Hardware instancing again, but for gltfio

[gibssa]

Hi again Filament team,

gltifo is missing an API for hardware instancing.

Problem

• Thanks for your previous help on manual instancing.
  • I got a basic sphere instanced via vertex shader.
  • Metal profiler confirms reduction from 100 draw calls -> 1 draw call.
• gltfio's FilamentInstance != GPU instancing.
  • Metal profiler shows many repeated draw calls.

Solution?

• Is it reasonable to use an approach similar to the internal Wireframe.h?
  • like this?

  mVertexBufer = // somehow copy from `gltfio` output
  mIndexBuffer = // somehow copy from `gltfio` output

  RenderableManager::Builder(1)
      .instances(100)
      .geometry(0, RenderableManager::PrimitiveType::TRIANGLES, mVertexBuffer, mIndexBuffer)
      .build(*engine, mEntity);

Or is there a better solution to get hardware instancing working on gltfio models?

[bejado]

I don't think gltfio will support hardware instancing without some work, but if you're building Filament from source you can hack this. Essentially what you want to do though is call .instances on RenderableManager::Builder each time gltfio creates a Renderable. We merged a new API a few weeks ago:

Builder& instances(size_t instanceCount, InstanceBuffer* instanceBuffer) noexcept;

This allows you to supply an InstanceBuffer with transforms to automatically offset the position of each instance, so you don't need to change the vertex shader.

[gibssa]

I am indeed building Filament from source. I'll try out an approach like this and close this out if I can figure it out.

Thanks again team!

[gibssa]

(deleted my previous response, was half-baked)

For RenderableManger.Builder.instances(instance_buffer), it looks like InstanceBuffer supports dynamic transforms at runtime:

void setLocalTransforms(math::mat4f const* localTransforms, size_t count, size_t offset = 0);

But for my use case, I need to set other uniforms dynamically too (e.g. baseColorFactor).

Does the Filament team have any plans to either extend the InstanceBuffer struct to support more uniforms, or alternatively have an OKR or similar down the road for adding custom vertex shaders / similar hacks to get hardware instancing in gltfio?

(thanks again, as always!)

[bejado]

We don't have any plans for this at the moment. If you want to pass other per-instance uniforms, I recommend creating a parameter array in your material, for example:

{
    type : float4[16],
    name : baseColorFactor
}

Then you can index into it using getInstanceIndex(): materialParams_baseColorFactor[getInstanceIndex()].

[gibssa]

Sounds good, no worries.

I am indeed passing in a parameter array for a couple of my non-glTF instanced Renderables (per advice in #6739). I'll pick up where I left off on your initial advice to modify gltfio to call .instances(). I'll try to take this a step further and also wire up gltfio to load some of my filamat materials that I throw into the build.

Thanks again!

[gibssa]

Got it working with your advice to directly hack gltfio, by passing in both .instances() and my own .material().

Thanks @bejado 🙏