Web Specific Notes

Table of Contents

Known Limitations

Supported Features

With “OpenSiv3D for Web,” you can use the almost features which is supported in OpenSiv3D for Linux. For details, check out Implementation Status

File Systems

Your WebGL apps cannot access any files on users’ file system.

Those files required on running your WebGL apps, can be bundled on building with emcc’s --preload option. These bundled files are loaded into a virtual file system; then you can access these files on ordinal way.

Visual Studio

Open the project configuration window, and add preloaded file path into Preloaded Files (found in [Emscripten Linker] > [Input]) in the project configuration.



Open .vscode/Link.Debug.rsp or .vscode/Link.Release.rsp and add linker preloaded files option.


File Save Dialog

s3d::Dialog::SaveFile always returns None. Please use s3d::Platform::Web::DownloadFile to download files that stored in the virtual file system.

  // Not Supported in Web.
  // if (auto path = Dialog::SaveFile())
  // {
  //   image.save(*path);
  // }


No Sounds before User Actions

Some browser does not allow for WebGL apps to make any sounds before some user actions, such as typing keyboards, clicking, or tapping screen.

Cannot Switch to Fullscreen on iOS devices

iOS devices have no fullscreen support.

Features that Differs Other Platforms


Only connecting to external websocket server is supported. Make sure to use secured websocket server in secured (https://) pages.

  const IPv4Address ip = IPv4Address::Localhost();
  constexpr uint16 port = 50000;

  TCPClient client;

  client.connect(ip, port);

  Point serverPlayerPos{ 0, 0 };
  const Point clientPlayerPos = Cursor::Pos();
  // send

  // Not Supported that polling with `client.read`.
  // Polling will freeze the browser.
  // while (client.read(serverPlayerPos));

  // recv


OpenSiv3D for Web is designed to be run single-threaded, so AsyncTask or std::thread will not work as you expected.

  AsyncTask task
      Console << U"Done.";

Unsupported Texture Format

In mobile browsers, generating textures of format TextureFormat::R32_Float will be failed due to the mobile hardware limitation. Consider using TextureFormat::R16G16_Float formatted textures instead.

Features that Requires User Actions

Some features are required to use on user actions.

  • Dialog::*
  • ClipBoard::ReadText, SetText
  • Window::SetFullscreen
  • VideoReader (on Safari)
  • System::LaunchBrowser
// will enter fullscreen on first user action after invoking `Window::SetFullscreen`
// Window::SetFullscreen(true);

if (SimpleGUI::Button(U"Full Screen", Point{ 20, 20 }))
  // SimpleGUI::Button()` will be true on user `click` actions,
  // invocation of `Window::SetFullscreen` will work expectedly.

Wire-frame Drawing

Wire-frame Drawing is not available in the WebGL backend as WebGL 2.0 does not have wireframe drawing capabilities.

# include <Siv3D.hpp>

void Main()
	while (System::Update())
    // Ignored in Web
		const ScopedRenderStates2D rasterizer{ RasterizerState::WireframeCullNone };
		Shape2D::Heart(200, Scene::Center()).draw(Palette::Skyblue);