Material::compile() callback?

[gibssa]

Hi again Filament team,

I've got a handful of custom (GPU instanced) materials. They take ~10 seconds to compile on an iPhone 14 Pro Max:

I've pre-compiled (matc and resgen) and embedded them, like the samples:

material_ = Material::Builder()
      .package(RESOURCES_MY_SHADER_DATA, RESOURCES_MY_SHADER_SIZE)
      .build(*engine_);
material_instance_ = material_->createInstance();

... but I suspect that matc compilation is actually mostly SPIRV-Cross? It looks like MSL compilation is still on-device (based on Xcode logs).

As a workaround, I'm hoping to at least show some sort of progress bar to the user during those ~10 seconds of shader compilation.

So my questions are:

1. Would a callback passed into Material::compile() conflict with my existing Material::Builder::build() call? (that is, would that be "double" compilation?)
2. Alternatively, is there a way to pre-compile my shaders offline from .mat -> .filamat -> MSL -> .metallib and hack them into Filament somehow?
3. Am I correct in assuming that matc is indeed mostly SPIRV-Cross, with backend MSL compilation still happening at runtime?

Thanks as always!

[gibssa]

Closing the loop on this (forgot about this one):

1. Material::compile was indeed enough to get a callback working to show a loading screen (I tried to make sure shader variants that I requested at runtime matched what I actually use; hopefully I'm not doing extra work).
2. Doesn't look like there's a way to precompile to .metallib.
3. Looks like matc (filamat) is indeed targeting MSL, which still needs to get compiled on-device.

Hope this is helpful for posterity.