Input Event API Demo Guide

This document explains how to use the Input Event API demo in the standalone Cast reference implementations.

Overview

The Input Event API allows a Cast Receiver to send user interactions (mouse clicks, keyboard events, etc.) back to the Cast Sender. This demo specifically shows mouse click events being captured on the receiver and visualized on the sender.

Prerequisites

  • SDL2: The standalone receiver requires SDL2 for window management and input event capture.
  • FFmpeg/LibVPX/LibOpus: Required for media encoding and decoding.

Building the Demo

Ensure your GN args are configured for standalone build with external libraries. Then build the following targets:

autoninja -C out/Default cast_receiver cast_sender

Running the Demo

1. Start the Receiver

Run the receiver on a networked machine (or your local machine using the loopback interface). You must opt-in using the --enable-input-events flag.

./out/Default/cast_receiver --enable-input-events <interface_name>

Replace <interface_name> with your network interface (e.g., eth0, wlan0, or lo for local testing).

2. Start the Sender

Run the sender, pointing it to the receiver's address and a media file. Again, the --enable-input-events flag is required.

./out/Default/cast_sender --enable-input-events <receiver_ip_or_interface> <media_file>

Observing the Results

  1. A window will open on the Receiver's machine showing the mirrored video.

  2. Click anywhere inside the Receiver's video window.

  3. Visual Feedback: An animated “ping” (an expanding white ring) will appear directly in the video stream at the location where you clicked. This ring is drawn by the Sender on the raw frames before they are encoded.

  4. Console Logs: Check the Sender's terminal output. You should see logs indicating the received mouse events:

    [Input] Received MOUSE_DOWN at (450, 300) button=1 display=1920x1080
[Input] Received MOUSE_UP at (450, 300) button=1 display=1920x1080

How it Works

  1. Capture: The SDLEventLoopProcessor in cast_receiver captures SDL mouse events.
  2. Mapping: StreamingPlaybackController maps window coordinates to the video's logical coordinate space.
  3. Transport: The events are sent back to the sender via the InputProducer using the negotiated input_events RTP extension.
  4. Consumption: The LoopingFileCastAgent in cast_sender receives the events via InputConsumer.
  5. Overlay: LoopingFileSender draws an animated ring directly into the YUV pixel data of the next video frame before encoding.