c++ - 奇怪的体素锥体追踪结果

我目前正在使用 C++ 和 OpenGL 编写体素锥体跟踪渲染引擎。一切都进行得相当顺利，除了对于更宽的锥角我得到了相当奇怪的结果。

现在，出于测试的目的，我所做的就是垂直于片段法线射出一个单一的锥体。我只计算“间接光”。作为引用，这是我正在使用的相当简单的片段着色器:

#version 450 core

out vec4 FragColor;

in vec3 pos_fs;
in vec3 nrm_fs;

uniform sampler3D tex3D;

vec3 indirectDiffuse();

vec3 voxelTraceCone(const vec3 from, vec3 direction);

void main()
{
    FragColor = vec4(0, 0, 0, 1);
    FragColor.rgb += indirectDiffuse();
}

vec3 indirectDiffuse(){
    // singular cone in direction of the normal
    vec3 ret = voxelTraceCone(pos_fs, nrm);

    return ret;
}

vec3 voxelTraceCone(const vec3 origin, vec3 dir) {
    float max_dist = 1f;
    dir = normalize(dir);

    float current_dist = 0.01f;

    float apperture_angle = 0.01f; //Angle in Radians.

    vec3 color = vec3(0.0f);
    float occlusion = 0.0f;

    float vox_size = 128.0f; //voxel map size

    while(current_dist < max_dist && occlusion < 1) {
        //Get cone diameter (tan = cathetus / cathetus)
        float current_coneDiameter = 2.0f * current_dist * tan(apperture_angle * 0.5f);

        //Get mipmap level which should be sampled according to the cone diameter
        float vlevel = log2(current_coneDiameter * vox_size);

        vec3 pos_worldspace = origin + dir * current_dist;
        vec3 pos_texturespace = (pos_worldspace + vec3(1.0f)) * 0.5f; //[-1,1] Coordinates to [0,1]

        vec4 voxel = textureLod(tex3D, pos_texturespace, vlevel); //get voxel

        vec3 color_read = voxel.rgb;
        float occlusion_read = voxel.a;

        color = occlusion*color + (1 - occlusion) * occlusion_read * color_read;
        occlusion = occlusion + (1 - occlusion) * occlusion_read;

        float dist_factor = 0.3f; //Lower = better results but higher performance hit
        current_dist += current_coneDiameter * dist_factor; 
    }

    return color;
}

tex3D 制服是体素 3d 纹理。

在常规 Phong 着色器(计算体素值)下，场景如下所示: