# How should I understand path-space measurement equation?

I read the corresponding chapter of PBRT-v3, as well as Veach's thesis which introduced BDPT. However I didn't quite understand why we use the measurement equation. It seems that the measurement equation can be further formulated to obtain different light transport algorithms, such as photon map and bdpt. But why?

I've searched for other literature but didn't find anything really useful. Can anyone give some hint?

• Can you include the actual equation you're asking about, and maybe some more background in your question for the benefit of people who may not have those references handy? – Nathan Reed Jan 22 '18 at 18:52