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Raspberry Pie with a Taste of Chocolate, the funny Desktop Droste effect

June 28th, 2022

The original Droste Effect

Trying out the new Ubuntu 22.04 on my Raspberry Pi 400, I was surprised by the smoothness of the new Ubuntu distribution. Much better impression then the first time I tried Ubuntu on the RPI, I think that was the 20.10 release.

A funny thing to try out, especially if you love fractals or you are an admirer of the Dutch graphic artist Escher who’s work features features mathematical and even impossible objects, is the subject of this post.

Another returning phenomenon in his drawings and paintings is the Droste-effect , and I’ll shwo you how to create a Droste effect on your Raspberry Pi with a few mouse-clicks.

Creating a Droste effect on your Raspberry Pi

Yes, you can do that with just a couple of mouse-clicks, you don’t need a mathematical package or a graphical editor like Gimp or so.

Trying out the new Gnome Desktop Sharing feature, which let you share your desktop not only with the older VNC protocol, but also with the newer RDP protocol, gave me this idea.

To activate:

Settings -> sharing -> enable -> enable Remote Desktop -> and setup some authentication: username and password

For creating the Droste-effect we gonna do something silly: we gonna connect to our-self! Yes a Remote Desktop Connection with a local client.

A Remote Desktop Connection with a local client


Start up the default remote desktop client Remmina.

Quick setup a new connection, enter your IP-address and the authentication you just entered: username and password.

To find your IP-address, open a terminal, (CTRL ALT T) and type `ip address` return. Then you will find it in the output, or look it up under details in the network settings.

Save and connect in the Remmina dialog, and see the connection being made.

Click the `Toggle Scaled Mode` button to rescale the desktop (CTRL_R S), and there it is.

A nice Raspberry Pi Droste Effect of the Ubuntu Desktop in a local remote Desktop connection: 🙂

Remmina Droste effect

Remmina Droste effect


An alternative to `Remmina` is Gnome-connections. That program is in development, but like all Gnome apps, it does offer an very easy and intuitive approach.

Can all the settings in Remmina be overwhelming, gnome-connections is easy as it can be.

But the default resolution seems to be quite poor. And I could not find a scaling options, so you end up with a more spacey psychedelic form of computer art.

The Gnome-Connections Art

The Gnome-Connections Art

Cool as well.

Update: Actually there is a scale setting for Gnome-Connections, a bit hidden, under properties once you established a connection. Using Gnome-connections for managing my Pi400 from another Ubuntu 22.04 is working quite well, although I had to restart the Pi400 to get control working.

So maybe Gnome-connections is lacking a lot of settings, it’s working out of the box surprisingly well in Ubuntu 22.04.

Give it a try, if you own a Pi.

How does the Pi create a Droste effect?

You open up a program that shows your complete desktop scaled including the program that shows your desktop scaled, etc etc.

Actually I was expecting a crash, or out of memory error, you will probably get that when you let it run for hours, but the Raspberry Pi kept being responsive for the couple of minutes I tried. Enough time to take a screenshot.

So it seems Ubuntu and Gnome are much more optimized for running on less powerful hardware in 2022 then a couple of years ago.

This funny showcase of the Droste-effect is the prove, and that is all a big win.

Please let me know what you think in the comments.

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Adding virtual columns in MySQL/MariaDB

June 22nd, 2022

A small MariaDB database with a simple table with just a couple of columns (id, name, `description` ) drives a PHP website. The table lacks a `slug` column.

You could still get away with spaces in an URL although it’s against the specs.

`” Doe”` will work if you parse “John Doe” to a variable and do a search in your database.

select * from `people` where `name` like 'John Doe' limit 1

But you can’t copy the url, it will lose Doe.

Spitting out a better url with a slug/url PHP function should output something like `””`

But then there is a problem in the database:

select * from `people` where `name` like 'john-doe' limit 1

doesn’t return anything.

So we need to add a `slug` column to the database, and why not try out a VIRTUAL column. A virtual column doesn’t exist in memory of disk, but is generated on the fly.

Can that work?

To sluggify a string in MariaDB, is a bit problematic, probably you should write you own custom function but I took the easy road. The REPLACE function which can only REPLACE one character a time, it doesn’t have the more powerful TRANSLATE function, but you can nest the REPLACE function so that should do the job.

Adding a virtual slug column to a Maria/MySQL database

Not really clean and concise but let’s try:

ALTER TABLE `users` add `slug` VARCHAR(50) AS (LOWER(REPLACE(REPLACE(REPLACE(`name`," ","-"),"(",""),")",""))) VIRTUAL;

Filtering out spaces, and brackets.

Doesn’t work. Doesn’t even execute.

Got this error in  PHPMyAdmin:


Static analysis:

3 errors were found during analysis.

 	A new statement was found, but no delimiter between it and the previous one. (near "replace" at position 52)

 	Unrecognized alter operation. (near "," at position 68)
 	Unrecognized alter operation. (near "" at position 0)


Took me some time to find out was wrong.


It’s a bug in PHPMyAdmin!

Executing the same command in a MySQL console worked perfectly fine.

A search on the internet gave me this issue, guess my PHPMyAdmin is too old on my local server.

Anyhow for the moment I’m testing a virtual `slug` column instead of a real one. Should save some MB on expensive disk-space.


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Setting several properties of dataset of any DOM element, writing to the dataset property

June 21st, 2022

One of the greatest additions in writing JS of the last years for writing clean code are arrow functions, classes and several ways of Destructuring assingment.

Arrow functions let you skip, several verbose terms like the function keyword, return keyword and {} character, while maintaining readability. Easy, once you mastered the new syntax!.

What is Destructuring assingment?

Let’s assume you fetch a stock json:

let json= {name:"SHELL PLC",isin:"GB00BP6MXD84",price:"25.00","volume":"2500000",time:"2022-06-22:09-06"}

They old way of dealing with this was to write everything out to assign values to variables:

var name =;
var isin= json.isin
var price = json.price;
var volume=json.volume;
var time = json.time;

Destructuring assingment let you do this more succinct:

let {name,isin,price,volume,time}=json

That is nice code.

Destructuring arrays, or csv rows:

let {name,isin,price,volume,time}=row.split(",")

Sure there is more on parsing cvs rows than just splitting on “,” but this is an example and you get the drift. When you totally control the csv file, using “,” only as separator, it should do the job.

Writing to the datatset property?

I dearly miss something for setting the dataset property of an element concisely.

The dataset read-only property of the HTMLElement interface provides read/write access to custom data attributes (data-*) on elements.

Note: The dataset property itself can be read, but not directly written. Instead, all writes must be to the individual properties within the dataset, which in turn represent the data attributes.

So you can’t simply do something like this:


You have to write it all out, and that is  verbose:

let elStock = document.querySelector("#stock");

Well there is a workaround.

Imitate writing to the dataset property directly, Object.assign()

According to the specs you can’t directly write the dataset, but you can do this:



<body data-name="SHELL PLC" data-isin="GB00BP6MXD84" data-price="25.00" data-volume="2500000" data-time="2022-06-22:09-06">

A piece of cake. That is as easy as writing the directly.

To extract dataset into variables destructuring assignment does work.

let {name,isin,price}=document.body.dataset

I used `Object.assign` in an older project for writing several properties to the style attribute, but it works as well for the dataset property.

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How to check the Signing Certificate on an Android app / apk (II)

June 17th, 2022

This is an 2022 update of an older post.

If you own a Android Phone, and you want to use Signal instead of Whatsapp or Telegram for privacy matters, and, for the same privacy matters, you prefer to use open source Android AOSP instead of the commercial Android variant that is enriched spoiled with proprietary Google services,  or you don’t have a Google Account on your phone, or you don’t use Google Play but the free F-Droid software-store, there is a solution. You can download the Signal APK from their website.

To verify that the signing certificate on the APK matches the SHA256 fingerprint on the Signal website you can use the following one-liner.

As Matthew (kudos) pointed out, the certification file has a new name in recent Signal APK’s.

So the one-liner changed a bit:

f="Signal-Android-website-prod-universal-release-5.40.4.apk" ; unzip -p "$f" $(unzip -l "$f" | grep '.RSA' | awk '{print $4}') | keytool -printcert

Hopefully this version will be future proof as the one-liner now uses his suggestion to search/grep for a .RSA file.


Owner: CN=Whisper Systems, OU=Research and Development, O=Whisper Systems, L=Pittsburgh, ST=PA, C=US
Issuer: CN=Whisper Systems, OU=Research and Development, O=Whisper Systems, L=Pittsburgh, ST=PA, C=US
Serial number: 4bfbebba
Valid from: Tue May 25 17:24:42 CEST 2010 until: Tue May 16 17:24:42 CEST 2045
Certificate fingerprints:
SHA1: 45:98:9D:C9:AD:87:28:C2:AA:9A:82:FA:55:50:3E:34:A8:87:93:74
SHA256: 29:F3:4E:5F:27:F2:11:B4:24:BC:5B:F9:D6:71:62:C0:EA:FB:A2:DA:35:AF:35:C1:64:16:FC:44:62:76:BA:26
Signature algorithm name: SHA1withRSA
Subject Public Key Algorithm: 1024-bit RSA key
Version: 3

As you can see, still the same fingerprint.


Kodi on Bullseye, playing 4K on the RPI4

March 18th, 2022

Getting the most out of your (cheap) hardware is always a challenge. Selling hardware is easier then supporting hardware.

The Raspberry Pi 4 has strong multimedia capabilities, it can playing 4K media 60fps, but getting it done isn’t a piece of cake. Even playing 1080HD content on YouTube can be a problem, but that’s probably because YouTube is more about tracking and selling adds than playing media.

A bold statement? Think about it this way. Any 1080p movie will play fine outside a browser in Kodi, any movie will play fine without DRM (Digital Rights Management). It chokes on the DRM en-/decryption. By definition: DRM is tracking.

Historically LibreElec is the best distribution for a Raspberry Pi as a multimedia device. LibreElec’s goal: just enough OS to support Kodi, and it is highly optimized for that.

Yes it’s powerful. You can install add-ons like MPD for music, or RetroPie-alike  for games. Also docker containers are available for HomeAssistant, MQTT and Nginx. So a RPI with LibreElec can be pretty powerful.

Still LibreElec is limited, and the RPI4 is capable of doing more.

Kodi on Raspberry used to work pretty well, until big changes in the 5.10 kernel came. It more or less stopped working on Buster. Compiling it yourself was quite a hassle, that actually failed more than it succeeded. I wrote about that before and it failed in most cases for most users including myself, I must admit.

But now luckily RPI-engineers stepped up:  Kodi in Rapsberry Pi OS is more or less supported again.

At least it’s easy again to install Kodi in Bullseye. But you still need a bit more tweaks to get it running smoothly.

To install Kodi in Bullseye

No OS can beat this 🙂 :

sudo apt install kodi

Install addons

Two important add-ons that can’t be installed from within Kodi like in LibreElec, but you have to resort to apt again:

TVHeadend-client (DVB-T tv)

sudo apt install kodi-pvr-hts

Only the client is installed with this command, this assumes a TV-Headend server is running on another local IP-address.

Inputstream-adaptive helper to play DRM protected (Widevine) content

sudo apt-get install kodi-inputstream-adaptive

The add-on then will extract (and update) the needed libs from internet automatically.

To enable HEVC HW decoding, you have to tweak /boot/config.txt

Add this line (only for RPI4)


For 4K HEVC playback tweak this line:

# Enable DRM VC4 V3D driver

Mount NFS shares

Somehow Kodi on Bullseye stopped discovering NFS by default. Don’t worry to much, just give it a little manual bump.

  • Choose Browse for new share -> Add network location -> Protocol -> Network File System (NFS)
  • Entering the your ip-address as Server address and Remote path manually, adding up to something like nfs://
  • Click OK, then it will list.
  • Select and click OK.


Changes in `/boot/config.txt` require a reboot.

After all these steps, playing 4K HEVC content with HW-acceleration should work fine on a Raspberry Pi 4.

That’s all. Let me know if it’s working for you.


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Powering a Pi Zero (2) from your laptop

November 12th, 2021

The new quad-core Pi Zero 2 has a lot more horsepower than the original Zero and as a consequence it does require more power than the original Zero.

But that doesn’t necessarily means that you cannot power it from an USB port from your laptop.

Can you still power a Raspberry Pi Zero from an USB port of a laptop?

Let’s try. Be reminded all Pi’s are cleverly designed to throttle down, when they experience a power shortage.

To check if your Pi has throttled down:

> vcgencmd get_throttled

If you see some other output then 0x0 yes then you’ve had power problems. Otherwise you’re OK.

For the moment, running Raspberry Pi OS Bullseye on a Raspberry Pi Zero 2 powered form an USB port doesn’t show problems by just installing programs and updating the OS.