Building and installing OpenCV 2.3 on Ubuntu 11.04 -
1. Install the OpenCV dependencies by typing following commands.

sudo apt-get install build-essential libgtk2.0-dev libavcodec-dev libavformat-dev libjpeg62-dev libtiff4-dev cmake libswscale-dev libjasper-dev
To cover all the dependencies install -

sudo apt-get install build-essential
sudo apt-get install cmake
sudo apt-get install pkg-config
sudo apt-get install libpng12-0 libpng12-dev libpng++-dev libpng3
sudo apt-get install libpnglite-dev libpngwriter0-dev libpngwriter0c2
sudo apt-get install zlib1g-dbg zlib1g zlib1g-dev
sudo apt-get install libjasper-dev libjasper-runtime libjasper1
sudo apt-get install pngtools libtiff4-dev libtiff4 libtiffxx0c2 libtiff-tools
sudo apt-get install libjpeg8 libjpeg8-dev libjpeg8-dbg libjpeg-progs
sudo apt-get install ffmpeg libavcodec-dev libavcodec52 libavformat52 libavformat-dev
sudo apt-get install libgstreamer0.10-0-dbg libgstreamer0.10-0  libgstreamer0.10-dev
sudo apt-get install libxine1-ffmpeg  libxine-dev libxine1-bin
sudo apt-get install libunicap2 libunicap2-dev
sudo apt-get install libdc1394-22-dev libdc1394-22 libdc1394-utils
sudo apt-get install swig
sudo apt-get install libv4l-0 libv4l-dev
sudo apt-get install python-numpy

Further optional dependencies could be checked and installed from the configure option during building. For more info go to OpenCV site.

2. Download the OpenCV 2.3 source from here or type following command -
   wget http://sourceforge.net/projects/opencvlibrary/files/opencv-unix/2.3.1/OpenCV-2.3.1a.tar.bz2/

3. Extract the OpenCV tar file (preferably in your home directory)
    tar -xvf OpenCV-2.3.1a.tar.bz2

4. Now create the build directory for opencv
    $ cd OpenCV-2.3.1
    $ mkdir opencv.build
    $ cd opencv.build

5. Configure, make and install the code -
    $ cmake ..
    $ make
    $ sudo make install

6. Open the .bashrc file from your home folder
    $ gedit .bashrc
     and add the following at the end of the file -

    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/home//OpenCV-2.3.1/opencv.build/lib
    export PKG_CONFIG_PATH=/home//OpenCV-2.3.1/lib/opencv.build/pkgconfig
    alias gcv="g++ `pkg-config --cflags opencv` `pkg-config --libs opencv`"

7. Now test building the samples -
    $ cd /home//OpenCV-2.3.1/samples/c
    $ chmod a+x build_all.sh
    $ ./build_all.sh
   If the samples are successfully compiled, then your opencv is ready to use.

8. Since we added an alias in the bashrc file named "gcv", you can compile any opencv code just by using this alias as -
    $ gcv hellocv.cpp