forge-pbr-shading

name	forge-pbr-shading
description	Add Cook-Torrance PBR shading with GGX, Schlick-GGX, and Schlick Fresnel to an SDL GPU project alongside forge_scene.h

Add physically-based rendering to an SDL GPU project. Implements a Cook-Torrance microfacet BRDF fragment shader that uses the same texture bindings and uniform layout as forge_scene.h's built-in model pipeline, enabling side-by-side comparison with Blinn-Phong.

When to use

• Replacing Blinn-Phong with physically-based shading
• Adding metallic-roughness material support
• Comparing different lighting models on the same geometry
• Creating a custom pipeline that coexists with forge_scene.h

Key API calls

• forge_scene_create_shader() — create vertex/fragment shaders from bytecode
• SDL_CreateGPUGraphicsPipeline() — create custom pipeline matching model vertex layout
• SDL_BindGPUFragmentSamplers() — bind per-material textures with fallbacks
• SDL_PushGPUFragmentUniformData() — push material uniform data to fragment shader
• forge_scene_load_model() — load pipeline-processed model
• forge_scene_draw_model() — draw with built-in Blinn-Phong for comparison

Correct order

1. Load model via forge_scene_load_model()
2. Create PBR fragment shader from compiled bytecode
3. Create vertex shader from scene_model_vert_spirv/dxil/msl (available via FORGE_SCENE_IMPLEMENTATION)
4. Create pipeline with model vertex layout using sizeof(ForgeSceneModelVertex) for pitch and offsetof() for attribute offsets
5. Release shaders after pipeline creation
6. Per frame: bind custom pipeline, iterate model submeshes, push uniforms with overridden PBR parameters, bind textures, draw

Cook-Torrance BRDF components

/* GGX Normal Distribution */
float D = alpha2 / (PI * pow(NdotH * NdotH * (alpha2 - 1.0) + 1.0, 2.0));

/* Schlick-GGX Geometry (Smith method) */
float k = (roughness + 1.0) * (roughness + 1.0) / 8.0;
float G = (NdotV / (NdotV * (1-k) + k)) * (NdotL / (NdotL * (1-k) + k));

/* Schlick Fresnel */
float3 F = F0 + (1.0 - F0) * pow(1.0 - VdotH, 5.0);

/* Full specular: D * G * F / (4 * NdotV * NdotL) */
/* Energy-conserving diffuse: (1 - F) * (1 - metallic) * albedo / PI */

Common mistakes

• F0 for metals must use albedo color — lerp(0.04, albedo, metallic), not a fixed 0.04 for all materials
• alpha = roughness^2, not roughness — GGX uses squared roughness
• k differs for direct vs IBL lighting — direct: (r+1)^2/8, IBL: r^2/2
• Divide-by-zero guard — clamp NdotV above zero (e.g. 0.001) in the Cook-Torrance denominator
• Diffuse must divide by pi — without this, the surface reflects more energy than it receives

Reference

See Lesson 51 — PBR Shading Model for the full implementation.