# Forge Indirect Drawing > > - **Type:** Skill - **Install:** `agentstack add skill-nebulavenus-forge-gpu-forge-indirect-drawing` - **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior - **Seller:** [Nebulavenus](https://agentstack.voostack.com/s/nebulavenus) - **Installs:** 0 - **Category:** [Agent Skills](https://agentstack.voostack.com/c/agent-skills) - **Latest version:** 0.1.0 - **License:** Zlib - **Upstream author:** [Nebulavenus](https://github.com/Nebulavenus) - **Source:** https://github.com/Nebulavenus/forge-gpu/tree/main/.claude/skills/forge-indirect-drawing ## Install ```sh agentstack add skill-nebulavenus-forge-gpu-forge-indirect-drawing ``` Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent. ## About # Indirect Drawing with Compute Frustum Culling Add GPU-driven rendering to an SDL GPU project where a compute shader performs frustum culling and writes indirect draw commands. The CPU uploads object data once; the GPU decides what to draw every frame. Based on [GPU Lesson 38](../../../lessons/gpu/38-indirect-drawing/). ## When to use - You have many instances of the same mesh and want the GPU to cull invisible ones - You want to eliminate per-object CPU draw decisions entirely - You need frustum culling without CPU-side bounding volume tests - You want a debug visualization showing which objects are visible vs. culled ## Core data structures ### ObjectGPUData storage buffer Each object's transform, color, bounding sphere, and draw arguments are packed into a single struct stored in a GPU storage buffer. Both the compute shader (for culling) and the vertex shader (for rendering) read from this buffer. ```c /* Must match the HLSL ObjectData struct exactly. * Total: 112 bytes per object, 16-byte aligned. */ typedef struct ObjectGPUData { mat4 model; /* offset 0: 64 bytes — model-to-world transform */ float color[4]; /* offset 64: 16 bytes — base color (RGBA) */ float bounding_sphere[4]; /* offset 80: 16 bytes — xyz=center(world), w=radius */ Uint32 num_indices; /* offset 96: 4 bytes — index count for this object */ Uint32 first_index; /* offset 100: 4 bytes — first index in shared index buffer */ Sint32 vertex_offset; /* offset 104: 4 bytes — added to each index */ Uint32 _pad; /* offset 108: 4 bytes — align to 16 bytes */ } ObjectGPUData; /* 112 bytes */ ``` ### IndirectCommand (matches SDL_GPUIndexedIndirectDrawCommand) ```c /* 20 bytes per command — 5 x uint32. Written by the compute shader. */ struct IndirectCommand { uint num_indices; uint num_instances; /* 1 = visible, 0 = culled (hardware no-op) */ uint first_index; int vertex_offset; uint first_instance; /* = object index, used for storage buffer lookup */ }; ``` ### CullUniforms ```c typedef struct CullUniforms { float frustum_planes[6][4]; /* 6 planes, each (nx, ny, nz, d) */ Uint32 num_objects; Uint32 enable_culling; /* 1 = cull, 0 = draw everything */ float _pad[2]; /* 16-byte alignment */ } CullUniforms; ``` ## Buffer creation and usage flags Each buffer requires specific usage flags to allow cross-pipeline access: ```c /* Object data — read by compute (culling) and vertex shader (transforms) */ object_data_buf = upload_gpu_buffer(device, SDL_GPU_BUFFERUSAGE_COMPUTE_STORAGE_READ | SDL_GPU_BUFFERUSAGE_GRAPHICS_STORAGE_READ, objects, num_objects * sizeof(ObjectGPUData)); /* Indirect draw commands — written by compute, consumed by indirect draw */ SDL_GPUBufferCreateInfo bi; SDL_zero(bi); bi.usage = SDL_GPU_BUFFERUSAGE_INDIRECT | SDL_GPU_BUFFERUSAGE_COMPUTE_STORAGE_WRITE; bi.size = num_objects * 20; /* 5 x uint32 per command */ indirect_buf = SDL_CreateGPUBuffer(device, &bi); /* Visibility flags — written by compute, read by debug fragment shader */ SDL_zero(bi); bi.usage = SDL_GPU_BUFFERUSAGE_COMPUTE_STORAGE_WRITE | SDL_GPU_BUFFERUSAGE_GRAPHICS_STORAGE_READ; bi.size = num_objects * sizeof(Uint32); visibility_buf = SDL_CreateGPUBuffer(device, &bi); /* Instance ID buffer — [0, 1, 2, ..., N-1] as a vertex buffer */ Uint32 ids[NUM_OBJECTS]; for (int i = 0; i objects : register(t0, space0); RWStructuredBuffer indirect_commands : register(u0, space1); RWStructuredBuffer visibility : register(u1, space1); cbuffer CullUniforms : register(b0, space2) { float4 frustum_planes[6]; uint num_objects; uint enable_culling; float2 _pad; }; bool sphere_vs_frustum(float3 center, float radius) { [unroll] for (int i = 0; i = num_objects) return; ObjectData obj = objects[idx]; float3 center = obj.bounding_sphere.xyz; float radius = obj.bounding_sphere.w; bool visible = (enable_culling == 0) || sphere_vs_frustum(center, radius); IndirectCommand cmd; cmd.num_indices = obj.num_indices; cmd.num_instances = visible ? 1 : 0; cmd.first_index = obj.first_index; cmd.vertex_offset = obj.vertex_offset; cmd.first_instance = idx; /* object index for storage buffer lookup */ indirect_commands[idx] = cmd; visibility[idx] = visible ? 1 : 0; } ``` ## Vertex shader with storage buffer lookup The vertex shader reads per-object transforms from a `StructuredBuffer` using the `object_id` attribute passed via the instance vertex buffer: ```hlsl struct ObjectTransform { column_major float4x4 model; float4 color; float4 bounding_sphere; uint num_indices; uint first_index; int vertex_offset; uint _pad; }; StructuredBuffer object_data : register(t0, space0); cbuffer VertUniforms : register(b0, space1) { column_major float4x4 vp; column_major float4x4 light_vp; }; struct VSInput { float3 position : TEXCOORD0; float3 normal : TEXCOORD1; float2 uv : TEXCOORD2; uint object_id : TEXCOORD3; /* from instance vertex buffer */ }; VSOutput main(VSInput input) { VSOutput output; ObjectTransform obj = object_data[input.object_id]; float4 world = mul(obj.model, float4(input.position, 1.0)); output.clip_pos = mul(vp, world); output.world_pos = world.xyz; output.color = obj.color; /* ... normal transform, shadow coords, etc. ... */ return output; } ``` ## Gribb-Hartmann frustum plane extraction Extract 6 frustum planes from the view-projection matrix. Each plane is `(nx, ny, nz, d)` where `dot(pos, n) + d` gives the signed distance. **Vulkan [0,1] depth convention:** The near plane is `row2` alone, not `row3 + row2` as in OpenGL's `[-1, 1]` depth range. Getting this wrong causes the near plane to be positioned incorrectly, culling objects that should be visible. ```c static void extract_frustum_planes(mat4 vp, float planes[6][4]) { float m[16]; SDL_memcpy(m, &vp, sizeof(m)); /* Left: row3 + row0 */ planes[0][0] = m[3] + m[0]; planes[0][1] = m[7] + m[4]; planes[0][2] = m[11] + m[8]; planes[0][3] = m[15] + m[12]; /* Right: row3 - row0 */ planes[1][0] = m[3] - m[0]; planes[1][1] = m[7] - m[4]; planes[1][2] = m[11] - m[8]; planes[1][3] = m[15] - m[12]; /* Bottom: row3 + row1 */ planes[2][0] = m[3] + m[1]; planes[2][1] = m[7] + m[5]; planes[2][2] = m[11] + m[9]; planes[2][3] = m[15] + m[13]; /* Top: row3 - row1 */ planes[3][0] = m[3] - m[1]; planes[3][1] = m[7] - m[5]; planes[3][2] = m[11] - m[9]; planes[3][3] = m[15] - m[13]; /* Near: row2 only (Vulkan [0,1] depth — NOT row3+row2) */ planes[4][0] = m[2]; planes[4][1] = m[6]; planes[4][2] = m[10]; planes[4][3] = m[14]; /* Far: row3 - row2 */ planes[5][0] = m[3] - m[2]; planes[5][1] = m[7] - m[6]; planes[5][2] = m[11] - m[10]; planes[5][3] = m[15] - m[14]; /* Normalize each plane for correct sphere-vs-plane distances */ for (int i = 0; i 0.0001f) { planes[i][0] /= len; planes[i][1] /= len; planes[i][2] /= len; planes[i][3] /= len; } } } ``` ## Compute dispatch The compute pass runs before any render passes. It reads object data, writes indirect commands and visibility flags: ```c /* Fill cull uniforms on the CPU */ CullUniforms cull_uniforms; SDL_zero(cull_uniforms); cull_uniforms.num_objects = NUM_OBJECTS; cull_uniforms.enable_culling = culling_enabled ? 1 : 0; extract_frustum_planes(cam_vp, cull_uniforms.frustum_planes); SDL_PushGPUComputeUniformData(cmd, 0, &cull_uniforms, sizeof(cull_uniforms)); /* Bind read-write storage buffers for the compute pass */ SDL_GPUStorageBufferReadWriteBinding rw_bindings[2]; SDL_zero(rw_bindings); rw_bindings[0].buffer = indirect_buf; rw_bindings[0].cycle = true; rw_bindings[1].buffer = visibility_buf; rw_bindings[1].cycle = true; SDL_GPUComputePass *compute = SDL_BeginGPUComputePass( cmd, NULL, 0, rw_bindings, 2); if (compute) { SDL_BindGPUComputePipeline(compute, cull_pipeline); /* Object data as read-only storage buffer */ SDL_GPUBuffer *ro_buf = object_data_buf; SDL_BindGPUComputeStorageBuffers(compute, 0, &ro_buf, 1); Uint32 groups = (NUM_OBJECTS + WORKGROUP_SIZE - 1) / WORKGROUP_SIZE; SDL_DispatchGPUCompute(compute, groups, 1, 1); SDL_EndGPUComputePass(compute); } ``` ## Indirect draw call After the compute pass fills the indirect buffer, the render pass consumes it. Bind the mesh vertex buffer in slot 0 and the instance ID buffer in slot 1, then issue a single indirect draw call for all objects: ```c SDL_BindGPUGraphicsPipeline(pass, indirect_pipeline); /* Push uniforms */ VertUniforms vu = { .vp = cam_vp, .light_vp = light_vp }; SDL_PushGPUVertexUniformData(cmd, 0, &vu, sizeof(vu)); /* Bind object data as vertex storage buffer */ SDL_GPUBuffer *obj_buf = object_data_buf; SDL_BindGPUVertexStorageBuffers(pass, 0, &obj_buf, 1); /* Bind mesh vertex buffer (slot 0) + instance ID buffer (slot 1) */ SDL_GPUBufferBinding vb[2]; SDL_zero(vb); vb[0].buffer = mesh_vertex_buffer; vb[1].buffer = instance_id_buf; SDL_BindGPUVertexBuffers(pass, 0, vb, 2); /* Bind index buffer */ SDL_GPUBufferBinding ib; SDL_zero(ib); ib.buffer = mesh_index_buffer; SDL_BindGPUIndexBuffer(pass, &ib, SDL_GPU_INDEXELEMENTSIZE_16BIT); /* THE KEY CALL: GPU reads NUM_OBJECTS draw commands from the indirect * buffer. Culled objects have num_instances=0 — hardware skips them * at zero cost, no CPU round-trip needed. */ SDL_DrawGPUIndexedPrimitivesIndirect(pass, indirect_buf, 0, NUM_OBJECTS); ``` ## Compute pipeline creation ```c #define WORKGROUP_SIZE 64 SDL_GPUComputePipelineCreateInfo comp_ci; SDL_zero(comp_ci); comp_ci.code = cull_shader_bytecode; comp_ci.code_size = cull_shader_size; comp_ci.entrypoint = "main"; comp_ci.format = shader_format; comp_ci.num_samplers = 0; comp_ci.num_readonly_storage_textures = 0; comp_ci.num_readonly_storage_buffers = 1; /* object data */ comp_ci.num_readwrite_storage_textures = 0; comp_ci.num_readwrite_storage_buffers = 2; /* indirect commands + visibility */ comp_ci.num_uniform_buffers = 1; /* frustum planes + flags */ comp_ci.threadcount_x = WORKGROUP_SIZE; comp_ci.threadcount_y = 1; comp_ci.threadcount_z = 1; SDL_GPUComputePipeline *cull_pipeline = SDL_CreateGPUComputePipeline(device, &comp_ci); ``` ## Key API calls - `SDL_DrawGPUIndexedPrimitivesIndirect()` — reads an array of indexed draw commands from a GPU buffer. Each command is 20 bytes (5 x uint32). - `SDL_BeginGPUComputePass()` — starts a compute pass with read-write storage buffer bindings for the indirect and visibility buffers. - `SDL_BindGPUComputeStorageBuffers()` — binds read-only storage buffers to compute shader slots. - `SDL_DispatchGPUCompute()` — launches compute workgroups. Use `(num_objects + WORKGROUP_SIZE - 1) / WORKGROUP_SIZE` groups. - `SDL_BindGPUVertexStorageBuffers()` — binds the object data storage buffer for the vertex shader to read per-instance transforms. ## Common mistakes - **Using `SV_InstanceID` instead of the instance vertex buffer** — `SV_InstanceID` does not include `first_instance` on all backends (notably Vulkan). The instance ID buffer with `first_instance` offset is the portable solution. - **Setting `instance_step_rate = 1`** — In SDL3 GPU, `instance_step_rate` must be 0 for all vertex buffer descriptions. Set `input_rate` to `SDL_GPU_VERTEXINPUTRATE_INSTANCE` instead. A non-zero step rate triggers an assertion failure in `SDL_CreateGPUGraphicsPipeline`. - **Using OpenGL near plane extraction** — OpenGL uses `[-1, 1]` depth, so the near plane is `row3 + row2`. SDL GPU uses Vulkan's `[0, 1]` depth, so the near plane is `row2` alone. Getting this wrong places the near plane at the wrong distance and culls visible objects. - **Forgetting to normalize frustum planes** — The sphere-vs-plane test `dot(center, n) + d` only produces correct world-space distances if the plane normal has unit length. Un-normalized planes cause incorrect culling at oblique angles. - **Missing INDIRECT usage flag** — The indirect draw buffer must have `SDL_GPU_BUFFERUSAGE_INDIRECT` in addition to `SDL_GPU_BUFFERUSAGE_COMPUTE_STORAGE_WRITE`. Without it, the buffer cannot be used as an indirect draw source. - **Missing GRAPHICS_STORAGE_READ on object data** — The object data buffer needs both `COMPUTE_STORAGE_READ` (for the cull shader) and `GRAPHICS_STORAGE_READ` (for the vertex shader). Missing the graphics flag prevents the vertex shader from reading transforms. - **Not cycling read-write buffers** — Set `cycle = true` on the compute pass read-write bindings to allow the GPU to double-buffer. Without cycling, the compute and render passes may contend on the same memory. ## Cross-references - [GPU Lesson 38 — Indirect Drawing](../../../lessons/gpu/38-indirect-drawing/) for the full walkthrough with dual-camera split-screen debug visualization - [GPU Lesson 13 — Instanced Rendering](../../../lessons/gpu/13-instanced-rendering/) for the basic instanced rendering pattern that indirect drawing extends - [GPU Lesson 11 — Compute Shaders](../../../lessons/gpu/11-compute-shaders/) for compute pipeline fundamentals - [Math Lesson 05 — Matrices](../../../lessons/math/05-matrices/) for the view-projection matrix that frustum planes are extracted from ## Source & license This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code. - **Author:** [Nebulavenus](https://github.com/Nebulavenus) - **Source:** [Nebulavenus/forge-gpu](https://github.com/Nebulavenus/forge-gpu) - **License:** Zlib Install and usage instructions live in the source repository linked above. ## Pricing - **Free** — Free ## Security capabilities Automated source analysis of v0.1.0 — what this tool can access: - **Network access:** no - **Filesystem access:** no - **Shell / process execution:** no - **Environment & secrets:** no - **Dynamic code execution:** no *"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.* ## Versions - **0.1.0** — security scan: passed — Imported from the upstream source. ## Links - Listing page: https://agentstack.voostack.com/l/skill-nebulavenus-forge-gpu-forge-indirect-drawing - Seller: https://agentstack.voostack.com/s/nebulavenus - Browse the marketplace: https://agentstack.voostack.com/browse --- Listed on AgentStack — the marketplace for AI agent skills and MCP servers. 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