Help With Drawing To Canvas In Pthreads

[Andrew Lay]

Can someone please help with out?

I am having trouble correctly drawing to Canvas on Pthreads.

There is nothing being drawn to the canvas on the screen while the application is running, however if I freeze the code with the Dev tools in the debugger the Canvas updates.

I have created an example in this git hub repo: 

https://github.com/alayGit/DummyEmscriptenCanvasForumExample

It is not pretty but I hope it will demonstrate the issue.

If anything is not clear please let me know.

[Sam Clegg]

Seems likely related to `explicitSwapControl`, `renderViaOffscreenBackBuffer` and `proxyContextToMainThread` which are part of `EmscriptenWebGLContextAttributes`:

https://emscripten.org/docs/api_reference/html5.h.html?highlight=gamepad#c.EmscriptenWebGLContextAttributes.explicitSwapControl

Have you read the documentation related to those?

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[Andrew Lay]

Hi Sam,
Thank you for your quick response.

I have seen the struct params that you speak of, but all of the examples that I have seen relate to GLES2 C api.  For example I looked at 'webgl_draw_triangle.c' example file in the Emscripten folder.

When I saw the complexity of the library, I thought it seemed like overkill for what I wanted to do, simply draw a buffer of rgba values to a canvas. 

This is why I tried to call the ordinary JavaScript API from an Emscripten PThread. See here: https://github.com/alayGit/DummyEmscriptenCanvasForumExample/blob/main/Emscripten/EmscriptenCanvasWorker.cpp

I am wasting my time trying to do things this way?

Should I use the GLES2 library, or are the things you pointed me too and the stuff in the draw triangle example needed for the JavaScript API method too?

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[Sam Clegg]

On Fri, Sep 2, 2022 at 5:46 AM Andrew Lay <andrew...@gmail.com> wrote:

Hi Sam,
Thank you for your quick response.

I have seen the struct params that you speak of, but all of the examples that I have seen relate to GLES2 C api.  For example I looked at 'webgl_draw_triangle.c' example file in the Emscripten folder.

When I saw the complexity of the library, I thought it seemed like overkill for what I wanted to do, simply draw a buffer of rgba values to a canvas. 

This is why I tried to call the ordinary JavaScript API from an Emscripten PThread. See here: https://github.com/alayGit/DummyEmscriptenCanvasForumExample/blob/main/Emscripten/EmscriptenCanvasWorker.cpp

I am wasting my time trying to do things this way?

I would stick to using the emscripten opengl implementation, unless you really know what you are doing and want to get into the weeds.  Otherwise, you likely end up running into issues that emscripten has already solved in its implementation.   I imagine this is one of those cases but I'm not an expert on this part of the code.

 

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[Andrew Lay]

Hi Sam,

Thanks for replying. When I have time I will try the Emscripten way.

I will let you know how I go.

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[Andrew Lay]

Hi Sam,
No luck with this approach either. Same problem.
I outlined it in this forum. https://github.com/emscripten-core/emscripten/issues/17816

Let me know if you have any ideas.

Also can you let me know if there is a better place to get technical assistance?

Am frustrating people in the way I write my questions, causing people to skip over them?

Thanks,
Andrew Lay

[Andrew Lay]

Oh, by the way, I am assuming I don't need the 'renderViaOffScreenBackBuffer' as I am using an offscreen canvas. I looked at the other two attributes though.

[Sam Clegg]

On Fri, Sep 9, 2022 at 4:14 AM Andrew Lay <andrew...@gmail.com> wrote:

Hi Sam,
No luck with this approach either. Same problem.
I outlined it in this forum. https://github.com/emscripten-core/emscripten/issues/17816

Let me know if you have any ideas.

Also can you let me know if there is a better place to get technical assistance?

Am frustrating people in the way I write my questions, causing people to skip over them?

No, this forum, and github issues are the recommended support channels and your question seem reasonable to me :)

You could also try to discord server.

I myself have very little knowledge of how the offscreen canvas stuff works so I don't have any easy answers for you I'm afraid.  One thing I would recommend is looking through `test/test_browser.py` for examples/tests that use the offscreen canvas setting.  Do they work as expected for you?  

cheers,

sam

 

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[Andrew Lay]

Ok. Thanks for your help

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