/* pbr.glsl -- Contains everything you need for PBR
 *
 * Copyright (c) 2025-2026 Le Juez Victor
 *
 * This software is provided 'as-is', without any express or implied warranty.
 * For conditions of distribution and use, see accompanying LICENSE file.
 */

#include "./math.glsl"

/* === Functions === */

float PBR_DistributionGGX(float cosTheta, float alpha)
{
    // Standard GGX/Trowbridge-Reitz distribution - optimized form

    float a = cosTheta * alpha;
    float k = alpha / (1.0 - cosTheta * cosTheta + a * a);
    return k * k * (1.0 / M_PI);
}

float PBR_GeometryGGX(float NdotL, float NdotV, float roughness)
{
    // Hammon's optimized approximation for GGX Smith geometry term
    // This version is an efficient approximation that:
    // 1. Avoids expensive square root calculations
    // 2. Combines both G1 terms into a single expression
    // 3. Provides very close results to the exact version at a much lower cost
    // SEE: https://www.gdcvault.com/play/1024478/PBR-Diffuse-Lighting-for-GGX

    return 0.5 / mix(2.0 * NdotL * NdotV, NdotL + NdotV, roughness);
}

float PBR_SchlickFresnel(float u)
{
    float m = 1.0 - u;
    float m2 = m * m;
    return m2 * m2 * m; // pow(m,5)
}

vec3 PBR_ComputeF0(float metallic, float specular, vec3 albedo)
{
    // use (albedo * metallic) as colored specular reflectance at 0 angle for metallic materials
    // SEE: https://google.github.io/filament/Filament.md.html

    float dielectric = 0.16 * specular * specular;
    return mix(vec3(dielectric), albedo, vec3(metallic));
}