Feb 17

Augmented Reality on libQGLViewer and OpenCV-OpenGL tips [w/code]

You already know I love libQGLViewer. So here a snippet on how to do AR in a QGLViewer widget. It only requires a couple of tweaks/overloads to the plain vanilla widget setup (using the matrices properly, disable the mouse binding) and it works.
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Feb 07

Simplest 20-lines OpenCV video stabilizer [w/ code]

Just sharing a simple recipe for a video stabilizer in OpenCV based on goodFeaturesToTrack() and calcOpticalFlowPyrLK().
Well... it's a bit more than 20 lines, but it is short. And it doesn't work for every kind of video (although the results are funny anyway! :).
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Feb 05

Using Hidden Markov Models for staff line removal (in OMR) [w/code]

So lately I'm into Optical Music Recognition (OMR), and a central part of that is doing staff line removal. That is when you get rid of the staff lines that obscure the musical symbols to make recognition much easier. There are a lot of ways to do it, but I'm going to share with you how I did it (fairly easily) with Hidden Markov Models (HMMs), which will also teach us a good lesson on this wonderfully useful approach.

OMR has been around for ages, and if you're interested in learning about it [Fornes 2014] and [Rebelo 2012] are good summary articles.
The matter of Staff Line Removal has occupied dozens of researchers for as long as OMR exists; [Dalitz 2008] give a good overview. Basically the goal is to remove the staff lines that obscure the musical symbols, so they would be easier to recognize.
Screen Shot 2015-01-24 at 10.11.00 PM
But, the staff lines are connected to the symbols, so simply removing them will cut up the symbols and make them hardly recognizable.
So let's see how we could do this with HMMs.
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Jan 21

Run length encoding in OpenCV [w/code]

RLE exampleSharing a simple code snippet for run-length encoding with OpenCV...
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Jan 14

Simplest Color Balance with OpenCV [w/code]

Color balanceI came across an extremely simple color balancing algorithm here. And I thought I'll quickly transcode it to OpenCV.
Here's the gist:
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Dec 31

New Year, New Look

Hi Everybody

Another look at our blog, made us think: Why do we still look so 90's?
This is when we decided to do some cosmetic and functional changes:
We changed the theme (We though about buying a wordpress theme, but for some reason, they're way too expensive),
We installed a new commend system called "Disqus"
We have a new logo,
And we generally want to make your stay more comfortable.
We hope you like it.


Roy and Arnon


Oct 13

Touch up your sound with SoundTouch [w/code]

Screen Shot 2014-10-13 at 11.55.42 AM
So I needed to speed up / slow down an audio stream I had (speech generated with Flite TTS) and naively I thought it would suffice to simply sample it at the right intervals and interpolate.
I quickly discovered that just re-sampling won't do because changing frequency also changes pitch proportionally. And then I discovered the world of Time Scaling in audio and it's many algorithms and approaches to change the tempo without changing pitch.
To my surprise there were a number of ready made free libraries that do it, but the first one I tried - RubberBand - did not work out, it had too many dependencies I simply couldn't be bothered compiling it for the Mac. But SoundTouch, well it had a Homebrew formula so it won by default.
I wrote a little simple wrapper around it, that interfaces nicely with Qt.
Let's see what's going on there
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Sep 30

FFMpeg with Lame MP3 and streaming for the Arduino Yun

So, I've been trying to stream audio off of a USB microphone connected to an Arduino Yun.
Looking into it online I found some examples using ffserver & ffmpeg, which sounded like they could do the trick.
However right from the start I've had many problems with playing the streams on Android and iOS devices.
Seems Android likes a certain list of codecs (http://developer.android.com/guide/appendix/media-formats.html) and iOS like a different set of codecs (Link here), but they do have on codec in common - good ol' MP3.
Unfortunately, the OpenWRT on the Arduino Yun has an ffmpeg build which does not provide MP3 encoding... it does have the MP3 muxer/container format, but streaming anything other then MP3 in it (for example MP2, which the Yun-ffmpeg does have) simply doesn't work on the Android/iOS.
From experiments streaming from my PC a ffmpeg/libmp3lame MP3 stream, it looks like the mobile devices are quite happy with it - so I will need to recompile ffmpeg with Lame MP3 support to be able to stream it.
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Jul 23

Small footprint Bluetooth-enabled Arduino clones

Just a bit of online research on a small footprint Bluetooth-enabled Arduino clone for prototyping:


Jul 01

Push a file to your Smartphone

This is a quick tip. If you need to push a notification to your smartphone, or even send a file, I recently stumbled upon "Pushbullet".
I was looking for a solution to send a file (with automatic download) to my phone, via bash script on my raspberry pi. I've been using pushover for a while, but as far as I know, it doesn't support files

I have read the Pushbullet API, and came to realise that sending a file have 3 steps

1. Getting credentials to send the file
2. Upload the file to a remote server
3. Send a notification of "file" type with the download url

Of course, you can do 1 and 2 youself if you have access to a place which stores the file (I also managed to send a dropbox download link)

You need to sign up for Pushbullet via web, to get the api key, and also install the app on your smartphone (obviously)

So here is the bash script (It's not elegant - but it works)
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