QuaternionEngine/docs/IBL.md

Image-Based Lighting (IBL)

Overview

• IBL assets (environment maps + BRDF LUT + SH coefficients) are managed by IBLManager (src/core/assets/ibl_manager.{h,cpp}) and exposed to passes via EngineContext::ibl.
• Shaders share a common include, shaders/ibl_common.glsl, which defines the IBL bindings for descriptor set 3 and helper functions used by deferred, forward, and background passes.
• The engine currently supports:
  • Specular environment from an equirectangular 2D texture with prefiltered mips (sampler2D iblSpec2D).
  • Diffuse irradiance from 2nd‑order SH (9 coefficients baked on the CPU).
  • A 2D BRDF integration LUT used for the split‑sum approximation.

Data Flow

• Init:
  • VulkanEngine::init_vulkan() creates an IBLManager, calls init(context), and publishes it via EngineContext::ibl.
  • The engine optionally loads default IBL assets (IBLPaths in src/core/engine.cpp), typically a BRDF LUT plus a specular environment .ktx2.
• Loading (IBLManager):
  • IBLManager::load(const IBLPaths&) (synchronous, mostly used in tools/tests):
    • Specular:
      • Tries ktxutil::load_ktx2_cubemap first. If successful, uploads via ResourceManager::create_image_compressed_layers with VK_IMAGE_CREATE_CUBE_COMPATIBLE_BIT.
      • If cubemap loading fails, falls back to 2D .ktx2 via ktxutil::load_ktx2_2d and create_image_compressed. The image is treated as equirectangular with prefiltered mips.
      • When the specular .ktx2 is HDR (R16G16B16A16_SFLOAT or R32G32B32A32_SFLOAT) and 2:1 aspect, IBLManager computes 9 SH coefficients on the CPU:
        • Integrates the environment over the sphere using real SH basis functions (L2) with solid‑angle weighting.
        • Applies Lambert band scaling (A0 = π, A1 = 2π/3, A2 = π/4).
        • Uploads the result as vec4 sh[9] in a uniform buffer (_shBuffer).
    • Diffuse:
      • If IBLPaths::diffuseCube is provided and valid, loads it as a cubemap via load_ktx2_cubemap + create_image_compressed_layers.
      • Current shaders only use the SH buffer for diffuse; the diffuse cubemap is reserved for future variants.
    • BRDF LUT:
      • Loaded as 2D .ktx2 via ktxutil::load_ktx2_2d and uploaded with create_image_compressed.
    • Fallbacks:
      • If diffuseCube is missing but a specular env exists, _diff is aliased to _spec.
  • IBLManager::load_async(const IBLPaths&) + IBLManager::pump_async() (runtime path used by the engine):
    • load_async runs KTX2 file I/O and SH bake on a worker thread and stores a prepared CPU-side description (PreparedIBLData).
    • pump_async is called on the main thread once per frame (after the previous frame is idle) to:
      • Destroy old IBL images/SH via destroy_images_and_sh().
      • Create new GPU images with create_image_compressed(_layers) and upload the SH buffer.
    • This avoids stalls in the main/game loop when switching IBL volumes or loading the default environment at startup.
  • IBLManager::unload() releases GPU images, the SH buffer, and the descriptor set layout.
• Descriptor layout:
  • IBLManager::ensureLayout() builds a descriptor set layout (set=3) with:
    • binding 0: COMBINED_IMAGE_SAMPLER — specular environment (2D equirect).
    • binding 1: COMBINED_IMAGE_SAMPLER — BRDF LUT 2D.
    • binding 2: UNIFORM_BUFFER — SH coefficients (vec4 sh[9]).
  • Passes request this layout from EngineContext::ibl and plug it into their pipeline set layouts:
    • Background: vk_renderpass_background.cpp (set 3 used for env background).
    • Lighting: vk_renderpass_lighting.cpp (deferred lighting pass, set 3).
    • Transparent: vk_renderpass_transparent.cpp (forward/transparent materials, set 3).

Shader Side (shaders/ibl_common.glsl)

• Bindings:
  • layout(set=3, binding=0) uniform sampler2D iblSpec2D;
  • layout(set=3, binding=1) uniform sampler2D iblBRDF;
  • layout(std140, set=3, binding=2) uniform IBL_SH { vec4 sh[9]; } iblSH;
• Helpers:
  • vec3 sh_eval_irradiance(vec3 n):
    • Evaluates the 9 SH basis functions (L2) at direction n using the same real SH basis as the CPU bake.
    • Multiplies each basis value by the corresponding iblSH.sh[i].rgb coefficient and sums the result.
    • Coefficients are already convolved with the Lambert kernel on the CPU; the function returns diffuse irradiance directly.
  • vec2 dir_to_equirect(vec3 d):
    • Normalizes d, computes (phi, theta) and returns equirectangular UV in [0,1]².
    • Used consistently by background, deferred, and forward pipelines.
  • float ibl_lod_from_roughness(float roughness, float levels):
    • Computes the mip LOD for specular IBL using roughness² * (levels - 1).
    • This biases mid‑roughness reflections towards blurrier mips and avoids overly sharp reflections.

Usage in Passes

• Deferred lighting (shaders/deferred_lighting.frag and shaders/deferred_lighting_nort.frag):
  • Include:
    • #include "input_structures.glsl"
    • #include "ibl_common.glsl"
  • IBL contribution (per pixel):
    • Specular:
      • vec3 R = reflect(-V, N);
      • float levels = float(textureQueryLevels(iblSpec2D));
      • float lod = ibl_lod_from_roughness(roughness, levels);
      • vec2 uv = dir_to_equirect(R);
      • vec3 prefiltered = textureLod(iblSpec2D, uv, lod).rgb;
      • vec2 brdf = texture(iblBRDF, vec2(max(dot(N,V),0.0), roughness)).rg;
      • vec3 specIBL = prefiltered * (F0 * brdf.x + brdf.y);
    • Diffuse:
      • vec3 diffIBL = (1.0 - metallic) * albedo * sh_eval_irradiance(N);
    • Combined:
      • color += diffIBL + specIBL;
• Forward/transparent (shaders/mesh.frag):
  • Same include and IBL logic as deferred, applied after direct lighting.
  • Uses the same ibl_lod_from_roughness helper for LOD selection.
• Background (shaders/background_env.frag):
  • Includes ibl_common.glsl and uses dir_to_equirect(worldDir) + textureLod(iblSpec2D, uv, 0.0) to render the environment at LOD 0.

Authoring IBL Assets

• Specular environment:
  • Preferred: prefiltered HDR cubemap in .ktx2 (BC6H or R16G16B16A16_SFLOAT) with multiple mips.
  • Alternative: prefiltered equirectangular 2D .ktx2 with width = 2 × height and full mip chain.
  • Make sure the mip chain is generated with a GGX importance sampling tool so the BRDF LUT + mip chain match.
• BRDF LUT:
  • A standard 2D preintegrated GGX LUT (RG), usually stored as R8G8_UNORM or BC5.
  • The LUT is sampled with (NoV, roughness) coordinates.
• Diffuse:
  • The engine currently uses SH coefficients baked from the specular equirectangular map. If you provide a separate diffuse cubemap, the CPU SH bake still uses the specular HDR; you can adjust this in IBLManager if you want SH to come from a different source.

Implementation Notes

• CPU SH bake:
  • Implemented in IBLManager::load using libktx to access raw HDR pixel data from .ktx2.
  • Uses a simple nested loop over pixels with solid‑angle weighting and the same SH basis as sh_eval_irradiance.
• Fallbacks:
  • Lighting and transparent passes create small fallback textures so that the IBL descriptor set is always valid, even when no IBL assets are loaded.
  • Background pass builds a 1×1×6 black cube as a fallback env.