# Vulkan® 1.4.304 - A Specification (with all registered extensions)

The Khronos® Vulkan Working Group
version 1.4.304, 2025-01-13 20:32:51Z
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## 26. Cluster Culling Shading

This shader type has an execution environment similar to that of a compute shader, where a collection of shader invocations form a workgroup and cooperate to perform coarse level geometry culling and LOD selection. A shader invocation can emit a set of built-in output variables via a new built-in function. The cluster culling shader organizes these emitted variables into a drawing command used by the subsequent rendering pipeline.

### 26.1. Cluster Culling Shader Input

The only inputs available to the cluster culling shader are variables identifying the specific workgroup and invocation.

### 26.2. Cluster Culling Shader Output

If a cluster survives after culling in a cluster culling shader invocation, a drawing command to draw this cluster should be emitted by this shader invocation for further rendering processing. There are two types of drawing command, indexed mode and non-indexed mode. Both type of drawing commands consist of a set of built-in output variables which have a similar definition to VkDrawIndexedIndirectCommand and VkDrawIndirectCommand members.

Cluster culling shaders have the following built-in output variables:

- built-in variable `IndexCountHUAWEI` is the number of vertices to draw.

- built-in variable `VertexCountHUAWEI` is the number of vertices to draw.

- built-in variable `InstanceCountHUAWEI` is the number of instances to draw.

- built-in variable `FirstIndexHUAWEI` is the base index within the index buffer.

- built-in variable `FirstVertexHUAWEI` is the index of the first vertex to draw

- built-in variable `VertexOffsetHUAWEI` is the value added to the vertex index before indexing into the vertex buffer.

- built-in variable `FirstInstanceHUAWEI` is the instance ID of the first instance to draw.

- built-in variable `ClusterIDHUAWEI` is the index of cluster being rendered by this drawing command. When cluster culling shader is enabled, `ClusterIDHUAWEI` will replace `gl_DrawID` pass to vertex shader.

- built-in variable `ClusterShadingRate` is the shading rate of cluster being rendered by this drawing command.

### 26.3. Cluster Culling Shader Cluster Ordering

- When a cluster culling shader is used, all output clusters generated by `DispatchClusterHUAWEI`() in a given workgroup are passed to subsequent pipeline stage before any cluster generated from subsequent workgroup.

- In a workgroup, the order of output clusters generated by `DispatchClusterHUAWEI`() is specified by the local invocation id, from lower to higher values.

- If any cluster culling invocation in the workgroup does not call `DispatchClusterHUAWEI`(), no cluster will be sent to the subsequent rendering pipeline.

- Any cluster culling shader invocation may also call `DispatchClusterHUAWEI`() many times as shown below:

    // Cluster Culling Shader sample code:
            ......
        DispatchClusterHUAWEI();  // dispatch 0
            ......
        DispatchClusterHUAWEI();  // dispatch 1
            ......
        DispatchClusterHUAWEI();  // dispatch 2
            ......

In this case, the output sequence of clusters in a workgroup are specified as shown below ( in case of 32 shader invocations in a workgroup):

    1. shader invocation0.dispatch0
    2. shader invocation1.dispatch0,
                ..........
    32. shader invocation31.dispatch0
    33. shader invocation0.dispatch1
    34. shader invocation1.dispatch1
                ..........
    64. shader invocation31.dispatch1
    65. shader invocation0.dispatch2
    66. shader invocation1.dispatch2
                ..........
    96. shader Invocation31.dispatch2

### 26.4. Cluster Culling Shader Primitive Ordering

Following guarantees are provided for the relative ordering of primitives produced by a cluster culling shader, as they pertain to primitive order.

- Limited guarantees are provided for the relative ordering of primitives produced by a cluster culling shader, as they pertain to primitive order.

- The order of primitives in a given cluster is specified by the content of

  - `DispatchClusterHUAWEI`() with indexed output built-in variables, vertices sourced from a lower index buffer addresses to higher addresses.

  - `DispatchClusterHUAWEI`() with non-indexed output built-in variables, from vertices with a lower numbered vertexIndex to a higher numbered vertexIndex.