How to create a simple server to transmit images and text

[prashant-saxena]

Hello,
libdatachannel 0.23.0
I am completely new to webRTC and other related stuff and trying to create a simple server in c++ and a client in python.
This is my first try and it's not working at all and I have no idea why. If someone point the right direction then it would be really helpful.

#include <iostream>
#include <opencv2/opencv.hpp>
#include <rtc/rtc.hpp>

int main() {
    cv::VideoCapture cap("output.mp4");
    if (!cap.isOpened()) {
        std::cerr << "Error opening video.\n";
        return 1;
    }

    rtc::InitLogger(rtc::LogLevel::Info);
    
    // Create peer connection
    rtc::Configuration config;
    auto pc = std::make_shared<rtc::PeerConnection>(config);

    // Create a data channel
    auto dc = pc->createDataChannel("video");

    // Handle command messages
    dc->onMessage([dc](rtc::message_variant msg) {
        std::string cmd = std::get<std::string>(msg);
        std::cout << "Command received: " << cmd << "\n";

        // Optional: process commands like "rgb2bgr" etc.
        dc->send("ok");
    });

    // Create and print offer
    pc->onLocalDescription([](const rtc::Description& desc) {
        std::cout << "\n--- SDP OFFER ---\n" << std::string(desc) << "\n";
    });

    pc->onGatheringStateChange([pc](rtc::PeerConnection::GatheringState state) {
        if (state == rtc::PeerConnection::GatheringState::Complete) {
            std::cout << "\nPaste the client's SDP answer:\n";
        }
    });

    auto offer = pc->createOffer();
    // Print type info to confirm
    std::cout << "Type: " << static_cast<int>(offer.type()) << "\n";
    std::cout << "SDP:\n" << std::string(offer) << "\n";
    pc->setLocalDescription(offer);  // ✅ THIS IS THE RIGHT WAY

    std::string answer;
    std::getline(std::cin, answer);
    pc->setRemoteDescription(rtc::Description(answer, "answer"));

    // Send loop
    while (true) {
        cv::Mat frame;
        if (!cap.read(frame)) break;

        std::vector<uchar> buf;
        cv::imencode(".jpg", frame, buf);

        if (dc->isOpen())
            dc->send(std::string(buf.begin(), buf.end()));

        std::this_thread::sleep_for(std::chrono::milliseconds(33)); // ~30 FPS
    }

    return 0;
}

[paullouisageneau]

Please be more descriptive than "it's not working", otherwise I can only guess what your problem is.

    auto offer = pc->createOffer();
    // Print type info to confirm
    std::cout << "Type: " << static_cast<int>(offer.type()) << "\n";
    std::cout << "SDP:\n" << std::string(offer) << "\n";
    pc->setLocalDescription(offer);  // ✅ THIS IS THE RIGHT WAY

This is not the right way. Where did you get this code from? Does it even compile? The setLocalDescription() method does not expect a description but an optional type. Calling createOffer() is useless. You should remove this whole paragraph, as createDataChannel() will automatically initiate the negotiation by default.

Then you have to fix the whole logic: the local description should be printed on GatheringState::Complete, then you must wait for the answer. You can remove the onLocalDescription callback.

As a side node, sending JPEG images over a data channel is a very poor way to send a real-time video. You should instead encode it as H264 and send it over a media track.