Off-screen canvas rendering

[Andrea M]

Hi,

This is uncharted territory for me so once again apologies if I am asking dull questions.

Is it possible for emscripten to handle 2 canvas elements? One would be used to render the GUI the other one for off-screen rendering of textures/images.

When I look at the Module singleton I get the impression is that there is meant to be at most one canvas element, i.e. `Module['canvas']`.

This is part of me trying to port Maplibre Native to emscripten, whereby the map would be incorporated into my Dear Imgui based GUI - however Maplibre does do async 'headless' rendering of the tiles beforehand.

I hope I have provided enough information, let me know if not.

Thank you!

Andrea

[Alon Zakai]

There is a currently-active context, which GL commands work on, and is stored on Module.canvas. But you can switch the current context between multiple ones. Specifically, when you create a context you can tell it which DOM canvas to use:

https://emscripten.org/docs/api_reference/html5.h.html#c.emscripten_webgl_create_context

and you can set the current context with this:

https://emscripten.org/docs/api_reference/html5.h.html#c.emscripten_webgl_make_context_current

However, some APIs may not fully support multiple canvases, I'm not sure.

- Alon

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[Alon Zakai]

(Oops, I wrote Module.canvas there, but that's the canvas, not the context. Module.canvas is a way to set the default canvas.)

[Andrea M]

Thank you, will try it out!

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