how much knowledge of C++ I need to start reading opencv book ??

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OpenCV is getting easier and easier in each version. In the past you wouldn't be able to write efficient opencv programs without a fair knowledge about pointers, memory allocation and release and other rather complicated stuff. Right now, opencv handles most complicated low level stuff and allows you to just keep thinking about the actual computer vision.

However, it is important to have notion about variables, loops, data types, functions, classes and the rest of the basic stuff, therefore I recommend you read a C++ for beginners book before diving into OpenCV programming.

Good luck!