USB camera encoded data stream - c++

Question

Currently, I am working on a streaming project. I need to grab frames from USB camera and send them over TCP.
To open USB camera video stream I'm using cv::VideoCapture. This allows me to read already decoded frames. According to this question I understood that there is no way to get encoded frame data using cv::VideoCapture and I need to encode each frame again and send it whatever I need using cv::imencode. The problem is that I can encode frames to some specific format listed here, and, in case, I use either .jpg or .png the file size still quite big and on receiving side frame rate very poor.
My question is: Is there any way to get mjpeg or h264 encoded data directly
or maybe you can suggest a better way to encode frames.
OpenCV 3.4.3, camera RICOH THETA V, language C++.
My code:
void Streamer::start()
{
cv::Mat img;
cv::VideoCapture cap(0);
cap.set(CV_CAP_PROP_FOURCC, CV_FOURCC('H', '2', '6', '4'));
if (!cap.isOpened())
throw std::invalid_argument("No device found.");
std::vector<int> format_params;
format_params.push_back(CV_LOAD_IMAGE_COLOR);
format_params.push_back(CV_IMWRITE_PNG_STRATEGY);
for (;;)
{
cap.read(img);
cv::imencode(".png", img, buffer_, format_params);
std::string strbuf(buffer_.begin(), buffer_.end());
server_->sendString(socket, strbuf);
}
cap.release();
}