# Reverse PDF updates for s-2 or t-2 vertices in MIS for BDPT

MIS weights for BDPT are calculated with ratios of PDFs for each vertex under different types of light transport (radiance vs. importance). These probabilities are regenerated for frontier vertices during subpath connection, which makes sense - the direction from the $t-1$-th camera vertex to the $s-1$-th light vertex will be different to the direction between the $t-1$-th and $t$th camera vertices.

PBRT also reccomends updating reverse probabilities for previous vertices, though. Why? The probability of tracing a ray from $s-1$ to $s-2$ won't be affected by the probability of tracing a ray between $s-1$ and $t-1$.

• can you post the line number in the pbrt code too – ali Jun 6 '18 at 13:32
• one thing is that you need to reevaluate both back and forth pdfs of the end s and end t vertices of the subpath when computing MIS. rendering-memo.blogspot.com/2016/03/… – ali Jun 6 '18 at 13:53