   Question about importance function in bidirectional ray tracing
  I am trying to implement bidirectional path tracing algorithm based on PBRT.I am using Veach's Ph.D thesis as a reference. The hard part is that I don't understand the importance function very well. The importance (We in the thesis) emitted from eye into the scen, what exactly is it?From information I gathered, it seems related to the filter function on the pixel if the measurement is the pixel value, i.e. the image synthesis. But then, what is the meaning of the directional part and positional part of the importance? For a single ray, how to calculate its importance? Does the camera has to be modelled in the scene (i.e. can be intersectable) to calculate it?Please clarify on it.   