Input Data Channels

CloudGaming uses dedicated WebRTC data channels to transmit user input from the client to the host with minimal latency. Each input type uses an optimized channel configuration based on its reliability requirements.

Channel Configuration

keyPressChannel (Keyboard Input)

The keyboard channel uses ordered, reliable delivery to ensure no key events are lost or received out of order.

// Client: Create keyboard channel
dataChannel = peerConnection.createDataChannel("keyPressChannel", { 
  ordered: true  // Maintain event order
});

Source: Client/html-server/index.html:1369 Why ordered and reliable?

Key sequences must be preserved (e.g., Ctrl+C)
Missing key events cause stuck keys
Order matters for text input and shortcuts

mouseChannel (Mouse Input)

The mouse channel uses unordered, unreliable delivery with no retransmissions for lowest possible latency.

// Client: Create mouse channel with maxRetransmits=0
mouseChannel = peerConnection.createDataChannel("mouseChannel", { 
  ordered: false,       // Order not critical
  maxRetransmits: 0     // Never retransmit lost packets
});

Source: Client/html-server/index.html:1370 Why unordered and unreliable?

Mouse movements are rapidly superseded by newer positions
Retransmitting old mouse positions causes lag
Missing one mouse event is imperceptible
Immediate delivery is more important than reliability

Message Schemas

Keyboard Events

All keyboard messages include these fields:

interface KeyboardMessage {
  type: "keydown" | "keyup";     // Event type
  code: string;                   // JavaScript KeyboardEvent.code
  key: string;                    // JavaScript KeyboardEvent.key
  client_send_time: number;       // Client timestamp (ms)
}

Schema Constants (Host/InputSchema.h:5-22):

namespace InputSchema {
    static constexpr const char* kType = "type";
    static constexpr const char* kCode = "code";
    static constexpr const char* kClientSendTime = "client_send_time";
    
    // Event type values
    static constexpr const char* kKeyDown = "keydown";
    static constexpr const char* kKeyUp   = "keyup";
}

Example: Keydown Event

{
  "type": "keydown",
  "code": "KeyW",
  "key": "w",
  "client_send_time": 1709481234567
}

Client code (Client/html-server/index.html:1527):

function handleKeyDown(event) {
  if (!event.code || event.repeat) return;
  
  event.preventDefault();
  keyState.add(event.code);
  const keyData = { 
    key: event.key, 
    code: event.code, 
    type: 'keydown', 
    client_send_time: Date.now() 
  };
  sendKeyPress(keyData);
}

Example: Keyup Event

{
  "type": "keyup",
  "code": "KeyW",
  "key": "w",
  "client_send_time": 1709481235678
}

Client code (Client/html-server/index.html:1536):

function handleKeyUp(event) {
  if (!event.code) return;
  if (!keyState.has(event.code)) return;
  
  event.preventDefault();
  keyState.delete(event.code);
  const keyData = { 
    key: event.key, 
    code: event.code, 
    type: 'keyup', 
    client_send_time: Date.now() 
  };
  sendKeyPress(keyData);
}

Mouse Events

All mouse messages include these fields:

interface MouseMessage {
  type: "mousemove" | "mousedown" | "mouseup";
  x: number;                      // X coordinate in canvas space
  y: number;                      // Y coordinate in canvas space
  button?: number;                // Button index (0=left, 1=middle, 2=right)
  client_send_time: number;       // Client timestamp (ms)
}

Schema Constants (Host/InputSchema.h:11-21):

namespace InputSchema {
    // Mouse fields
    static constexpr const char* kX = "x";
    static constexpr const char* kY = "y";
    static constexpr const char* kButton = "button";
    
    // Event type values
    static constexpr const char* kMouseMove = "mousemove";
    static constexpr const char* kMouseDown = "mousedown";
    static constexpr const char* kMouseUp   = "mouseup";
}

Example: Mouse Move

{
  "type": "mousemove",
  "x": 960,
  "y": 540,
  "client_send_time": 1709481234567
}

Client code with throttling (Client/html-server/index.html:1031-1046):

// Throttle mouse moves to ~125Hz (8ms) to reduce bandwidth
const throttledSendMouseMove = throttle((event) => {
  const rect = canvas.getBoundingClientRect();
  const scaleX = canvas.width / rect.width;
  const scaleY = canvas.height / rect.height;
  
  const mouseData = {
    type: "mousemove",
    x: Math.round((event.clientX - rect.left) * scaleX),
    y: Math.round((event.clientY - rect.top) * scaleY),
    client_send_time: Date.now(),
  };

  sendMouseEvent(mouseData);
  mousePosition.x = mouseData.x;
  mousePosition.y = mouseData.y;
}, 8);

Example: Mouse Button Events

// Mouse down
{
  "type": "mousedown",
  "x": 960,
  "y": 540,
  "button": 0,
  "client_send_time": 1709481234567
}

// Mouse up
{
  "type": "mouseup",
  "x": 960,
  "y": 540,
  "button": 0,
  "client_send_time": 1709481235678
}

Client code (Client/html-server/index.html:1545-1578):

function handleMouseDown(event) {
  event.preventDefault();
  mouseButtonState.add(event.button);
  
  const rect = canvas.getBoundingClientRect();
  const scaleX = canvas.width / rect.width;
  const scaleY = canvas.height / rect.height;
  
  const mouseData = {
    type: 'mousedown',
    x: Math.round((event.clientX - rect.left) * scaleX),
    y: Math.round((event.clientY - rect.top) * scaleY),
    button: event.button,
    client_send_time: Date.now(),
  };
  sendMouseEvent(mouseData);
}

function handleMouseUp(event) {
  event.preventDefault();
  if (!mouseButtonState.has(event.button)) return;
  mouseButtonState.delete(event.button);
  
  const rect = canvas.getBoundingClientRect();
  const scaleX = canvas.width / rect.width;
  const scaleY = canvas.height / rect.height;
  
  const mouseData = {
    type: 'mouseup',
    x: Math.round((event.clientX - rect.left) * scaleX),
    y: Math.round((event.clientY - rect.top) * scaleY),
    button: event.button,
    client_send_time: Date.now(),
  };
  sendMouseEvent(mouseData);
}

Host-Side Processing

Keyboard Processing

The host receives keyboard events through a blocking queue for deterministic processing (Host/KeyInputHandler.cpp:518-686):

void messagePollingLoop() {
    while (isRunning.load() && !ShutdownManager::IsShutdown()) {
        std::string message;
        
        // Blocking wait for message
        {
            std::unique_lock<std::mutex> lock(queueMutex);
            queueCondition.wait(lock, [&]() {
                return shutdownRequested || !keyboardMessageQueue.empty();
            });
            
            if (!keyboardMessageQueue.empty()) {
                message = keyboardMessageQueue.front();
                keyboardMessageQueue.pop();
            }
        }
        
        // Parse and inject
        json j = json::parse(message);
        if (j.contains(InputSchema::kCode) && j.contains(InputSchema::kType)) {
            std::string jsCode = j[InputSchema::kCode].get<std::string>();
            std::string jsType = j[InputSchema::kType].get<std::string>();
            bool isKeyDown = (jsType == "keydown");
            
            // Convert JavaScript key code to Windows virtual key
            WORD vkCode = MapJavaScriptCodeToVK(jsCode);
            
            // Inject via SendInput API
            SimulateWindowsKeyEvent(jsCode, isKeyDown);
        }
    }
}

Mouse Processing

The mouse handler processes events with priority elevation for low latency (Host/MouseInputHandler.cpp:440-716):

void mouseMessagePollingLoop() {
    // Elevate thread priority for input injection
    ThreadPriorityManager::ScopedPriorityElevation priorityElevation;
    
    while (isRunning.load() && !ShutdownManager::IsShutdown()) {
        std::string message;
        
        // Blocking wait
        {
            std::unique_lock<std::mutex> lock(queueMutex);
            queueCondition.wait(lock, [&]() {
                return shutdownRequested || !mouseMessageQueue.empty();
            });
            
            if (!mouseMessageQueue.empty()) {
                message = mouseMessageQueue.front();
                mouseMessageQueue.pop();
            }
        }
        
        // Parse and route
        json j = json::parse(message);
        std::string jsType = j[InputSchema::kType].get<std::string>();
        
        if (jsType == InputSchema::kMouseMove) {
            int x = j[InputSchema::kX].get<int>();
            int y = j[InputSchema::kY].get<int>();
            simulateWindowsMouseEvent(jsType, x, y, -1);
        }
        else if (jsType == InputSchema::kMouseDown || jsType == InputSchema::kMouseUp) {
            int x = j[InputSchema::kX].get<int>();
            int y = j[InputSchema::kY].get<int>();
            int button = j[InputSchema::kButton].get<int>();
            simulateWindowsMouseEvent(jsType, x, y, button);
        }
    }
}

Input Injection

Windows SendInput API

Both keyboard and mouse events are injected using the Windows SendInput API for system-level control (Host/KeyInputHandler.cpp:407-516, Host/MouseInputHandler.cpp:204-438):

// Keyboard injection
void SimulateWindowsKeyEvent(const std::string& eventCode, bool isKeyDown) {
    INPUT input = { 0 };
    input.type = INPUT_KEYBOARD;
    
    WORD virtualKeyCode = MapJavaScriptCodeToVK(eventCode);
    WORD scanCode = MapJavaScriptCodeToScanCode(eventCode);
    
    input.ki.wScan = scanCode;
    input.ki.dwFlags = KEYEVENTF_SCANCODE;
    
    if (IsExtendedKey(virtualKeyCode)) {
        input.ki.dwFlags |= KEYEVENTF_EXTENDEDKEY;
    }
    if (!isKeyDown) {
        input.ki.dwFlags |= KEYEVENTF_KEYUP;
    }
    
    SendInput(1, &input, sizeof(INPUT));
}

// Mouse injection with absolute positioning
void simulateWindowsMouseEvent(const std::string& eventType, int x, int y, int button) {
    INPUT input = { 0 };
    input.type = INPUT_MOUSE;
    
    if (eventType == "mousemove") {
        // Transform client coordinates to absolute virtual desktop coordinates
        auto transformResult = MouseCoordinateTransform::globalTransformer
            .transformClientToAbsolute(x, y, targetWindow, clientWidth, clientHeight);
        
        input.mi.dx = transformResult.absoluteX;
        input.mi.dy = transformResult.absoluteY;
        input.mi.dwFlags = MOUSEEVENTF_MOVE | MOUSEEVENTF_ABSOLUTE | MOUSEEVENTF_VIRTUALDESK;
    }
    else if (eventType == "mousedown") {
        input.mi.dwFlags = (button == 0) ? MOUSEEVENTF_LEFTDOWN : 
                          (button == 1) ? MOUSEEVENTF_MIDDLEDOWN : 
                          MOUSEEVENTF_RIGHTDOWN;
    }
    else if (eventType == "mouseup") {
        input.mi.dwFlags = (button == 0) ? MOUSEEVENTF_LEFTUP : 
                          (button == 1) ? MOUSEEVENTF_MIDDLEUP : 
                          MOUSEEVENTF_RIGHTUP;
    }
    
    SendInput(1, &input, sizeof(INPUT));
}

Performance Optimizations

Mouse Move Coalescing

The host coalesces rapid mouse move events to reduce processing overhead (Host/MouseInputHandler.cpp:27-75):

struct CoalescedMouseMove {
    int x = -1;
    int y = -1;
    bool hasPendingMove = false;
    std::chrono::steady_clock::time_point lastUpdateTime;
    std::chrono::steady_clock::time_point windowStartTime;
    
    // 4ms coalescing window to prevent visible lag
    static constexpr std::chrono::milliseconds COALESCE_WINDOW_MS{4};
    static constexpr int MAX_COALESCED_EVENTS = 10;
    int coalescedEventCount = 0;
    
    void update(int newX, int newY) {
        auto now = std::chrono::steady_clock::now();
        
        // Start new window if expired or max events reached
        if (!hasPendingMove ||
            (now - windowStartTime) > COALESCE_WINDOW_MS ||
            coalescedEventCount >= MAX_COALESCED_EVENTS) {
            windowStartTime = now;
            coalescedEventCount = 0;
        }
        
        // Update with latest coordinates (supersedes previous)
        x = newX;
        y = newY;
        hasPendingMove = true;
        lastUpdateTime = now;
        coalescedEventCount++;
    }
};

Thread Priority Elevation

The mouse processing thread runs at elevated priority to minimize input latency (Host/MouseInputHandler.cpp:443-449):

void mouseMessagePollingLoop() {
    // Elevate thread priority for input injection
    ThreadPriorityManager::ScopedPriorityElevation priorityElevation;
    if (!priorityElevation.isElevated()) {
        std::cerr << "Failed to elevate thread priority" << std::endl;
    }
    // ... process messages
}

State Management

Key State Tracking

Both client and host maintain key state to prevent duplicate events: Client (Client/html-server/index.html:715-717):

const keyState = new Set();
const mouseButtonState = new Set();
const mousePosition = {x: 0, y: 0};

Host (Host/KeyInputHandler.cpp:44-48):

static std::set<WORD> clientReportedKeysDown;
static std::mutex clientKeysMutex;
static std::unordered_map<WORD, std::string> vkDownToJsCode;
static std::unordered_map<uint16_t, std::string> scanIdDownToJs;

Cleanup on Disconnect

When the connection closes, all held keys/buttons are automatically released (Host/KeyInputHandler.cpp:689-714, Host/MouseInputHandler.cpp:695-716):

// Keyboard cleanup
for (const auto& jsCodeToRelease : codesToRelease) {
    LOG_INFO("Cleanup: Releasing '" + jsCodeToRelease + "'");
    SimulateWindowsKeyEvent(jsCodeToRelease, false);
}

// Mouse cleanup  
for (int buttonToRelease : buttonsToRelease) {
    simulateWindowsMouseEvent("mouseup", cleanupX, cleanupY, buttonToRelease);
}

Best Practices

Client-Side

Always check channel state before sending:

if (dataChannel && dataChannel.readyState === 'open' && 
    dataChannel.bufferedAmount < 65536) {
    dataChannel.send(message);
}

Throttle mouse moves to reasonable rate (8ms = 125Hz):

const throttledSendMouseMove = throttle(sendMouseMove, 8);

Prevent default events to avoid browser interference:

function handleKeyDown(event) {
    event.preventDefault();
    // ... send to host
}

Track state locally to filter duplicates:

if (!keyState.has(event.code)) {
    keyState.add(event.code);
    sendKeyPress(keyData);
}

Host-Side

Use blocking queues for deterministic event ordering
Validate all input before injection
Clamp coordinates to screen bounds
Release all input on cleanup
Use scancode injection for layout independence
Elevate thread priority for time-sensitive input

Feedback Channels - Video stats and latency measurement
Configuration Reference - Input injection configuration options
Performance Tuning - Optimizing input latency

Matchmaking API

WebRTC Signaling

Data Channels

Input Data Channels

Input Data Channels

Channel Configuration

keyPressChannel (Keyboard Input)

mouseChannel (Mouse Input)

Message Schemas

Keyboard Events

Example: Keydown Event

Example: Keyup Event

Mouse Events

Example: Mouse Move

Example: Mouse Button Events

Host-Side Processing

Keyboard Processing

Mouse Processing

Input Injection

Windows SendInput API

Performance Optimizations

Mouse Move Coalescing

Thread Priority Elevation

State Management

Key State Tracking

Cleanup on Disconnect

Best Practices

Client-Side

Host-Side

Build docs developers (and LLMs) love

Matchmaking API

WebRTC Signaling

Data Channels

Documentation Index

​Input Data Channels

​Channel Configuration

​keyPressChannel (Keyboard Input)

​mouseChannel (Mouse Input)

​Message Schemas

​Keyboard Events

​Example: Keydown Event

​Example: Keyup Event

​Mouse Events

​Example: Mouse Move

​Example: Mouse Button Events

​Host-Side Processing

​Keyboard Processing

​Mouse Processing

​Input Injection

​Windows SendInput API

​Performance Optimizations

​Mouse Move Coalescing

​Thread Priority Elevation

​State Management

​Key State Tracking

​Cleanup on Disconnect

​Best Practices

​Client-Side

​Host-Side

​Related Documentation

Build docs developers (and LLMs) love

Input Data Channels

Channel Configuration

keyPressChannel (Keyboard Input)

mouseChannel (Mouse Input)

Message Schemas

Keyboard Events

Example: Keydown Event

Example: Keyup Event

Mouse Events

Example: Mouse Move

Example: Mouse Button Events

Host-Side Processing

Keyboard Processing

Mouse Processing

Input Injection

Windows SendInput API

Performance Optimizations

Mouse Move Coalescing

Thread Priority Elevation

State Management

Key State Tracking

Cleanup on Disconnect

Best Practices

Client-Side

Host-Side

Related Documentation