Please add an SDL3 port

[MarkCallow]

It is pretty mature now and apparently can be built with Emscripten. They have made a number of their samples available via it. I'd like to switch my projects to SDL3.

Sorry if this is the wrong place to file this. Issues are not enabled under emscripten-ports and the SDL2 port repo has been "archived" and consequently no new issues can be added.

[kripken]

As of 4.0.4, there is an experimental port of SDL3:

https://github.com/emscripten-core/emscripten/blob/main/ChangeLog.md#404---022525

Code is here:

https://github.com/emscripten-core/emscripten/blob/main/tools/ports/sdl3.py

I believe it is not complete yet, e.g. an SDL3_image port PR was just opened now: emscripten-core/emscripten#24634 - but perhaps you can start to test it.

[fredheidrich]

Base SDL3 is functional as far as I can tell. I've been (slowly) adding support for the other libraries. So far there hasn't been any issues with those either. Got SDL3_ttf sitting on a PR here: emscripten-core/emscripten#24601

[MarkCallow]

The only optional component my apps use is vulkan so I can use the port in its current state. It is working the same as the native port with one exception. In SDL2 the default bit size of the alpha channel in the color buffer is 0. In SDL3 it is 8. My app has what looks like a bug. It's fragment shader writes the texture's alpha value to fragcolor.a. In SDL2 this was a NOP. In SDL3 running natively on macOS there is also no issue. With Emscripten's GLES, any fragment where fragcolor.a is 0 is rendered as black. In other words some blending of my output and the underlying canvas is going on.

Either always writing 1.0f to fragcolor.a or calling SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 0); fixes the problem.

The only concern is why is the behaviour different on Emscripten/Web vs macOS native OpenGL. macOS is getting a color buffer with an 8-bit alpha channel too.

P.S. Any chance of an assimp port?

[MarkCallow]

The WebGL spec says "If defined, the alpha channel is used by the HTML compositor to combine the color buffer with the rest of the page" meanwhile special setup is required to create a transparent window on Windows and likely on macOS too. I haven't checked the latter. So on desktop the alpha channel is ignored absent special transparent window set up and the different behaviour with Emscripten/WebGL is to be expected.

I don't understand why I see black in the fully transparent parts though. From this image of Emscripten's default page it looks like background behind the WebGL canvas is white.

But I doubt that is an issue with the SDL port. If it is an issue, i.e if the background is white, then it is an issue for Emscripten's OpenGL ES emulation.

[sbc100]

@Daft-Freak might know about the SDL2 vs SDL3 issue.

@juj might know about why you are seeing back rather than which for the transparent region.