在learnopengl specular IBL tutorial中，为了解决预过滤器卷积中的亮点，教程参考this article和预过滤器计算如下：

float3 PreFilterEnvMap(TextureCube envMap,sampler samEnv,float roughness,float3 R)
{
    float3 res = (float3)0.0f;  
    float totalWeight = 0.0f;   

    float3 normal = normalize(R);
    float3 toEye = normal;

    //roughness = max(0.02f,roughness);

    static const uint NUM_SAMPLES = 512;
    for(uint i=0;i<NUM_SAMPLES;++i)
    {
        float2 xi = hammersley_seq(i,NUM_SAMPLES); 

        float3 halfway = ImportanceSampleGGX(xi,roughness,normal);
        float3 lightVec = 2.0f * dot( toEye,halfway ) * halfway - toEye;

        float NdotL = saturate ( dot( normal,lightVec ) ) ;
        //float NdotV = saturate ( dot( normal,toEye ) ) ;
        float NdotH =  saturate ( dot( normal,halfway ) ) ;
        float HdotV = saturate ( dot( halfway,toEye ) ) ;

        if( NdotL > 0 )
        {
            float D = DFactor(roughness,NdotH);
            float pdf = (D * NdotH / (4 * HdotV)) + 0.0001f  ;

            float saTexel = 4.0f * CH_PI / (6.0f * CONV_SPEC_TEX_WIDTH * CONV_SPEC_TEX_WIDTH);
            float saSample = 1.0f / (NUM_SAMPLES * pdf + 0.00001f);

            float mipLevel = roughness == 0.0f ? 0.0f :  0.5f * log2( saSample / saTexel )  ;

            res += envMap.SampleLevel( samEnv,lightVec,mipLevel ).rgb *NdotL;     
            totalWeight += NdotL;
        }
    }

    return res / max(totalWeight,0.001f);
} 

这是我的代码：

#version 330 core
out vec4 FragColor;

in vec3 localPos;

uniform float roughness;
uniform samplerCube environmentMap;

const float Pi = 3.14159265359;

float RadicalInverse_VdC(uint bits){
    bits = (bits << 16u) | (bits >> 16u);
    bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);
    bits = ((bits & 0x33333333u) << 2u) | ((bits & 0xCCCCCCCCu) >> 2u);
    bits = ((bits & 0x0F0F0F0Fu) << 4u) | ((bits & 0xF0F0F0F0u) >> 4u);
    bits = ((bits & 0x00FF00FFu) << 8u) | ((bits & 0xFF00FF00u) >> 8u);
    return float(bits) * 2.3283064365386963e-10; // / 0x100000000
}

vec2 Hammersley(uint i,uint N){
    return vec2(float(i) / float(N),RadicalInverse_VdC(i));
}

vec3 ImportanceSampleGGX(vec2 Xi,vec3 N,float roughness){
    float a = roughness * roughness;

    float Phi = 2 * Pi * Xi.x;
    float cosTheta = sqrt((1.0 - Xi.y) / (1.0 + (a*a - 1.0) * Xi.y));
    float sinTheta = sqrt(1.0 - cosTheta * cosTheta);

    vec3 H;
    H.x = sinTheta * cos(Phi);
    H.y = sinTheta * sin(Phi);
    H.z = cosTheta;

    vec3 up = N.z < 0.999 ? vec3(0.0,0.0,1.0) : vec3(1.0,0.0);
    vec3 tangent = normalize(cross(up,N));
    vec3 bitangent = normalize(cross(N,tangent));

    vec3 sampleVec = tangent * H.x + bitangent * H.y + N * H.z;
    return normalize(sampleVec);
}

float DistributionGGX(vec3 N,vec3 H,float roughness){
    float a      = roughness * roughness;    // according to Disney and Epic game,squaring looks more correct
    float a2     = a * a;
    float NdotH  = max(dot(N,H),0.0);
    float NdotH2 = NdotH * NdotH;

    float denom = NdotH2 * (a2 - 1.0) + 1.0;
    denom = Pi * denom * denom;

    return a2 / max(denom,0.00000001);
//    return a2 / denom;
}

vec3 prefilterEnvironmentMap(vec3 N,vec3 V,float roughness){
    const uint SAMPLE_COUNT = 512u;
    vec3 prefilterColor = vec3(0.0);
    float totalWeight = 0.0;
    for(uint i = 0u; i < SAMPLE_COUNT; ++i){
        vec2 Xi = Hammersley(i,SAMPLE_COUNT);
        vec3 H = ImportanceSampleGGX(Xi,N,roughness);
        vec3 L = normalize(2.0 * dot(V,H) * H - V);

        float NdotH = max(dot(N,0.0);
        float NdotL = max(dot(N,L),0.0);
        float HdotV = max(dot(H,V),0.0);

        if(NdotL > 0.0){
            float D   = DistributionGGX(N,H,roughness);
            float pdf = (D * NdotH / (4.0 * HdotV)) + 0.0001;

            float resolution = 512.0; // resolution of source cubemap (per face)
            float saTexel  = 4.0 * Pi / (6.0 * resolution * resolution);
            float saSample = 1.0 / (float(SAMPLE_COUNT) * pdf + 0.0001);

            float mipLevel = roughness == 0.0 ? 0.0 : 0.5 * log2(saSample / saTexel);

//            prefilterColor += texture(environmentMap,L).rgb * NdotL;
            prefilterColor += textureLod(environmentMap,L,mipLevel).rgb;
            totalWeight += NdotL;
        }
    }
    return prefilterColor / max(totalWeight,0.0);
}

void main(){
    vec3 N = normalize(localPos);
    vec3 R = N;
    vec3 V = R;

    vec3 color = prefilterEnvironmentMap(N,V,roughness);

    FragColor = vec4(color,1.0);
}

当我仅在球体上显示预过滤器颜色时，它看起来像这样：