Looking at the glass spheres, we can see the checkered floor in the top of the spheres. This is caused by refraction through the glass material of the spheres. From the demo in class, a solid glass sphere will result in this effect, while a hollow glass sphere does not refract the light enough for the image to become inverted.
I'm wondering what kinds of scenes would we need to render using a bidirectional integrator, as opposed to using just unidirectional path tracing? ~where the geometry is more complex and the light is not big enough? Also, how might we combine methods of IOR-based reflection and refraction for camera paths with bidirectional path tracing for transparent/ straight paths for shadowing? How would the bidirectional integrator and lights work together in terms of implementation?