102 lines
6.5 KiB
Markdown
102 lines
6.5 KiB
Markdown
Image-Based Lighting (IBL)
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Overview
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- 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`.
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- 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.
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- The engine currently supports:
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- Specular environment from an equirectangular 2D texture with prefiltered mips (`sampler2D iblSpec2D`).
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- Diffuse irradiance from 2nd‑order SH (9 coefficients baked on the CPU).
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- A 2D BRDF integration LUT used for the split‑sum approximation.
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Data Flow
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- Init:
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- `VulkanEngine::init_vulkan()` creates an `IBLManager`, calls `init(context)`, and publishes it via `EngineContext::ibl`.
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- The engine optionally loads default IBL assets (`IBLPaths` in `src/core/engine.cpp`), typically a BRDF LUT plus a specular environment `.ktx2`.
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- Loading (IBLManager):
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- `IBLManager::load(const IBLPaths&)`:
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- Specular:
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- Tries `ktxutil::load_ktx2_cubemap` first. If successful, uploads via `ResourceManager::create_image_compressed_layers` with `VK_IMAGE_CREATE_CUBE_COMPATIBLE_BIT`.
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- 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.
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- When the specular `.ktx2` is HDR (`R16G16B16A16_SFLOAT` or `R32G32B32A32_SFLOAT`) and 2:1 aspect, `IBLManager` computes 9 SH coefficients on the CPU:
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- Integrates the environment over the sphere using real SH basis functions (L2) with solid‑angle weighting.
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- Applies Lambert band scaling (A0 = π, A1 = 2π/3, A2 = π/4).
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- Uploads the result as `vec4 sh[9]` in a uniform buffer (`_shBuffer`).
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- Diffuse:
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- If `IBLPaths::diffuseCube` is provided and valid, loads it as a cubemap via `load_ktx2_cubemap` + `create_image_compressed_layers`.
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- Current shaders only use the SH buffer for diffuse; the diffuse cubemap is reserved for future variants.
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- BRDF LUT:
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- Loaded as 2D `.ktx2` via `ktxutil::load_ktx2_2d` and uploaded with `create_image_compressed`.
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- Fallbacks:
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- If `diffuseCube` is missing but a specular env exists, `_diff` is aliased to `_spec`.
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- `IBLManager::unload()` releases GPU images, the SH buffer, and the descriptor set layout.
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- Descriptor layout:
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- `IBLManager::ensureLayout()` builds a descriptor set layout (set=3) with:
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- binding 0: `COMBINED_IMAGE_SAMPLER` — specular environment (2D equirect).
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- binding 1: `COMBINED_IMAGE_SAMPLER` — BRDF LUT 2D.
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- binding 2: `UNIFORM_BUFFER` — SH coefficients (`vec4 sh[9]`).
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- Passes request this layout from `EngineContext::ibl` and plug it into their pipeline set layouts:
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- Background: `vk_renderpass_background.cpp` (set 3 used for env background).
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- Lighting: `vk_renderpass_lighting.cpp` (deferred lighting pass, set 3).
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- Transparent: `vk_renderpass_transparent.cpp` (forward/transparent materials, set 3).
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Shader Side (`shaders/ibl_common.glsl`)
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- Bindings:
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- `layout(set=3, binding=0) uniform sampler2D iblSpec2D;`
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- `layout(set=3, binding=1) uniform sampler2D iblBRDF;`
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- `layout(std140, set=3, binding=2) uniform IBL_SH { vec4 sh[9]; } iblSH;`
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- Helpers:
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- `vec3 sh_eval_irradiance(vec3 n)`:
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- Evaluates the 9 SH basis functions (L2) at direction `n` using the same real SH basis as the CPU bake.
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- Multiplies each basis value by the corresponding `iblSH.sh[i].rgb` coefficient and sums the result.
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- Coefficients are already convolved with the Lambert kernel on the CPU; the function returns diffuse irradiance directly.
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- `vec2 dir_to_equirect(vec3 d)`:
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- Normalizes `d`, computes `(phi, theta)` and returns equirectangular UV in `[0,1]²`.
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- Used consistently by background, deferred, and forward pipelines.
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- `float ibl_lod_from_roughness(float roughness, float levels)`:
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- Computes the mip LOD for specular IBL using `roughness² * (levels - 1)`.
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- This biases mid‑roughness reflections towards blurrier mips and avoids overly sharp reflections.
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Usage in Passes
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- Deferred lighting (`shaders/deferred_lighting.frag` and `shaders/deferred_lighting_nort.frag`):
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- Include:
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- `#include "input_structures.glsl"`
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- `#include "ibl_common.glsl"`
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- IBL contribution (per pixel):
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- Specular:
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- `vec3 R = reflect(-V, N);`
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- `float levels = float(textureQueryLevels(iblSpec2D));`
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- `float lod = ibl_lod_from_roughness(roughness, levels);`
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- `vec2 uv = dir_to_equirect(R);`
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- `vec3 prefiltered = textureLod(iblSpec2D, uv, lod).rgb;`
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- `vec2 brdf = texture(iblBRDF, vec2(max(dot(N,V),0.0), roughness)).rg;`
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- `vec3 specIBL = prefiltered * (F0 * brdf.x + brdf.y);`
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- Diffuse:
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- `vec3 diffIBL = (1.0 - metallic) * albedo * sh_eval_irradiance(N);`
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- Combined:
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- `color += diffIBL + specIBL;`
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- Forward/transparent (`shaders/mesh.frag`):
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- Same include and IBL logic as deferred, applied after direct lighting.
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- Uses the same `ibl_lod_from_roughness` helper for LOD selection.
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- Background (`shaders/background_env.frag`):
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- Includes `ibl_common.glsl` and uses `dir_to_equirect(worldDir)` + `textureLod(iblSpec2D, uv, 0.0)` to render the environment at LOD 0.
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Authoring IBL Assets
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- Specular environment:
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- Preferred: prefiltered HDR cubemap in `.ktx2` (BC6H or `R16G16B16A16_SFLOAT`) with multiple mips.
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- Alternative: prefiltered equirectangular 2D `.ktx2` with width = 2 × height and full mip chain.
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- Make sure the mip chain is generated with a GGX importance sampling tool so the BRDF LUT + mip chain match.
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- BRDF LUT:
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- A standard 2D preintegrated GGX LUT (RG), usually stored as `R8G8_UNORM` or BC5.
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- The LUT is sampled with `(NoV, roughness)` coordinates.
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- Diffuse:
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- 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.
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Implementation Notes
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- CPU SH bake:
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- Implemented in `IBLManager::load` using libktx to access raw HDR pixel data from `.ktx2`.
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- Uses a simple nested loop over pixels with solid‑angle weighting and the same SH basis as `sh_eval_irradiance`.
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- Fallbacks:
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- Lighting and transparent passes create small fallback textures so that the IBL descriptor set is always valid, even when no IBL assets are loaded.
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- Background pass builds a 1×1×6 black cube as a fallback env.
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