"negative radius" support in sphere

[idcrook]

thanks for sharing this project code.

what I describe is not a problem per se, but allows a sphere with a negative radius to get a valid bounding box.

// sphere.cu
 RT_PROGRAM void getBounds(int pid, float result[6])
 {
     optix::Aabb* aabb = (optix::Aabb*)result;
-    aabb->m_min = center - radius;
-    aabb->m_max = center + radius;
+    aabb->m_min = center - abs(radius);
+    aabb->m_max = center + abs(radius);
 }

// scene.h
                     {
                         geometryList.push_back(new ioSphere(x,y,z, 0.2f));
                         materialList.push_back(new ioDielectricMaterial(1.5f));
+                        geometryList.push_back(new ioSphere(x,y,z, -(0.2f-0.007f)));
+                        materialList.push_back(new ioDielectricMaterial(1.5f));
                     }

This allows, for example, dielectric nested spheres to appear similar to a bubble. I have made this change and it works as expected.

https://github.com/idcrook/weeker_raytracer/blob/edd68e9dbaddee2d34388ed2cbcf9b28ecbcc573/src/OptiX/TheNextWeek/geometry/sphere.cu#L30-L31