Hi,
Fo rthe use-case, you can call createNvBuffer() an dmap the buffer to cv::Mat, please refer to the patch:
NVBuffer (FD) to opencv Mat - #6 by DaneLLL
And refer to the samples for calling createNvBuffer()/copyToNvBuffer():
/usr/src/jetson_multimedia_api/samples/09_camera_jpeg_capture/
/usr/src/jetson_multimedia_api/samples/13_multi_camera/