How to convert images to PDF with paper and image size, without ImageMagick

ImageMagick’s convert tool is handy for converting a series of images into a PDF. Just for future reference, here is one method how you can achieve the same without convert. It is useful if you have 1-bit PBM images (e.g. scanned text) at hand:

This command concatenates all .pbm files, pipes the data to pnmtops to create an […]

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Digitizing books 
(International Dunhuang Project, CC BY-SA 3.0)

Digitize books: Searchable OCR PDF with text overlay from scanned or photographed books on Linux

Here is my method to digitize books. It is a tutorial about how to produce searchable, OCR (Optical Character Recognition) PDFs from a hardcopy book using free software tools on Linux distributions. You probably can find more convenient proprietary software, but that’s not the objective of this post. Important: I should not need to mention that depending […]

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'Hashed' brown potatoes. Hashing is important on more than just one level (picture by Jamie Davids, CC-BY-2.0)

Hashing passwords: SHA-512 can be stronger than bcrypt (by doing more rounds)

On a server, user passwords are usually stored in a cryptographically secure way, by running the plain passwords through a one-way hashing function and storing its output instead. A good hash function is irreversible. Leaving dictionary attacks aside and by using salts, the only way to find the original input/password which generated its hash, is to simply try all possible […]

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100% HTTPS in the internet? Non-Profit makes it possible!

HTTPS on 100% of websites in the internet? This just has gotten a lot easier! Let’s Encrypt is a free, automated, and open certificate authority (CA), run for the public’s benefit. Let’s Encrypt is a service provided by the Internet Security Research Group (ISRG), a Section 501(c)(3) Non-Profit entity dedicated to reduce financial, technological, and education barriers to […]

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DIY Piezo-Electric Touch Probe with high sensitivity

I built this mechanical piezo-electric touch probe recently as an extension to a CNC machine to sense the depth of surfaces. It works with the grbl CNC controller and probably others. The goal was to flip a TTL (5V) signal whenever the probe was touching a surface mechanically. Unlike classical mechanical switches which have moving parts, this probe is ‘solid state’ and […]

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Simple OpenGL output using pyglpainter library

OpenGL programming in Python: pyglpainter

This was a recent hobby programming project of mine for use in a CNC application, using Python and OpenGL. The source code is available at https://github.com/michaelfranzl/pyglpainter . This Python module provides the class PainterWidget, extending PyQt5’s QGLWidget class with boilerplate code neccessary for applications which want to build a classical orthagnoal 3D world in which the user […]

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Unprivileged Unix Users vs. Untrusted Unix Users. How to harden your server security by confining shell users into a minimal jail

As a server administrator, I recently discovered a severe oversight of mine, one that was so big that I didn’t consciously see it for years. What can Unprivileged Unix Users do on your server? Any so-called “unprivileged Unix users” who have SSH access to a server (be it simply for the purpose of rsync’ing files) is not really “unprivileged” as the […]

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The Bosch BMP180 on an Arduino UNO board

Infrasound Barograph with FFT spectrum analysis: Arduino UNO + Bosch BMP180 Sensor + Qt5 + HTML5

Infrasound sources are ubiquitous in our environment, but humans cannot hear them. In recent years, there has been rising interest and collaboration in infrasound monitoring, for reasons from health concerns regarding large wind turbines to detection of atmospheric nuclear bomb testing, not to mention that the Chelyabinsk meteor of Feb 2013 which created intense shock waves of about 10 […]

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