BSDF #

float ior;          // 折射率，空气≈1.0，玻璃≈1.5
float transmission; // 透射能量占比（“透明度”）

// 维护透射变量
float etaExt = 1.0;     // 外侧的 η­­
float etaInt = 1.0;     // 进入物体的 η
bool  inside = false;   // 当前光线是否在介质内

float transmission = mat.transmission;
if(!inside) {
    etaInt = mat.ior;       // 若在外侧（空气中）准备进入物体
}
bool sampledTransmission = false;   // 此次反弹有没有采样透射

// --- 用Fresnel估算各分支能量占比---
vec3 F0 = mix(vec3(0.04), albedo, metallic);
vec3 F_approx;
if(metallic > 0.0)
    F_approx = fresnelSchlickRoughness(clamp(dot(N, V), 0.0, 1.0), F0, roughness);
else
    F_approx = vec3(fresnelDielectric(clamp(dot(N, V), 0.0, 1.0), etaExt, etaInt));

// 镜面 / 透射 / 漫反射 强度（标量化）
float wS = dot(F_approx, vec3(0.2126,0.7152,0.0722));              // 镜面（亮度）
float wT = (1.0 - metallic) * transmission
         * dot(vec3(1.0) - F_approx, vec3(0.2126,0.7152,0.0722));   // 透射
float wD = (1.0 - metallic) * (1.0 - transmission)
         * dot(vec3(1.0) - F_approx, vec3(0.2126,0.7152,0.0722));   // 漫反射
float sumW = wS + wD + wT + 1e-6;

// 混合采样分支概率 （让 选择权重 与能量占比一致）
float  pdfSelSpec    = wS / sumW;   // 镜面分支
float  pdfSelTrans   = wT / sumW;   // 透射分支
float  pdfSelDiffuse = wD / sumW;   // 漫反射分支

if (rTech < pdfSelSpec)      // --- 镜面分支：GGX importance sample ---
{
    H = ImportanceSampleGGX(Xi, N, roughness);
    L2     = normalize(reflect(-V, H));                               
} else if(rTech < pdfSelSpec + pdfSelDiffuse) // --- 漫反射分支：余弦加权 ---
{
    L2     = cosineSampleHemisphere(Xi, N);
    H      = normalize(V + L2); // 计算 H 向量    
} else                         // --- 透射分支：GGX importance sample ---
{
    sampledTransmission = true;
    H = ImportanceSampleGGX(Xi, N, roughness);
    float eta = etaExt / etaInt;
    vec3 T = refract(-V, H, eta);
    if (length(T) == 0.0) {      // 回退为漫反射/镜面分支
        sampledTransmission = false;
        L2     = normalize(reflect(-V, H));  
    }else{
        L2     = T;
        H = normalize(V * eta + T);  // 透射 Half-vector 定义
        if (dot(N, H) < 0.0) H = -H;    // 保证 N·H ≥ 0
    }
}

if(sampledTransmission){
    vec3 F = vec3(fresnelDielectric(cosTheta, etaExt, etaInt)); 
    // 透射项
    // ---Physically Based Rendering 4ed (eq 9.37 & 9.40)---
    float eta = etaExt / etaInt;
    float VdotH = dot(V, H);
    float HdotL2 = dot(H, L2);
    float denom = (VdotH + HdotL2/eta);
    denom       = max(1e-4, denom*denom);
    float nom = abs(dot(V,N)) * abs(dot(L2,N));
    vec3 trans  = abs(VdotH) * abs(HdotL2) * albedo * (1.0 - F) * G * NDF / (denom * nom);  // (eq 9.40)
    float NdotH = abs(dot(N, H));
    float pdfTrans = (NDF * NdotH * abs(dot(L2,H))) / denom; //(eq 9.37)

    // ---简化近似版---
    // float transmissionWeight = transmission      
    //          * (1.0 - metallic); 
    // // 可给玻璃一点着色 (albedo)，也可用 vec3(1.0)
    // vec3 trans = transmissionWeight * vec3(1.0) / PI;
    //float pdfTrans = abs(dot(N, L2)) / PI; 

    f_bsdf = trans;
    pdfCombined = pdfSelTrans * pdfTrans;
    misW = 1.0f;
}

// 接上
else{
    vec3 F;
    if(metallic > 0.0)
        F = fresnelSchlickRoughness(cosTheta, F0, roughness);
        
    else
        F = vec3(fresnelDielectric(cosTheta, etaExt, etaInt));
    vec3 spec   = (NDF * G * F) / (4.0 * NdotV * NdotL2 + 1e-4);
    vec3 kD = vec3(1.0) - F;
    kD *= (1.0 - metallic) * (1.0 - transmission);    
    vec3 diff   = kD * albedo / PI;
    f_bsdf = spec + diff;
    float NdotH = max(dot(N, H), 0.0);
    float HdotV = max(dot(H, V), 0.0);
    float pdfSpec = NDF * NdotH / (4.0 * HdotV + 1e-4);     
    float pdfDiff      = abs(dot(N, L2)) / PI;  
    pdfCombined = pdfSelSpec*pdfSpec + pdfSelDiffuse*pdfDiff;
    misW    = (pdfCombined*pdfCombined) /
              (pdfCombined*pdfCombined + pdfLight*pdfLight);
}

// 透射更新
if (sampledTransmission) {
    inside = !inside;
    // 交换内外折射率，供下一次 bounce 使用
    float tmp = etaExt;
    etaExt = etaInt;
    etaInt = tmp;
}

struct DirectionalLight {
    vec3 direction; // 单位向量
    vec3 color;
    float intensity;
    float angularRadius;
};

LightSample sampleDirectionalLight(vec2 Xi)  
{
    DirectionalLight d = directionalLights[0];              // 仅支持 1 个方向光
    float thetaMax     = d.angularRadius;                    // 半角
    float cosThetaMax  = cos(thetaMax);
    float coneSolidAngle = 2.0 * PI * (1.0 - cosThetaMax);

    LightSample ls;
    ls.radiance = d.color * d.intensity / coneSolidAngle; 

    // 阴影：在圆锥内均匀采样
    float pdfSA;
    vec3  lDir = sampleCone(-d.direction, cosThetaMax, Xi, pdfSA);  // 指向光源，锥形采样
    ls.pos      = vec3(0.0);      // “无限远”光源，位置无意义
    ls.normal   = lDir;      
    ls.pdfArea  = pdfSA;            // solid-angle PDF
    ls.isDirectional = 1;         // 方向光
    return ls;
}

if (!hitInfo.hit) {        
    // 将光线方向转换为等距柱状投影的UV坐标
    // phi (longitude): 范围 [-PI, PI]，从X轴开始
    // theta (latitude): 范围 [0, PI]，从Y轴开始
    float phi   = atan(rayDir.z, rayDir.x); // 
    float theta = acos(rayDir.y);  

    // u: [-PI, PI] -> [0, 1]
    // v: [0, PI] -> [0, 1]
    float u = phi / (2.0 * PI) + 0.5; 
    float v = theta / PI;    

    vec3 envRadiance = texture(textures[skyboxIndex], vec2(u, v)).rgb;
    radiance += throughput * envRadiance;
    break;
}