Monday, August 22, 2011

OpenCV and a webcam stream: a fail and a win

I managed to compile OpenCV with FFmpeg support for the Chumby. Unfortunately this did not solve my problem, because FFmpeg is not able to decode the MJPEG stream from my web camera via mjpg_streamer. I've found an another solution that works, though! More on that in the end of this post.

OpenCV with FFmpeg does work with video files (I tested with an AVI DIVX file) and might work with the stream generated by some other web camera, so I'm including some notes on how to repeat the build procedure:

  • I got the latest FFmpeg source code using git clone git:// This was perhaps a mistake, because it turned out that compatibility with OpenCV 2.2.0 had been broken at some point. Fortunately I found patches that fixed this issue in the OpenCV source code.
  • FFmpeg had to be configured with the --enable-shared option. I used the command 
~/chumby-sw/OpenCV-2.2.0/release$ sb2 ./configure --prefix=/home/<my-username>/chumby-buildroot/usr --enable-shared
  • I had to apply another patch to OpenCV-2.2.0 source code to fix some linker errors (../../lib/ undefined reference to `cv::SIFT::SIFT(double, bool, bool, int, int, int, int)' etc.)
  • I used the following commands to configure and build OpenCV:
~/chumby-sw/OpenCV-2.2.0/release$ sb2 cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/home/<my-username>/chumby-buildroot/usr -D BUILD_PYTHON_SUPPORT=OFF -D BUILD_NEW_PYTHON_SUPPORT=OFF .. 
~/chumby-sw/OpenCV-2.2.0/release$ sb2 make
The working solution I mentioned above is simply to decode the motion-JPEG stream generated by mjpg_streamer manually and use some functions from libjpeg together with OpenCV's cvCreateImageHeader and cvCreateData to convert the individual JPG images into OpenCV's IplImages. I found some sample code that I could easily modify to suit my purposes.

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