More Than Technical

Archive for the ‘linux’ Category

Hi
In the past few weeks I have been working hard at a few projects for end-of-term at Uni. One of the projects is what I called “SmartHome”, for Embedded computing [link] course, is a home monitoring [link] application. In the course the students were given an LPC2148 arm7-MCU (NXP) based education board, implemented by Embedded Artists [link]. My partner Gil and I decided to work with ZigBee extension modules [link] to enable remote communication.

Here are the steps we took to bring this project to life.
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As my search for the best platform to roll-out my new face detection concept continues, I decided to give ol’ Qt framework a go.

I like Qt. It’s cross-platform, a clear a nice API, straightforward, and remindes me somewhat of Apple’s Cocoa.

My intention is to get some serious face detection going on mobile devices. So that means either the iPhone, which so far did a crummy job performance-wise, or some other mobile device, preferably linux-based.
This led me to the decision to go with Qt. I believe you can get it to work on any linux-ish platform (limo, moblin, android), and since Nokia baught Trolltech – it’s gonna work on Nokia phones soon, awesome!

Lets get to the details, shall we?
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I recently had to build a demo client that shows short video messages for Ubuntu environment.
After checking out GTK+ I decided to go with the more natively OOP Qt toolbox (GTKmm didn’t look right to me), and I think i made the right choice.

So anyway, I have my video files encoded in some unknown format and I need my program to show them in a some widget. I went around looking for an exiting example, but i couldn’t find anything concrete, except for a good tip here that led me here for an example of using ffmpeg’s libavformat and libavcodec, but no end-to-end example including the Qt code.

The ffmpeg example was simple enough to just copy-paste into my project, but the whole painting over the widget’s canvas was not covered. Turns out painting video is not as simple as overriding paintEvent()…

Firstly, you need a separate thread for grabbing frames from the video file, because you won’t let the GUI event thread do that.
That makes sense, but when the frame-grabbing thread (I called VideoThread) actually grabbed a frame and inserted it somewhere in the memory, I needed to tell the GUI thread to take that buffered pixels and paint them over the widget’s canvas.

This is the moment where I praise Qt’s excellent Signals/Slots mechanism. So I’ll have my VideoThread emit a signal notifying some external entity that a new frame is in the buffer.
Here’s a little code:

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