You can probably use VLC to play RTSP streams in a Windows application. This is discussed here:
As far as I know, VLC doesn't integrate easily with DirectShow, so I'm not sure if it will be easy to get the video buffers to process them yourself.
If you can't find a free solution and want to consider a commercial library, I recommend going with an SDK that has a free evaluation such as LEADTOOLS Multimedia (https://www.leadtools.com/sdk/engine/multimedia). The toolkit has support for streaming, playback,
saving to multiple formats and buffer processing, among other features. Please note I work for the company that developed this library.