How to Fall in Love with Receipts
Or: How to print out album art of the music you’re listening to in real time
Many people hate the technology that goes into printing a receipt. It’s a known controversial topic.
But how does it work? How do they print? If you’d prefer to watch a talk version of this, check out my passion talk from Offline Camp below, otherwise read on for more detail.
The paper 📄
With a thermal printer, the ink isn’t printed onto the paper from a cartridge. Instead the ink is in the paper itself.
This ink is thermochromatic, meaning it changes colour when it changes temperature.
The thermal printer heats up small areas of the paper changing the ink colour to black.
You can see here what happens when you heat it from a flame. You may have also noticed old receipts left in your wallet end up going darker due to heat.
Hack friendly printers 🛠
Rather than rip apart a till, there are some thermal printers you can buy that are ready to be hacked with. The one I have (pictured below) is the Mini Thermal Receipt Printer from Adafruit.
I wanted to make something
I had to think of a project with this: What if, while listening to music, the album art of what I was listening to printed out in real time?
Sounded achievable, but I needed to work out two things:
I’ll be using JavaScript and Node.js for this project.
Get music data 🖼 🎶
So first I decided to use Last.fm. Last.fm tracks the music you listen to, such as streaming from Spotify.
They have a pretty flexible API that lets you get all sorts of data. You can sign up for the API and read the docs here: http://www.last.fm/api
Here’s an example of a endpoint you can access:
Going to this URL and using the “get recent tracks” method with my username, I can get the tracks I’m currently listening to!
The most recent track I listened to was a song by Bear Cavalry.
Here is a simplified version of what it returns. It brings back the song info and even has a image!
Using Last.fm in node
While this API endpoint is pretty accessible already, there is a node module that allows you to create a stream of tracks that are playing.
This will be helpful when sending these images and information to the printer later.
Print text and images 🖨 🔥
Now we have the data we need to work this printer out.
Thermal printer commands
Thermal printers can receive commands through a serial interface. You can send a command to print characters, print barcodes or even feed the paper.
Thankfully you don’t need to do this manually as there are node modules for this: serialport and thermalprinter.
First attempt
I tried to print the album art. On the left you can see what I tried to print, and on the right how it came out when printing.
It’s splodgy like a Rorschach test and has lost all of the depth of the picture…
This is because once the colour reaches a certain threshold the printer will either print it black or white.
Getting depth with dithering
Dithering is intentionally adding noise (here we’re adding dots) to an image to create colours. As we have black and white, we can achieve different grey colours.
These greys will help us achieve a sense of depth we’re missing.
Second attempt
After applying dithering to the image you can see the print out has a lot more depth! Yesss!
GraphicsMagick
To create the images and do the dithering you can use GraphicsMagick and the node module gm.
The end result
If we put all these parts together, we have something that can grab the music currently playing, create images and print them!
As that’s easier said than done, you can find code and some documentation over on GitHub of how this was done.
Below you can see a shoddy video of what I ended up with:
Thanks for reading. If you have any questions you can feel free to ask me on Twitter. :-)
Editor’s Note: This passion talk is one of many shared at Offline Camp, where a small group of campers with diverse interests come together to discuss Offline First. We hope to see you at an upcoming event!
