Mix.install([
{:kino, "~> 0.13.1"},
{:ex_webrtc, "~> 0.3.0"},
{:req, "~> 0.5"}
])We illustrate the SFU server ExWebRTC by broadcasting our webcam via WebRTC.
ExWebRTC is an Elixir port of the WebRTC API.
This is a low latency protocole running on UDP.
We are not using the "standard" peer-to-peer WebRTC connection but we are connecting the input feed - the browser's built-in webcam - to an SFU server written in Elixir. The SFU will forward the signal to other connected clients.
We transform the input directly in the browser. Since it is mandatory to start with the "hello world" of computer vision, namely face detection, we showcase two libraries:
-
the library
face-api. -
the library
MediaPipefrom Tensorflow.js.The
mediaPipelibrary gives excellent results.
The transformed stream will be sent to the SFU server via WebRTC.
The Elixir server will broadcast it back, into another <video> element of our page in this demo.
You can run this first Livebook which uses face-api. You will notice that the results are not so good. You have a lot of negative findings.
This second Livebook uses MediaPipe. This is much more performant. Recall that each frame (30 fps) are detected and rebuilt and broadcasted to the Elixir server to broadcast back in the video element below the input.
The webcam is captured and displayed in a first <video> element.
The browser and the Elixir SFU will establish a WebRTC PeerConnection via a signaling channel: the Livebook WebSocket.
They will exchange SDP and ICE candidates.
Once connected, the Elixir SFU will receive the streams from the <video 1> via a UDP connection. In this Echo configuration, it will send back the streams to the connected peer via UDP and display them in the other <video 2> element.
The "magic" lines:
def handle_info(
{:ex_webrtc, pc, {:rtp, client_track_id, _rid, packet}},
%{client_video_track: %{id: client_track_id, kind: :video}} = state
) do
PeerConnection.send_rtp(pc, state.serv_video_track.id, packet)
{:noreply, state}
end- The signaling process (instantiate the PeerConnection):
graph LR;
B1[Browser <br> PeerConnection]-- ws-->L[Livebook];
L-- ws -->Ex[Elixir SFU <br> PeerConnection];
Ex -- ws --> L
L -- ws --> B1
- The active connection:
graph LR;
B3[video 1]--stream <br> UDP -->ExSFU[Elixir SFU]
ExSFU --stream <br> UPD-->B4[video 2];
defmodule RtcServer do
use GenServer
alias ExWebRTC.{ICECandidate, PeerConnection, SessionDescription, MediaStreamTrack, RTPCodecParameters}
alias ExWebRTC.RTP.VP8.Depayloader
require Logger
defp ice_servers(), do: [%{urls: "stun:stun.l.google.com:19302"}]
defp video_codecs, do: [
%RTPCodecParameters{
payload_type: 96,
mime_type: "video/VP8",
clock_rate: 90_000
}
]
defp setup_transceivers(pc) do
media_stream_id = MediaStreamTrack.generate_stream_id()
video_track = MediaStreamTrack.new(:video, [media_stream_id])
{:ok, _sender} = PeerConnection.add_track(pc, video_track)
%{serv_video_track: video_track}
end
# public ----------------
def start_link(args), do: GenServer.start_link(__MODULE__, args, name: __MODULE__)
def connect, do: GenServer.call(__MODULE__, :connect)
def receive_signaling_msg({msg, sender}) do
GenServer.cast(__MODULE__, {:receive_signaling_msg, msg, sender})
end
def peek_state, do: GenServer.call(__MODULE__, :peek_state)
# callbacks-------------------------
@impl true
def init(_args) do
Logger.info("ExRtc PeerConnection started")
{:ok, %{
sender: nil,
pc: nil,
client_video_track: nil,
video_depayloader: Depayloader.new(),
i: 1,
t: System.monotonic_time(:microsecond)
}}
end
# Livebook calls
@impl true
def handle_call(:peek_state, _, state) do
{:reply, state, state}
end
def handle_call(:connect, _, state) do
{:ok, pc_pid} =
PeerConnection.start_link(ice_servers: ice_servers(), video_codecs: video_codecs())
state = %{state | pc: pc_pid} |> Map.merge(setup_transceivers(pc_pid))
{:reply, :connected, state}
end
# messages received from the client
@impl true
def handle_cast({:receive_signaling_msg, %{"type"=> "offer"} = msg, sender},state) do
with desc <-
SessionDescription.from_json(msg),
:ok <-
PeerConnection.set_remote_description(state.pc, desc),
{:ok, answer} <-
PeerConnection.create_answer(state.pc),
:ok <-
PeerConnection.set_local_description(state.pc, answer),
:ok <-
gather_candidates(state.pc) do
# the 'answer' is formatted into a struct, which can't be read by the JS client
answer = %{"type" => "answer", "sdp" => answer.sdp}
send(sender, {:signaling, answer})
Logger.warning("--> Server sends Answer to remote")
{:noreply, %{state | sender: sender}}
else
error ->
Logger.error("Server: Error creating answer: #{inspect(error)}")
{:stop, :shutdown, state}
end
end
def handle_cast({:receive_signaling_msg, %{"type"=> "ice"} = msg, _sender}, state) do
candidate = ICECandidate.from_json(msg["candidate"])
:ok = PeerConnection.add_ice_candidate(state.pc, candidate)
Logger.debug("--> Server processes remote ICE")
{:noreply, state}
end
def handle_cast({:receive_signaling_msg, {msg, _}}, state) do
Logger.warning("Server: unexpected msg: #{inspect(msg)}")
{:stop, :shutdown, state}
end
@impl true
def handle_info({:ex_webrtc, _pc, {:track, %{kind: :video} = client_video_track}}, state) do
{:noreply, %{state | client_video_track: client_video_track}}
end
# internal messages --------
def handle_info({:ex_webrtc, _pc, {:ice_candidate, candidate}}, state) do
candidate = ICECandidate.to_json(candidate)
send(state.sender, {:signaling, %{"type"=> "ice", "candidate" => candidate}})
Logger.debug("--> Server sends ICE to remote")
{:noreply, state}
end
def handle_info(
{:ex_webrtc, pc, {:rtp, client_track_id, _rid, packet}},
%{client_video_track: %{id: client_track_id, kind: :video}} = state
) do
PeerConnection.send_rtp(pc, state.serv_video_track.id, packet)
{:noreply, state}
end
def handle_info({:ex_webrtc, pc, {:connection_state_change, :connected}}, state) do
PeerConnection.get_transceivers(pc)
|> Enum.find(&(&1.kind == :video))
|> then(fn %{receiver: receiver} ->
Logger.warning("PeerConnection successfully connected, using #{inspect(receiver.codec.mime_type)}")
end)
{:noreply, state}
end
def handle_info({:ex_webrtc, _pc, _msg}, state) do
{:noreply, state}
end
defp gather_candidates(pc) do
receive do
{:ex_webrtc, ^pc, {:ice_gathering_state_change, :complete}} -> :ok
after
1000 -> {:error, :timeout}
end
end
endWe will start the RTC GenServer.
Supervisor.start_link([RtcServer], strategy: :one_for_one)We instantiate a new PeerConnection:
:connected = RtcServer.connect()This will instantiate a ExWebRTC.PeerConnection server-side.
It is waiting for a peer to connect and receive its offer. It will respond with an "answer", digest the peer's Ice candidates, and send to the peer its own ICE candidates.
RtcServer.peek_state()On connection, a new PeerConnection is created. Since the server is already connected, the client will send an "offer" via the signaling channel. The client expects an "answer" back. It will also send "Ice" candidates via the signaling channel, and expects to receive Ice candidates from the server.
Once the connection is set, the client will receive RTP packets and digest them into a <video> element.
We use a Kino.JS.Live since we obviously need a signaling channel (the Live WebSocket).
Kino expects us to code a "main.js" module that exports and
initfunction
Kino does not load files from an URL, but from a location. We supply the following helper module to make the Livebook work from GitHub, and clean the code.
defmodule Assets do
def fetch_js do
github_js_url = "https://raw.githubusercontent.com/dwyl/WebRTC-SFU-demo/main/lib/assets/main_mediapipe.js"
Req.get!(github_js_url).body
end
def fetch_html do
github_html_url = "https://raw.githubusercontent.com/dwyl/WebRTC-SFU-demo/main/lib/assets/index.html"
Req.get!(github_html_url).body
end
endThe Kino.JS.Live module:
defmodule VideoLive do
# GenServer to communicate between browser and Livebook server
use Kino.JS
use Kino.JS.Live
require Logger
@html Assets.fetch_html()
asset "main.js" do
Assets.fetch_js()
end
def run() do
Kino.JS.Live.new(__MODULE__, @html)
end
@impl true
def init(html, ctx) do
{:ok, assign(ctx, html: html)}
end
@impl true
def handle_connect(ctx) do
{:ok, ctx.assigns.html, ctx}
end
# received from the browser via the signaling WebSocket, call server
@impl true
def handle_event("offer",%{"sdp" => sdp}, ctx) do
RtcServer.receive_signaling_msg({sdp, self()})
{:noreply, ctx}
end
def handle_event("ice",%{"candidate" => candidate}, ctx) do
RtcServer.receive_signaling_msg({%{"type" => "ice", "candidate" => candidate}, self()})
{:noreply, ctx}
end
# received from the server, send to the browser via signaling WebSocket
@impl true
def handle_info({:signaling, %{"type" => "answer"} = msg}, ctx) do
broadcast_event(ctx, "answer", msg)
{:noreply, ctx}
end
def handle_info({:signaling, %{"type" => "ice"} = msg}, ctx) do
if msg["candidate"], do: broadcast_event(ctx, "ice",msg)
{:noreply, ctx}
end
endVideoLive.run()The PeerConnection server-side is now populated:
RtcServer.peek_state()It relies on the method requestVideoFrameCallback.
You can transform each frame of a video stream, at the rate of the video, namely 30fps.
We use this transformed stream and add it to the PeerConnection track. Et voilà.
The mediaPipe library uses `
import {
FaceDetector,
FilesetResolver,
} from "https://cdn.jsdelivr.net/npm/@mediapipe/[email protected]";
export async function init(ctx, html) {
ctx.importCSS("main.css");
ctx.root.innerHTML = html;
async function run() {
console.log("Starting.....");
const videoIn = document.getElementById("source"),
display = { width: videoIn.width, height: videoIn.height },
canvas = document.createElement("canvas"),
context = canvas.getContext("2d"),
stream = await window.navigator.mediaDevices.getUserMedia({
video: display,
audio: false,
});
videoIn.srcObject = stream;
await videoIn.play();
// -------------------mediaPipe-api ------------------
canvas.height = display.height;
canvas.width = display.width;
let faceDetector;
// Loads the MediaPipe Face Detector model and begins detecting faces in the input video.
const initializeFaceDetector = async () => {
const vision = await FilesetResolver.forVisionTasks(
"https://cdn.jsdelivr.net/npm/@mediapipe/[email protected]/wasm"
);
faceDetector = await FaceDetector.createFromOptions(vision, {
baseOptions: {
modelAssetPath: `https://storage.googleapis.com/mediapipe-models/face_detector/blaze_face_short_range/float16/1/blaze_face_short_range.tflite`,
delegate: "GPU",
},
runningMode: "VIDEO",
});
await predictWebcam();
};
async function predictWebcam() {
const detections = await faceDetector.detectForVideo(
videoIn,
performance.now()
);
displayVideoDetections(detections.detections);
window.requestVideoFrameCallback(predictWebcam);
}
function displayVideoDetections(detections) {
context.clearRect(0, 0, display.width, display.height);
context.drawImage(videoIn, 0, 0, display.width, display.height);
detections.forEach((detection) => {
const bbox = detection.boundingBox;
context.beginPath();
context.rect(bbox.originX, bbox.originY, bbox.width, bbox.height);
context.lineWidth = 2;
context.strokeStyle = "blue";
context.stroke();
detection.keypoints.forEach((keypoint) => {
context.beginPath();
context.arc(keypoint.x, keypoint.y, 3, 0, 2 * Math.PI);
context.fillStyle = "red";
context.fill();
});
});
}
await initializeFaceDetector();
videoIn.requestVideoFrameCallback(predictWebcam);
const transformedStream = canvas.captureStream(30);
//----------------------- WEBRTC-----------------------------
const iceConf = { iceServers: [{ urls: "stun:stun.l.google.com:19302" }] };
const pc = new RTCPeerConnection(iceConf);
// capture local MediaStream (from the webcam)
const tracks = transformedStream.getTracks();
tracks.forEach((track) => pc.addTrack(track, transformedStream));
// send offer to any peer connected on the signaling channel
pc.onicecandidate = ({ candidate }) => {
if (candidate === null) {
return;
}
ctx.pushEvent("ice", { candidate: candidate.toJSON(), type: "ice" });
};
// send offer to any peer connected on the signaling channel
pc.onnegotiationneeded = async () => {
const offer = await pc.createOffer();
await pc.setLocalDescription(offer);
console.log("--> Offer created and sent");
ctx.pushEvent("offer", { sdp: offer });
};
// received from the remote peer (Elixir SFU server here) via UDP
pc.ontrack = ({ streams }) => {
console.log("--> Received remote track");
const echo = document.querySelector("#echo");
echo.srcObject = streams[0];
};
// received from the remote peer via signaling channel (Elixir server)
ctx.handleEvent("ice", async ({ candidate }) => {
await pc.addIceCandidate(candidate);
});
ctx.handleEvent("answer", async (msg) => {
console.log("--> handled Answer");
await pc.setRemoteDescription(msg);
});
// internal WebRTC listener, for information or other action...
pc.onconnectionstatechange = () => {
console.log("~~> Connection state: ", pc.connectionState);
};
}
run();
}