Implementing Multiple Windows

Oct 13, 2025 by pynkocean.com 107 views

Creating Multiple Windows Not Possible on Emscripten: A Technical Analysis

Emscripten is a popular tool for compiling C and C++ code to JavaScript, allowing developers to run their applications in web browsers. However, one of the limitations of Emscripten is that it does not support creating multiple windows. This is because the code is written as if there will only be one canvas element in the document. In this article, we will delve into the technical details of this issue and explore the possible solutions.

Emscripten builds cannot create multiple windows because the code is written as if there will only be one canvas element in the document. This is a significant limitation, as many applications require multiple windows to function properly. To understand the root cause of this issue, we need to look at the code.

Upon analyzing the code, we can identify several tasks that need to be completed to fix this issue:

Remove Emscripten Hints and Make Them Properties

The first task is to remove Emscripten hints and make them properties. This is reflected in the issue #12509. By making these changes, we can ensure that the code is more flexible and can accommodate multiple windows.

Register Callbacks that Don't Take an Element as a Parameter

The second task is to register callbacks that don't take an element as a parameter. These callbacks include emscripten_set_fullscreenchange_callback, emscripten_set_focus_callback, and Emscripten_set_pointer_event_callbacks. To fix this issue, we need to update these callbacks to look through all of the canvases in the document and only handle the event for the focused canvas.

Update `Emscripten_UpdateWindowFramebuffer`

The third task is to update Emscripten_UpdateWindowFramebuffer to store multiple canvases and update the specified canvas based on the window_id of the window. This will only fix the software renderer.

For the OpenGLES2 renderer, there are a few possibilities to create multiple windows:

Update Emscripten

One possible solution is to update Emscripten such that a specific canvas can be set as the target canvas. Currently, Emscripten only renders to Module['canvas']. By making this change, we can ensure that the correct canvas is being rendered to.

Update SDL

Another possible solution is to update SDL so that it updates Module['canvas'] to the canvas of the renderer before any render calls. This will ensure that the correct canvas is being used for rendering.

Implement SDL_gpu with WebGL (or WebGPU)

The final possible solution is to implement SDL_gpu with WebGL (or WebGPU) and ensure that the correct canvas is being rendered to. This will provide a more efficient and flexible solution for creating multiple windows.

Creating multiple windows on Emscripten is not possible due to the code being written as if there will only be one canvas element in the document. However, by completing the tasks outlined above, we can fix this issue and enable multiple windows to be created. Additionally, we can explore the possibilities of updating Emscripten, SDL, or implementing SDL_gpu with WebGL (or WebGPU) to provide a more efficient and flexible solution.

To further improve the functionality of Emscripten, we can consider the following future work:

Implementing Multiple Windows: As discussed earlier, implementing multiple windows is a crucial feature that needs to be addressed.
Improving Performance: We can explore ways to improve the performance of Emscripten, such as optimizing the rendering pipeline or using more efficient algorithms.
Enhancing Security: We can also focus on enhancing the security of Emscripten, such as implementing sandboxing or using more secure communication protocols.

By addressing these areas, we can make Emscripten a more powerful and versatile tool for developers, enabling them to create complex and engaging applications with ease.

Note: The references provided are subject to change and may not be up-to-date. It is recommended to check the official documentation for the latest information.
Emscripten Multiple Windows: Frequently Asked Questions

In our previous article, we discussed the technical details of creating multiple windows on Emscripten and explored the possible solutions. In this article, we will answer some of the frequently asked questions related to Emscripten multiple windows.

Q: Why can't I create multiple windows on Emscripten?

A: Emscripten builds cannot create multiple windows because the code is written as if there will only be one canvas element in the document. This is a limitation of the current implementation.

Q: What are the possible solutions to create multiple windows on Emscripten?

A: There are several possible solutions to create multiple windows on Emscripten:

Remove Emscripten hints and make them properties: This involves removing Emscripten hints and making them properties, as reflected in the issue #12509.
Register callbacks that don't take an element as a parameter: This involves updating callbacks to look through all of the canvases in the document and only handle the event for the focused canvas.
Update Emscripten_UpdateWindowFramebuffer: This involves updating Emscripten_UpdateWindowFramebuffer to store multiple canvases and update the specified canvas based on the window_id of the window.
Update Emscripten: This involves updating Emscripten to render to a specific canvas, rather than the default Module['canvas'].
Update SDL: This involves updating SDL to update Module['canvas'] to the canvas of the renderer before any render calls.
Implement SDL_gpu with WebGL (or WebGPU): This involves implementing SDL_gpu with WebGL (or WebGPU) and ensuring that the correct canvas is being rendered to.

Q: Which solution is the most efficient?

A: The most efficient solution will depend on the specific use case and requirements. However, implementing SDL_gpu with WebGL (or WebGPU) is likely to provide the best performance and flexibility.

Q: Can I use multiple windows on Emscripten with the OpenGLES2 renderer?

A: Yes, it is possible to use multiple windows on Emscripten with the OpenGLES2 renderer. However, this will require updating Emscripten, SDL, or implementing SDL_gpu with WebGL (or WebGPU).

Q: What are the benefits of creating multiple windows on Emscripten?

A: Creating multiple windows on Emscripten can provide several benefits, including:

Improved user experience: Multiple windows can provide a more intuitive and user-friendly experience.
Increased productivity: Multiple windows can allow users to work on multiple tasks simultaneously.
Enhanced functionality: Multiple windows can provide additional features and functionality.

Q: What are the challenges of creating multiple windows on Emscripten?

A: Creating multiple windows on Emscripten can present several challenges, including:

Complexity: Implementing multiple windows can add complexity to the code and require significant changes.
Performance: Creating multiple windows can impact performance and require optimization.
Security: Multiple windows can introduce security risks and require additional measures to prevent unauthorized access.

Creating multiple windows on Emscripten is a complex task that requires careful consideration and planning. By understanding the possible solutions and challenges, developers can make informed decisions and create a more efficient and user-friendly experience.

To further improve the functionality of Emscripten, we can consider the following future work:

Implementing Multiple Windows: As discussed earlier, implementing multiple windows is a crucial feature that needs to be addressed.
Improving Performance: We can explore ways to improve the performance of Emscripten, such as optimizing the rendering pipeline or using more efficient algorithms.
Enhancing Security: We can also focus on enhancing the security of Emscripten, such as implementing sandboxing or using more secure communication protocols.

By addressing these areas, we can make Emscripten a more powerful and versatile tool for developers, enabling them to create complex and engaging applications with ease.

Note: The references provided are subject to change and may not be up-to-date. It is recommended to check the official documentation for the latest information.