Theoretical Computer Graphics - Ray Cast Algorithm

Question

I studied computer graphics exam using previous iterations and I came across this question

Please note the following:

"The ray casting algorithm is great for calculating predicted shadows, but computationally very expensive when the second time calculations are for visibility."

Can someone explain why it’s “computationally expensive to recalculate visibility”? Sorry for repeating the quote.

Thanks to everyone.

Answer 1

I suppose this refers to the fact that shadows can be easily (more or less) calculated from the vertices (corners) of a silhouette, while visibility requires checking all possible rays of light that can bounce off an object (and not just its edges).

Thus, the number of rays for shadows is much less than the number of rays for determining visibility.

It makes sense? I could paint a picture ...