Category: Tips

Scanning Lecture Notes – Separating Colors

Continuing my journey to prefect my scanned lecture notes, I’ll be reviewing my efforts for finding a good way to threshold scanned notes to black and white. I’ve spent several days experimenting with this stuff, and I think I’ve managed to improve on the basic methods used.

In the process of experimenting, I’ve come up with what I think are the 3 main hurdles into scanning notes (or text in general) to black and white.

  1. Bleeding. When using both sides of the paper the ink might be “bleed” through to the other side. Even if the ink doesn’t actually pass through, it might still be visible as kind of shadow, when scanning, just like when you hold a piece of paper in front of a light and you’re able to make out the text on the other side.
  2. Non-black ink. Photocopying blue ink, is notoriously messy. Scanning it to b&w, also imposes challenges.
  3. Skipping. This is an artifact that sometimes introduced when writing with a ballpoint pen. It’s a result of inconsistent ink flow, and is more rare with more liquid inks such as rollerballs or fountain pens.

Those issue can be visualized in the first three images. These images are the originals I’ve tested the various methods with. The other images are results of the various methods, explained in this post, and should convey the difference between them.
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Scanning Lecture Notes – Compression

A new semester is about to begin, hence I again set out to organize lecture notes and scan them. This time I intend to invest more time investigating and perfecting this process. Hopefully, I’ll present my conclusions in few posts, each focusing on a different aspect.

In the first post, I’ll discuss the various ways to compress the scanned lecture notes. Because lecture notes (at least mine) aren’t especially colorful has I only use one pen at the time, I want the result to be black and white (line art). This allows readable lecture notes in while preserving small size per page (as you can see in Some Tips on Scanning Lecture Notes).
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Sending Desktop Notification from Cron

Usually when one wants to keep track of one’s cron jobs, one tells the cron daemon to email the output of the commands. While this is probably the best solution for servers, on desktop machines is problematic. Many ISPs block outgoing traffic on port 25 (SMTP), and if you want to send the emails via external SMTP server (such as GMail) this requires you to store authentication details in plain text. A better solution for the desktop would be to harness the desktop notifications available in Ubuntu.

There is a useful tool called notify-send which is able to send desktop notifications directly from the command line. However, there are few caveats:

  • notify-send expects its input on the command line, it can’t read from stdin.
  • If you run from cron you must tell it which display to use.

The first issue can be worked around by using cat to pick up the input. The second issue is handled by adding a DISPLAY environment variable to the crontab. So your crontab will look something like this:

DISPLAY=:0
10 1 * * sun some_cool_command | notify-send "Backup Documents" "$(cat)"

The first argument to notify-send is the title of the notification. The second is the actual text to appear in it, in our case it’s whatever comes in the stdin. If you want to store the output in a log file as well as displaying it in a desktop notification, you can use tee, which basically saves its input to a given file and also pipes it again to stdout.

DISPLAY=:0
10 1 * * sun some_cool_command | tee -a ~/some_log.log | notify-send "Backup Documents" "$(cat)"

WiFi won’t connect on Samsung Galaxy S2 with CyanogenMod 9 (ICS 4.0.4)

After upgrading to CyanogenMod 9 (ICS 4.0.4) I’ve experience problems connecting to WiFi networks. Most of the time I’ve tried to connect to a WiFi network it would try to connect and then fail. Often it would also show the network I’ve just tried to connect to as “Not in range”. This was extremely irritating, as I could stand by my router and have the phone report that there is no signal, while another Galaxy S2 phone would connect immediately, the only difference was that the other phone was running Android 2.3.7. Needless to say, I haven’t made any major (or minor) changes to my WiFi equipment and the problem wasn’t confined just to my home. Sometimes, it would connect after being idle for a day or so.

Back when I was using CyanogenMod 7, I had no problems with the WiFi on my phone, and I could also replicate this behaviour on another Galaxy S2 I’ve upgraded to CyanogenMod 9. Initially I thought it was some bug in CyanogenMod, but I came up with nothing. Over the weekend, I’ve figured maybe the bug isn’t specific to CyanogenMod, and I’ve found this bug report WiFi freeze on Samsung Galaxy S2 with ICS 4.0.3. For some reason many people have problems with WiFi on Galaxy S2 with ICS. While the report relates to the WiFi freezing, which I didn’t experience, I’ve decided to sift through the comments. Some people mentioned the “Wi-Fi frequency band” feature available in Wi-Fi settings -> Advanced (available by pressing the menu button), so I’ve decided to try playing with the different settings there.

This setting allows you to choose the WiFi operating frequency between three options 5GHz, 2.4GHz and “Auto”, which as you can guess automatically figures out the right frequency, 5GHz (802.11a and 802.11n) and 2.4GHz(802.11b/g/n). However, it seems the phone (probably due to a problem in the driver) fails the “Auto” thing. Instead of guessing the right frequency, it fails to connect. So I’ve changed it to 2.4GHz, which is still more popular choice in routers over 5GHz, and instantly my phone connected to my router.

For conclusion: If you have trouble connecting to WiFi on Samsung Galaxy S2 with ICS, you should probably set the phone to use 2GHz only (or 5GHz only) instead of the default “Auto” mode.

Auto-Detect Dependencies when Building .debs Using CMake

CMake (via CPack) as a great feature that allows you to automatically generate Debian/Ubuntu (.deb) packages. One of the annoying things to do when you create a package is listing its dependencies. CMake asks you do it via the CPACK_DEBIAN_PACKAGE_DEPENDS variable. For example:

set (CPACK_DEBIAN_PACKAGE_DEPENDS "libc6 (>=2.7-18)")

But what happens when you work on a more complex project? Keeping track of all the dependencies by hand is a tedious task. Debian provides a tool named dpkg-shlibdeps which makes this task easier by updating the debian/control file with dependencies extracted from the dynamic libraries needed by a given executable. Luckily since CMake 2.8.3, CMake also supports running this tool automatically to figure out the required dependencies. The documentation is sparse, and I had hard time finding how to do so (I actually found it via a bug report and a commit message, but afterwards I’ve seen it the official documentation too). To enable it, you need to add the following line to your CMakeLists.txt file:

# autogenerate dependency information
set (CPACK_DEBIAN_PACKAGE_SHLIBDEPS ON)

Conditional Compilation in Autoconf and Automake

While working on my audio based random password generator (you view the source in github), I wanted to do some conditional compilation: Compiling certain parts of the program only in case some option is passed to the configure script. As it usually happens with GNU’s autotools, it kind of hell to do it. Documentation is spread across dozens of sources, each provides only a specific part of what to do. I’m writing it here in the blog, in hope I’ll never have to search how to do so again.
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Fixing virtualenv after Upgrading Your Distribution/Python

After you upgrade your python/distribution (specifically this happened to me after upgrading from Ubuntu 11.10 to 12.04), your existing virtualenv environments may stop working. This manifests itself by reporting that some modules are missing. For example when I tried to open a Django shell, it complained that urandom was missing from the os module. I guess almost any module will be broken.

Apparently, the solution is dead simple. Just re-create the virtualenv environment:

virtualenv /PATH/TO/EXISTING/ENVIRONMENT

or

virtualenv --system-site-packages /PATH/TO/EXISTING/ENVIRONMENT

(depending on how you created it in the same place). All the modules you’ve already installed should keep working as before (at least it was that way for me).

Bye Bye OmniCppComplete, Hello Clang Complete

For years OmniCppComplete has been the de facto standard for C++ completion in Vim. But as time progressed, I got more and more annoyed by it’s shortcomings. OmniCppComplete is based on tokenizing provided by ctags. The ctags parsing of C++ code is problematic, you can’t even run it on libstdc++ headers (you need to download modified headers). You want to use an external library? You’ll need to run ctags seperatly on each library. Not to mention it’s inablity to deduce types of anything more than trivial. The core of the problem is that OmniCppComplete isn’t a compiler and you can’t expect something that isn’t a compiler to fully understand code. This what makes Visual Studio’s IntelliSense so great: it uses the Visual C++ compiler for parsing, it isn’t making wild guess at types and what is the current scope – it knows it.
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Starting Djano upon Reboot using Cron

Three years ago I wrote about starting services as user via cron instead of init.d (specifically Trac). However this method has a serious downside: it has no support for dependencies. This really bothered me when I used cron to start a Django server, has it would attempt to load before MySQL was running. This made the cron useless, as always after rebooting I would receive an error email saying it couldn’t connect to the MySQL server, and I would have to login and start the Django Server manually. Yesterday I got sick of it, and decided to hack something that will work properly. So my crontab had the following line:

@reboot python ./manage.py runfcgi ...options...

which I changed into:

@reboot until /sbin/status mysql | grep start/running ; do echo "Mysql isn't running yet...>&2"; sleep 1; done; python ./manage.py runfcgi ...options...

Basically it loops and checks whether the MySQL service got started. If so, it would start Django as it did before. On the other hand, if MySQL isn’t running it would just sleep for a second and repeat the check.

A small issue is that if for some reason MySQL won’t start at all, it will loop forever. If this happens, it would mean that I’ll have to manually kill that cronjob, but I would have to login anyway to see what wrong with the MySQL. So, while this method can’t support dependencies like init.d does, it does provide a good-enough solution.

Update 2012-11-23: Fixed the crontab line (it would fail when mysql was in start/post-start state).