Agent skill
forge-vertex-pulling
Add vertex pulling (programmable vertex fetch) to an SDL GPU project. Replace fixed-function vertex input with storage buffer reads in the vertex shader using SV_VertexID and StructuredBuffer.
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SKILL.md
Vertex Pulling with SDL3 GPU
Replace the fixed-function vertex input assembler with manual storage buffer
reads in the vertex shader. The pipeline declares zero vertex attributes and
the shader fetches vertex data from a StructuredBuffer using SV_VertexID.
Based on GPU Lesson 33 — Vertex Pulling.
When to use
- Meshes with different vertex layouts sharing one pipeline
- Compute shaders writing vertex data for the vertex shader to read
- Compressed or packed vertex formats decoded in the shader
- GPU-driven rendering with indirect draw calls
- Reducing pipeline state complexity and pipeline object count
HLSL pattern
Vertex shader (pulled)
struct PulledVertex
{
float3 position;
float3 normal;
float2 uv;
};
/* Vertex storage buffer: register(t0, space0) for DXIL vertex shaders.
* Slot index follows sampled textures and storage textures (if any). */
StructuredBuffer<PulledVertex> vertex_buffer : register(t0, space0);
cbuffer SceneUniforms : register(b0, space1)
{
column_major float4x4 mvp;
column_major float4x4 model;
};
struct VSOutput
{
float4 clip_pos : SV_Position;
float3 world_pos : TEXCOORD0;
float3 world_nrm : TEXCOORD1;
float2 uv : TEXCOORD2;
};
VSOutput main(uint vertex_id : SV_VertexID)
{
VSOutput output;
PulledVertex v = vertex_buffer[vertex_id];
float4 world = mul(model, float4(v.position, 1.0));
output.clip_pos = mul(mvp, float4(v.position, 1.0));
output.world_pos = world.xyz;
output.world_nrm = normalize(mul((float3x3)model, v.normal));
output.uv = v.uv;
return output;
}
Fragment shader
No changes needed — fragment shaders work identically regardless of whether vertex data came from the input assembler or a storage buffer.
C-side pattern
1. Define the vertex struct (must match HLSL)
typedef struct PulledVertex {
vec3 position; /* 12 bytes */
vec3 normal; /* 12 bytes */
vec2 uv; /* 8 bytes */
} PulledVertex; /* 32 bytes */
2. Upload as storage buffer (not vertex buffer)
/* The only CPU-side difference: buffer usage flag */
SDL_GPUBuffer *storage_buf = upload_gpu_buffer(
device,
SDL_GPU_BUFFERUSAGE_GRAPHICS_STORAGE_READ, /* NOT VERTEX */
vertex_data,
vertex_count * sizeof(PulledVertex));
3. Create shader with storage buffer count
/* Vertex shader: declare 1 storage buffer */
SDL_GPUShaderCreateInfo info;
SDL_zero(info);
info.stage = SDL_GPU_SHADERSTAGE_VERTEX;
info.num_samplers = 0;
info.num_storage_buffers = 1; /* <-- vertex data storage buffer */
info.num_uniform_buffers = 1;
/* ... format, code, entrypoint ... */
4. Create pipeline with empty vertex input
/* No vertex buffer descriptions, no vertex attributes */
SDL_GPUVertexInputState vis;
SDL_zero(vis);
/* vis.num_vertex_buffers = 0; already zero */
/* vis.num_vertex_attributes = 0; already zero */
SDL_GPUGraphicsPipelineCreateInfo pi;
SDL_zero(pi);
pi.vertex_input_state = vis;
/* ... rest of pipeline setup ... */
5. Bind and draw
/* Bind storage buffer instead of vertex buffer */
SDL_GPUBuffer *bufs[1] = { storage_buf };
SDL_BindGPUVertexStorageBuffers(pass, 0, bufs, 1);
/* Index buffer binding is unchanged */
SDL_GPUBufferBinding ib = { index_buffer, 0 };
SDL_BindGPUIndexBuffer(pass, &ib, index_type);
SDL_DrawGPUIndexedPrimitives(pass, index_count, 1, 0, 0, 0);
SDL GPU register mapping
For DXIL vertex shaders, resources are bound in this order at space0:
- Sampled textures (
t0..tN) - Storage textures (
tN+1..) - Storage buffers (
tN+M+1..)
With zero sampled/storage textures, the first storage buffer is t0, space0.
For SPIR-V vertex shaders, storage buffers go in descriptor set 0 after sampled and storage textures.
Combining with other techniques
-
Compute → Vertex: Use
SDL_GPU_BUFFERUSAGE_GRAPHICS_STORAGE_READ | SDL_GPU_BUFFERUSAGE_COMPUTE_STORAGE_WRITEso a compute shader can write the buffer and the vertex shader can read it. -
Indirect drawing (Lesson 38): Vertex pulling pairs naturally with indirect draws for fully GPU-driven rendering.
-
Multiple vertex formats: One pipeline can render meshes with different vertex layouts by defining multiple
StructuredBufferstructs or using aByteAddressBufferwith manual offset calculations.
Common mistakes
| Mistake | Fix |
|---|---|
Using SDL_GPU_BUFFERUSAGE_VERTEX |
Use SDL_GPU_BUFFERUSAGE_GRAPHICS_STORAGE_READ |
Forgetting num_storage_buffers = 1 on shader |
Set it in SDL_GPUShaderCreateInfo |
Calling SDL_BindGPUVertexBuffers |
Use SDL_BindGPUVertexStorageBuffers |
| Mismatched C/HLSL struct layout | Ensure identical field order and sizes |
| Leaving vertex attributes in pipeline | Set num_vertex_buffers = 0, num_vertex_attributes = 0 |
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