QuaternionEngine/docs/materials.md

Materials and Textures Overview (PBR)

Current state (as of Nov 1, 2025)

• PBR textures bound per material (set=1):
  • binding=0: GLTFMaterialData (UBO)
  • binding=1: colorTex (albedo/base color) — sRGB
  • binding=2: metalRoughTex (G=roughness, B=metallic) — UNORM
  • binding=3: normalMap (tangent-space normal, UNORM)
• G‑Buffer writes world‑space normals. Tangent‑space normal maps are decoded with TBN using a sign‑correct bitangent (B = sign * cross(N, T)).
• Numeric fallbacks via MaterialConstants (CPU) / GLTFMaterialData (GPU):
  • colorFactors (RGBA). Defaults to 1 if zero.
  • metal_rough_factors (X=metallic, Y=roughness). Roughness is clamped to ≥ 0.04 in shaders.
  • extra[0].x = normalScale (scalar, default 1.0). Multiplies the XY of decoded normal.
• Defaults when a texture is missing:
  • Albedo → checkerboard error texture
  • MR → white (no effect)
  • Normal → 1×1 flat normal (0.5, 0.5, 1.0)

Implications for primitive meshes

• Primitives can use:
  • Albedo + MR + Normal textures, or
  • Numeric factors only, or
  • Any mix (missing textures fall back to defaults).

Texture conventions

• Albedo/base color: sRGB.
• Metallic-Roughness: UNORM; channels: G=roughness, B=metallic.
• Normal map: UNORM; expected +Y (green up). If your maps look inverted, flip the green channel offline.

Notes on tangent space

• Tangents are loaded from glTF when present (TANGENT attribute, vec4 where w is handedness).
• If missing, the engine generates tangents:
  • Default: robust Gram–Schmidt with handedness.
  • Preferred: MikkTSpace (enabled by CMake option ENABLE_MIKKTS=ON).

Notes on MikkTSpace

• Recommended for parity with content tools. Enable at configure time: -DENABLE_MIKKTS=ON.
• Falls back automatically to the internal generator if disabled or if MikkTS fails.

Usage Examples

• Adjust normal strength per material: set material.constants.extra[0].x (CPU) or normalTexture.scale in glTF.
• Primitive with PBR textures:
  • Set MeshMaterialDesc::Kind::Textured and fill albedoPath, metalRoughPath, and normalPath.