To use OpenCV with CodeBlocks, you need to tell CodeBlocks:

1. The directory which contains the OpenCV header files → c:\opencv\build\include
2. List of OpenCV libraries needed for linking → c:\opencv\build\x86\mingw\libs\libopencv_*.dll.a

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1. Add the OpenCV header files directory

Open Settings → Compiler and debugger... → Search directories tab → Compiler tab

Click add button for adding a new entry. In the popup dialog, type c:\opencv\build\include,

and click Ok.

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2. Add the OpenCV libraries needed for linking

Open Settings → Compiler and debugger... → Linker settings tab.

Click add for adding new entries and open a popup dialog.

Click the "..." button to open the File Open Dialog. Go to c:\opencv\build\x86\mingw\lib directory and select all files by pressing Ctrl-A.

Click Open to add the files,

Click Ok to save all settings.

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Now that you've set the include directory and the libraries for linking, you can compile and run your project by pressing F9 key.