OpenCV 4.0 is the evolution of OpenCV 3.0. We follow the same design principles and the same library layout as in OpenCV 3.0. So most of the code and most of the scripts should work fine, with a few notable exceptions:

• Most of the C API has been excluded. In particular, instead of cv::cvtColor(src, dst, CV_RGB2GRAY) one should use cv::cvtColor(src, dst, cv::COLOR_RGB2GRAY); instead of cv::VideoCapture cap(0); cap.set(CV_CAP_PROP_WIDTH, 640); one should use cv::VideoCapture cap(0); cap.set(cv::CAP_PROP_WIDTH, 640); etc. Also, the classical C data structures, such as CvMat, IplImage, CvMemStorage etc., as well as the corresponding functions, such as cvCreateMat(), cvThreshold() etc., are mostly excluded from API and will be completely excluded in further OpenCV 4.x updates. Please, replace them with the corresponding C++ structures and functions, like cv::Mat, std::vector, cv::threshold() etc.

The new engine for constructing efficient image processing has been introduced. See Graph API (G-API) for details.