Support WebGL2 and WebGPU in the same WASM file

[BD103]

What problem does this solve or what need does it fill?

Bevy currently forces you to choose either WebGL2 or WebGPU when building a project. This is one extra thing that developers need to think about when deploying their games, which is already a complicated enough process.

Since the web is a large, incredibly portable platform, we should attempt to make targeting it as painless as possible.

What solution would you like?

In gfx-rs/wgpu#5044, WGPU added support for detecting WebGL2 and WebGPU at runtime. I think integrating this into Bevy would be incredibly useful, and give WebGPU more exposure. This would be achieved through the existing webgl2 and webgpu features.

webgl2 enabled?	webgpu enabled?	Behavior
✅	✅	Detect API at runtime, preferring WebGPU and falling back to WebGL2 if not available
✅	❌	Use WebGL2, panicking if not available at runtime
❌	✅	Use WebGPU, panicking if not available at runtime
❌	❌	Fail to compile for WASM

Questions

• What crate(s) will need to implement this functionality? Is there a way we can abstract over both APIs to reduce the amount of logic necessary?
• Does WebGPU still need --cfg=web_sys_unstable_apis, and do we handle this already?
• How difficult will this change be? Is it something we can plan on finishing by Bevy 0.14?

What alternative(s) have you considered?

• Instead of panicking, using the error!(...) macro may be preferred because it will be visible in the Javascript console.
  • Do we have a panic handler for WASM that logs panics? If not, then error! may be preferred.

Additional context

• Discord message that inspired this feature request
• Update to wgpu 0.19 and raw-window-handle 0.6 #11280 (comment)

cc @Elabajaba, who originally migrated Bevy to the WGPU version that introduced this feature.

[mockersf]

Questions

• What crate(s) will need to implement this functionality? Is there a way we can abstract over both APIs to reduce the amount of logic necessary?

Only rendering part should be affected. mostly bevy_render and bevy_pbr

• Does WebGPU still need --cfg=web_sys_unstable_apis, and do we handle this already?

No, wgpu now vendors those so they're not needed anymore

• How difficult will this change be? Is it something we can plan on finishing by Bevy 0.14?

Biggest difficulty I think is that currently some const change value depending on WebGL2 / WebGPU / Native. Deciding them at runtime means they won't be const anymore.

Also I think WebGPU detection is hard to do perfectly, so we need a way to "force" fallback on WebGL2 even if WebGPU is detected

[BD103]

Biggest difficulty I think is that currently some const change value depending on WebGL2 / WebGPU / Native. Deciding them at runtime means they won't be const anymore.

Ok, that will require a bit of benchmarking then to ensure the performance hit is not too great. This will probably affect all of WASM, even if you don't runtime detection.

Also I think WebGPU detection is hard to do perfectly, so we need a way to "force" fallback on WebGL2 even if WebGPU is detected

You can toggle off the webgpu feature at compile time. If we want to do this at runtime, perhaps there can be a resource that configures it. Quick draft:

// This is a bad name, please ignore it
#[derive(Resource)]
enum WASMBackend {
    WebGPU,
    WebGL2,
    Available, // Prefers WebGPU if possible, could instead be named `PreferWebGPU`
}

[mockersf]

Ideally (parts of) Bevy should also support building without WebGL2 and WebGPU, and support targeting wasi (#4906 & #8520)

[rparrett]

Related: #11505

[lomirus]

Will this feature double the WASM file size for those who only want to use WebGPU or WebGL2, or just increase the size slightly?

[mockersf]

just increase slightly!