Hardware instancing

[gibssa]

Hi again Filament team,

I was hoping you could clarify this comment from your PR:

Specifies the number of draw instance of this renderable. The default is 1 instance and
the maximum number of instances allowed is 65535. 0 is invalid.
All instances are culled using the same bounding box, so care must be taken to make
sure all instances render inside the specified bounding box.
The material can use getInstanceIndex() in the vertex shader to get the instance index and
possibly adjust the position or transform.

I have an existing (repeated) transform that I'd like to batch into a single draw call. Here's my (naive) code:

for (int i = 0; i < 100; ++i) {
  const auto& instance = tm_.getInstance(entities_[i]);
  auto translation = mat4f::translation(float3(
      offsets.x,
      offsets.y,
      offsets.z
  ));

  float percentage = ...;
  auto scaling = mat4f::scaling(float3(
      0.020f * percentage,
      0.01f,
      0.020f * percentage
  ));
  tm_.setTransform(instance, translation * scaling);
}

I'm converting this to instead call RenderableManager::Builder.instances(100).

Here's my (unfinished) CPU + shader code:

RenderableManager::Builder(1)
      .boundingBox({{0}, {1}})
      .material(0, mMaterialInstance)
      .geometry(0,
                RenderableManager::PrimitiveType::TRIANGLES,
                geometry->vertexBuffer,
                geometry->indexBuffer)
      .instances(100)
      .culling(false)
      .build(engine, mRenderable);

material {
    name : Batched,
    parameters : [
        {
           type : float4,
           name : color
        }
    ],
    blending : transparent,
    culling : none,
    instanced: true,
    depthCulling : false,
    shadingModel : unlit
}

vertex {
    // https://google.github.io/filament/Materials.html#materialdefinitions/materialblock/general:instanced
    void materialVertex(inout MaterialVertexInputs material) {
        int instance_index = getInstanceIndex();

        material.worldPosition.xyz = ??
    }
}

It seems that the material.worldPosition accepts a position, but not a transform?

So my question is:

1. How can I pass a (translation + scaling) transform to a vertex shader to batch draw calls?

Thanks!

[bejado]

material.worldPosition sets the position of a vertex. By default (in the OBJECT vertex domain), it is computed like so:

mat4 transform = getWorldFromModelMatrix();
vec3 position = getPosition().xyz;
material.worldPosition = mulMat4x4Float3(transform, position);

So if you want to pass a unique transform per instance, you somehow need to customize transform depending on the instance. How you do this depends on what you're trying to achieve. If you have some matrices on the CPU you'd like to use, you could pass them as a float3×3 parameter array and use getInstanceIndex() to index into them.

[gibssa]

Thank you for the details, I'll try to implement this and close out this comment once I do.

[gibssa]

I think I got it, thanks @bejado !