Raymarching Clamping coordinate makes weird stretched artifacts

Question

我正在尝试使用光线行进渲染球体网格。 SDF 函数如下所示：

float sq_layer(vec3 p, vec3 bounding_box_min, vec3 bounding_box_max)
{
         float cell_size = 4.0 / 16.0;
         vec3 p0 = clamp(p.xyz, bounding_box_min, bounding_box_max);
         p0 = mod(p0, cell_size) - cell_size * 0.5f;
         float d_pt = sphereSDF(vec3(p0.x, p0.y, p0.z), 0.05f)
         return d_pt;
}

但我得到这样的东西：

为了更相关，我尝试稍微更改代码：

float sq_layer(vec3 p, vec3 bounding_box_min, vec3 bounding_box_max)
{
         vec3 cell_size = (bounding_box_max - bounding_box_min)  / 4.0;
         vec3 p0 = clamp(p.xyz, bounding_box_min, bounding_box_max);
         p0 = mod(p0, cell_size) - cell_size * 0.5f;
         float d_pt = boxSDF(p0, cell_size * 0.35);
         return d_pt;
}

我还是明白了，

Answer 1

以您的方式夹紧坐标会导致您的 sdf 返回非常接近表面的恒定距离，因此您将用尽迭代，从而导致您看到的工件。

您想与边界框大小的框 sdf 相交，而不是夹紧：

float sq_layer(vec3 p, vec3 bounding_box_min, vec3 bounding_box_max)
{
         float cell_size = 4.0 / 16.0;
         vec3 p0 = mod(p, cell_size) - cell_size * 0.5f;
         float d_pt = sphereSDF(p0, 0.05f);

         // the following is just a simplified box sdf
         vec3 bb_extents = (bounding_box_max - bounding_box_min) * .5; 
         vec3 bb_center = bounding_box_min + bb_extents;
         vec3 bb_p = max(abs(p-bb_center)-bb_extents,0.);
         float d_bb = max(max(bb_p.x,bb_p.y),bb_p.z);

         // intersect "sphere-field" with bounding box
         return max(d_pt,d_bb);
}