OpenCV不会从RTMP源捕获帧，而FFmpeg会捕获帧

my goal is to capture a frame from a rtmp stream every second, and process it using OpenCV. I'm using FFmpeg version N-71899-g6ef3426 and OpenCV 2.4.9 with the Java interface (but I'm first experimenting with Python). For the moment, I can only take the simple and dirty solution, which is to capture images using FFmpeg, store them in disk, and then read those images from my OpenCV program. This is the FFmpeg command I'm using:my goal is to capture a frame from a rtmp strea