Agent skill
forge-environment-mapping
Add cube map environment mapping with a skybox and reflective surfaces. Use when someone needs a 360-degree background, reflection mapping, or cube map textures in SDL3 GPU.
Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/other/other/forge-environment-mapping
SKILL.md
Environment Mapping — SDL3 GPU Skill
Add a cube map skybox and environment reflections to a 3D scene.
When to use this skill
- Rendering a 360-degree background (space, sky, indoor environment)
- Adding reflections to metallic or glossy surfaces
- Working with
SDL_GPU_TEXTURETYPE_CUBEtextures - Combining multiple graphics pipelines in one render pass
Cube map texture creation
/* Create a cube map texture with 6 faces. */
SDL_GPUTextureCreateInfo tex_info;
SDL_zero(tex_info);
tex_info.type = SDL_GPU_TEXTURETYPE_CUBE;
tex_info.format = SDL_GPU_TEXTUREFORMAT_R8G8B8A8_UNORM_SRGB;
tex_info.usage = SDL_GPU_TEXTUREUSAGE_SAMPLER;
tex_info.width = face_size;
tex_info.height = face_size;
tex_info.layer_count_or_depth = 6; /* MUST be 6 for cube maps */
tex_info.num_levels = 1;
SDL_GPUTexture *cubemap = SDL_CreateGPUTexture(device, &tex_info);
/* Upload each face as a separate layer (0=+X, 1=-X, 2=+Y, 3=-Y, 4=+Z, 5=-Z). */
for (int face = 0; face < 6; face++) {
/* Load face image, convert to ABGR8888, map transfer buffer... */
SDL_GPUTextureRegion dst;
SDL_zero(dst);
dst.texture = cubemap;
dst.layer = (Uint32)face; /* Face index = layer index */
dst.w = face_size;
dst.h = face_size;
dst.d = 1;
SDL_UploadToGPUTexture(copy_pass, &src, &dst, false);
}
Skybox rendering pattern
Vertex shader — depth = 1.0
cbuffer VertUniforms : register(b0, space1)
{
column_major float4x4 vp_no_translation;
};
struct VSOutput
{
float4 clip_pos : SV_Position;
float3 direction : TEXCOORD0;
};
VSOutput main(VSInput input)
{
VSOutput output;
output.direction = input.position;
float4 pos = mul(vp_no_translation, float4(input.position, 1.0));
output.clip_pos = pos.xyww; /* z = w → depth = 1.0 after divide */
return output;
}
Fragment shader — sample cube map
TextureCube skybox_tex : register(t0, space2);
SamplerState smp : register(s0, space2);
float4 main(PSInput input) : SV_Target
{
return skybox_tex.Sample(smp, input.direction);
}
Pipeline settings
/* Cull FRONT faces — we're inside the cube. */
pipe_info.rasterizer_state.cull_mode = SDL_GPU_CULLMODE_FRONT;
/* Depth test LESS_OR_EQUAL to pass at depth=1.0.
* Depth write DISABLED — skybox should never occlude anything. */
pipe_info.depth_stencil_state.enable_depth_test = true;
pipe_info.depth_stencil_state.enable_depth_write = false;
pipe_info.depth_stencil_state.compare_op = SDL_GPU_COMPAREOP_LESS_OR_EQUAL;
Rotation-only view matrix
/* Strip translation so the skybox always surrounds the camera. */
mat4 view_rot = view;
view_rot.m[12] = 0.0f;
view_rot.m[13] = 0.0f;
view_rot.m[14] = 0.0f;
mat4 vp_sky = mat4_multiply(proj, view_rot);
Reflection mapping pattern
Fragment shader with reflections
TextureCube env_tex : register(t1, space2);
/* Compute reflection direction */
float3 V = normalize(eye_pos.xyz - world_pos);
float3 R = reflect(-V, N);
float3 env_color = env_tex.Sample(env_smp, R).rgb;
/* Blend diffuse with environment reflection */
float3 blended = lerp(surface_color.rgb, env_color, reflectivity);
Render order
- Clear depth to 1.0
- Draw skybox (depth write off, passes at depth=1.0)
- Draw objects (depth write on, depth < 1.0 always passes)
Common mistakes
-
Forgetting
layer_count_or_depth = 6— cube maps require exactly 6 layers. Without this, texture creation fails silently or renders black. -
Wrong face order —
SDL_GPUCubeMapFaceenum order is +X, -X, +Y, -Y, +Z, -Z. Mixing up faces produces seams or incorrect reflections. -
Depth write enabled on skybox — the skybox writes depth=1.0 everywhere, occluding all subsequent geometry. Always disable depth write for the skybox.
-
Using full view matrix for skybox — includes camera translation, causing visible parallax when moving. Strip translation columns (m[12], m[13], m[14]).
-
Wrong cull mode for skybox — the camera is inside the cube, so front faces point inward (toward the camera). Cull front faces, not back faces.
-
Not negating V in reflect() —
reflect()expects the incident vector pointing toward the surface. The view direction V points away from the surface, so pass-V.
File layout
shaders/
├── skybox.vert.hlsl # Unit cube → clip space with depth=1.0
├── skybox.frag.hlsl # Sample TextureCube
├── object.vert.hlsl # Standard MVP + model (same as lighting)
└── object.frag.hlsl # Blinn-Phong + reflect() + cube map sample
References
- Lesson 14 — Environment Mapping
- Lesson 10 — Basic Lighting (Blinn-Phong)
- Math Lesson 01 — Vectors (dot product, reflection)
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