger pan/zoom; 3 finger gestures like changing pen colour

* Add images - when teaching, this is usually photos of student work taken on my phone and auto-sync'd to the whiteboard at the front of the room

* Add PDFs - when teaching, these are usually past paper exams which I then annotate over with the class

* Zen mode - 3 finger tap or hit the ∞ icon to hide the UI; something I use every lesson so students can focus on the actual maths (there's also a fullscreen button when not on iOS)

* Visualiser - often when teaching we'll work on paper with a webcam aka 'visualiser' pointing down at it; this projects that to the main whiteboard, with optional cropping, freeze-frame and snapshots

* Screencast - many teachers use eg PowerPoint to teach; instead, I'll use PowerPoint in edit/design mode rather than slideshow mode, with a locally-cast cropped portion of that on the main whiteboard at the front of the room. This enables me to eg edit my PowerPoint as I go and use all the PowerPoint tools not available in slideshow mode

* Instant replay - hit the play button to play back all the scribblings currently showing on the screen

* Magnet mode - when sync'd with another device, use the magnet icon so the other device follows you. Most of my teaching is now via a tablet-with-stylus anywhere in the room, and as I pan/zoom around with the tablet the main whiteboard comes with me... but only when I want it to by activating the magnet

* Student mini-whiteboards (MWBs) - if my students have devices and I want them to use them, I 'sync devices' then enable student MWBs and each student has a live copy of the whiteboard, and I can see what they write and can showcase any student instantly on the main board

* PWA support - install as a PWA and you can download whiteboards as .iwb files which can then be double-clicked to open/edit on desktop

* Free - I have no plans to charge for this

Other things you may find interesting from a tech perspective:

* The client is a single <5,000 lines HTML file, with JS, CSS, SVG-favicon all inlined (plus PDF.js lazily loaded if you add a PDF)

* This is vanilla Javascript with no frameworks or libraries (except PDF.js) and no minification or build scripts - just view-source and check out how ugly all my code is!

* 77.6kB for everything (except PDF support)... the size of 'modern' websites frankly disgusts me

* The server is just a single ~500 line Javascript file and runs on Deno (also ported to Bun but unstable for now) and really just serves some static files, deals with websockets and temporarily stores images people add

* Costs ~£5/month on Heroku

* There's no database or any long-term persistence - Heroku servers restart every 24h and nothing is saved beyond that; it's all ephemeral