We have an OpenCV problem of opening (and writing) file paths that contain non-Ascii characters on Windows. I saw questions OpenCV imread with foreign characters and imread
The way to do this without hacking the OpenCV source code is to use _wfopen (as Remy suggested) to read the whole file into a memory buffer. Then use OpenCV's function imdecode to create a cv::Mat from that buffer.
You can do the reverse too, if necessary - i.e. use imencode to write an image to a memory buffer, then use _wfopen to open a file with a UNICODE name and write the buffer to it (alternatively, you could just imwrite to a temporary file and then move/rename it using the appropriate API function).