• Print photos on the receipt printer (select a photo then click “Invoke”)
• Display images on the e-paper screen

Web of Things is a set of standards for Internet of Things use cases. At the Matrix Hackathon and FOSDEM 2026, I brought the following hardware. They support support Web of Things via HTTP and Websockets.

Current status: Ready

• Epson TM-T20III
• Native colors: 1-bit
  • #ffffff (white)
  • #000000 (black)
• Native resolution: 576 pixels wide, any height (will be converted to be divisible by 8 pixels)
• Supported image formats: PNG (best results), JPG

• URL for printing images: https://wot-wrench.chrpaul.de/#try=https://wot-why.jaller.de/receipt-printers-for-events/photo
  • URL for scripts: https://wot-why.jaller.de/receipt-printers-for-events/photo/actions/print-photo
• UI for printing text: https://wot-wrench.chrpaul.de/#try=https://wot-why.jaller.de/receipt-printers-for-events/text
  • URL for scripts: https://wot-why.jaller.de/receipt-printers-for-events/text/actions/print-text

Bash:

curl -X POST \
  -H "Content-Type: application/json" \
  --data "{\"image\":\"$(base64 image.png | tr -d '\n')\"}" \
  https://wot-why.jaller.de/receipt-printers-for-events/photo/actions/print-photo

Or in NodeJS / Bun / Deno:

import { readFile } from "node:fs/promise";
 
const image = await readFile("image.png");
const response = await fetch("https://wot-why.jaller.de/receipt-printers-for-events/photo/actions/print-photo", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    image: image.toString("base64"),
  }),
});
 
if (!response.ok) {
  throw Error(`Failed to queue print job. HTTP ${response.status}`);
}

Bash:

curl -X POST \
  --header "Content-Type: application/json" \
  --data '{"text":"Hello World!"}' \
  https://wot-why.jaller.de/receipt-printers-for-events/text/actions/print-text

Or in NodeJS / Bun / Deno:

const response = await fetch("https://wot-why.jaller.de/receipt-printers-for-events/text/actions/print-text", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    text: "Hello World!",
  }),
});
 
if (!response.ok) {
  throw Error(`Failed to queue print job. HTTP ${response.status}`);
}

Current status: Ready at https://wot-wrench.chrpaul.de/#try=https://wot.chrpaul.de/image-storage/LDN3dJE3vu

• 7.3inch ACeP 7-Color E-Paper Display
• Native colors: 7 colors
  • #000000 (black)
  • #ffffff (white)
  • #ff0000 (red)
  • #00ff00 (green)
  • #0000ff (blue)
  • #ffff00 (yellow)
  • #ff8000 (orange)
• Native resolution: 800×480 pixels
• Supported image formats: PNG (best results), JPG

Use these URLs to test your scripts. They will not display anything on the screen, but the Thing has a preview property.

• Test URL: https://wot-wrench.chrpaul.de/#try=https://wot.chrpaul.de/image-storage/fosdem
  • Image property for scripts: https://wot.chrpaul.de/image-storage/fosdem/properties/image

Use this URL to HTTP PUT an image onto the screen.

• Production URL for scripts: https://wot-wrench.chrpaul.de/#try=https://wot.chrpaul.de/image-storage/LDN3dJE3vu
  • Image property for scripts: https://wot.chrpaul.de/image-storage/LDN3dJE3vu/properties/image

To upload a local PNG file image.png via a Bash:

curl -X PUT \
  -H "Content-Type: application/json" \
  --data "\"$(base64 image.png | tr -d '\n')\"" \
  https://wot.chrpaul.de/image-storage/fosdem/properties/image

Or in NodeJS / Bun / Deno:

Have a look at this demo project: https://codeberg.org/jaller94/wot-fosdem-demo

import { readFile } from "node:fs/promise";
 
const image = await readFile("image.png");
const response = await fetch("https://wot.chrpaul.de/image-storage/fosdem/properties/image", {
  method: "PUT",
  headers: {
    "Content-Type": "application/json",
  },
  body: JSON.stringify(image.toString("base64")),
});
 
if (!response.ok) {
  throw Error(`Failed to upload image. HTTP ${response.status}`);
}

Images will be resized and dithered automatically. However, if you want full control over the result, you can send images in the right resolution using only the supported color palette.

For TypeScript, I recommend using my fork of dither-me-this. It supports custom color palettes which is optimal for the e-paper screens. Previously, I used my fork of canvas-dither. Its API is easier and more mature, but it only supports 1-bit dithering. It's most suitable for the receipt printer.