Visualizing the world space geometry or shading normals on the face-culled geometry of the DirectionalLight.gltf shows that they are pointing to the inside
(here: color = N * 0.5f + 0.5f;)
Also visualizing the front-face condition showed that this is actually the front-face so vertex winding and normal on front-face is consistent.
Consequently, switching the three materials to double-sided shows the expected normal orientation of the outside of the spheres:
This results in black rendering with a ray tracer because the inside of the sphere is hit and light is blocked by the face-culled geometry in front. Switching to double-sided materials produces the correct result.
Visualizing the world space geometry or shading normals on the face-culled geometry of the DirectionalLight.gltf shows that they are pointing to the inside (here: color = N * 0.5f + 0.5f;)
Also visualizing the front-face condition showed that this is actually the front-face so vertex winding and normal on front-face is consistent.
Consequently, switching the three materials to double-sided shows the expected normal orientation of the outside of the spheres:
This results in black rendering with a ray tracer because the inside of the sphere is hit and light is blocked by the face-culled geometry in front. Switching to double-sided materials produces the correct result.