Windows平台CPU路径追踪器输出过暗，疑似缺失全局光照

问题描述 #

在Windows平台开发CPU光线追踪器，场景为带两扇窗户的室内空间，仅设置一个方向光，光线反弹次数设为8次。直接光照表现正常，但输出画面过暗，疑似缺失全局光照：

我的路径追踪器输出：

Blender Cycles输出：

核心代码分析 #

路径追踪主函数 #

RGBColor pathTracing(const Math::Ray& ray, nbUint32 depth)
{
    // Find intersection using intel's Embree
    Intersector::IntersectionInfo isectInfo;
    m_intersector->intersect(ray, isectInfo);

    // Check if an intersection occured
    if (isectInfo.object == nullptr)
    {
        return BlackRGBColor;
    }

    // Get intersection properties
    const auto isectProps = buildIntersectionProperties(ray, isectInfo, m_currentScene);

    // The scene model
    const auto& model = m_currentScene->getModel();

    // Read the material
    const DatabaseMaterialPtr material = model->getMaterialPtr_FromEntity(isectInfo.object->getMaterialId());

    // Compute direct lighning
    RGBColor directCont = BlackRGBColor;
    {
        for (const EntityIdentifier& lightId : m_currentScene->getLights())
        {
            // Generate light sample
            const auto light = Entity::EntityDatabaseSingleton::instance()->getEntity<Light::BaseLight>(lightId);
            const auto sampleToLight = light->generateSampleToLight(isectProps.P);

            const nbFloat32 NoL = glm::dot(isectProps.BsdfProps.N, sampleToLight.L);
            if (NoL <= 0.0f)
                continue;

            // Compute occlusion
            Math::Ray shadowRay(isectProps.deltaP, sampleToLight.L, sampleToLight.length);
            const nbFloat32 occlusionStrength = m_intersector->occlusion(shadowRay);

            if (occlusionStrength != 1.0f)
            {
                // Build the bsdf sample struture
                Material::BaseMaterial::SampleBsdfData bsdfSample;
                bsdfSample.L = sampleToLight.L;
                bsdfSample.V = isectProps.V;
                bsdfSample.NoL = NoL;
                bsdfSample.geoProps = &isectProps.BsdfProps;

                // Sample the brdf to get the scale
                auto sampledBrdf = material.sampleBsdf(bsdfSample);

                // Multiply by light radiance
                sampledBrdf *= light->getColor();

                // Multiply by visibility
                sampledBrdf *= (1.0f - occlusionStrength);

                // Finally add light contribution
                directCont += sampledBrdf;
            }
        }
    }


    // Compute indirect lighning
    RGBColor indirectCont;
    if (++depth <= m_settings.m_maxDepth)
    {
        BaseMaterial::SpawnRayResult newRayData;
        newRayData = material->spawnRay(/* some parameters*/);

        // Build the bsdf sample struture
        Material::BaseMaterial::SampleBsdfData bsdfSample;
        bsdfSample.L = newRayData.ray.getDirection();
        bsdfSample.V = isectProps.V;
        bsdfSample.NoL = glm::dot(newRayData.ray.getDirection(), isectProps.BsdfProps.N);
        bsdfSample.geoProps = &isectProps.BsdfProps;

        // Indirect raytracing
        const Spectrum sampledBrdf = material->sampleBsdf(bsdfSample);
        const Spectrum sampledRadiance = sampledBrdf * pathTracing(newRayData.ray, depth);//recursion

        static const float normalizationFactor = 2.0f * Math::Pi.getValue();
        indirectCont = sampledRadiance * normalizationFactor;
    }

    return directCont + indirectCont;
}

SampleBsdf方法 #

RGBColor BaseMaterial::sampleBsdf(const SampleBsdfData& data) const
{
    DistributionFunction::SampleInput distSampleInput;
    distSampleInput.L = data.L;
    distSampleInput.V = data.V;
    distSampleInput.N = data.geoProps->N;
    distSampleInput.T = data.geoProps->T;
    distSampleInput.B = data.geoProps->B;
    distSampleInput.H = glm::normalize(data.L + data.V);

    distSampleInput.HoN = std::abs(glm::dot(data.geoProps->N, distSampleInput.H));
    distSampleInput.NoV = std::abs(glm::dot(data.geoProps->N, data.V));
    distSampleInput.NoL = data.NoL;


    return sampleBrdf(distSampleInput) * distSampleInput.NoL;
}

SampleBrdf方法 #

inline nbFloat32 lambert(const SampleInput& input)
{
    return Math::InvPi.getValue();
}

nbFloat32 blinnPhong(const SampleInput& input, nbFloat32 shininess)
{
    const nbFloat32 HoN = std::min(input.HoN, 0.999f);
    nbFloat32 normalizationFactor = (shininess + 8.0f) * Math::InvPi8.getValue();
    normalizationFactor = glm::clamp(normalizationFactor, 0.0f, 1.0f);

    return std::pow(HoN, shininess) * normalizationFactor;
}

#define NO_FRESNEL_VALUE 0.4f

nbFloat32 DefaultDielectric::sampleFresnel(nbFloat32 HoN) const
{
    if (m_fresnelEnabled)
    {
        // Schlick approximation.
        return glm::clamp(m_fresnel0 + (1.0f - m_fresnel0) * pow(1.0f - HoN, 5.0f), 0.0f, 1.0f);
    }

    return NO_FRESNEL_VALUE;
}

RGBColor DefaultDielectric::sampleBrdf(DistributionFunction::SampleInput& distSampleInput) const
{
    nbFloat32 fresnelFactor = sampleFresnel(distSampleInput.HoN);

    // Diffuse
    nbFloat32 diffuseFactor = lambert(distSampleInput) * (1.0f - fresnelFactor);
    auto diffuseLightning = m_diffuse * diffuseFactor;

    // Specular
    nbFloat32 specularFactor = blinnPhong(distSampleInput, m_glossiness) * fresnelFactor;
    auto specLightning = m_specular * specularFactor;

    return m_opacity * (diffuseLightning + specLightning);
}

问题根源及修复方案 #

1. 间接光照变量未初始化 #

RGBColor indirectCont;未初始化为BlackRGBColor，当光线深度超过最大值时，变量会携带垃圾值，导致画面异常。
修复：初始化时改为RGBColor indirectCont = BlackRGBColor;，或在深度不满足条件的分支显式赋值。

2. 未过滤背面的间接光线 #

采样的间接光线方向如果与法线夹角为负（光线射向物体内部），此时NoL <= 0，应直接舍弃该采样，避免引入负光照贡献。
修复：在计算间接光照前添加判断：

bsdfSample.NoL = glm::dot(newRayData.ray.getDirection(), isectProps.BsdfProps.N);
if (bsdfSample.NoL <= 0.0f)
{
    indirectCont = BlackRGBColor;
    return directCont + indirectCont;
}

3. 路径追踪权重计算错误（核心问题） #

路径追踪中，递归的光照贡献公式应为(BSDF值 / 采样PDF) * 递归返回的辐射度，原代码用固定的2π做归一化，完全不符合物理规律，这是全局光照缺失的主因。
修复：

• 给SpawnRayResult添加nbFloat32 pdf字段，存储采样对应的概率密度值
• 修改间接光照计算逻辑：

if (newRayData.pdf <= 0.0f)
{
    indirectCont = BlackRGBColor;
    return directCont + indirectCont;
}
const Spectrum sampledBrdf = material->sampleBsdf(bsdfSample);
const Spectrum sampledRadiance = (sampledBrdf / newRayData.pdf) * pathTracing(newRayData.ray, depth);
indirectCont = sampledRadiance;

注：如果spawnRay采用余弦加权半球分布（Lambert材质标准采样），其PDF为NoL / π，此时sampledBrdf / pdf的结果会自动抵消冗余系数，符合物理渲染逻辑。

4. 方向光阴影射线长度不合理 #

方向光是平行光，理论上无终点，原代码中sampleToLight.length如果是有限值，会导致阴影射线提前终止，无法正确判断远处遮挡。
修复：将方向光的阴影射线长度设为极大值（如1e6），确保覆盖场景内所有物体。

内容的提问来源于stack exchange，提问作者TheChamp