Sphere tracing

        sphere tracing algorithm [14], which marches along each pixel’s ray direction with the queried signed distance until the ray hits the surface, i.e., the signed distance equals to zero (Fig. 2). 

         where rays are shot from the camera pinhole along the direction of each pixel to search
for the surface level set according to the signed distance value. 

Sphere Tracing: A Geometric Method for the Antialiased Ray Tracing of Implicit
Surfaces

DIST: Rendering Deep Implicit Signed Distance Function with Differentiable Sphere Tracing