  Inverse path tracing has recently been applied to joint material and lighting
estimation, given geometry and multi-view HDR observations of an indoor scene.
However, it has two major limitations: path tracing is expensive to compute,
and ambiguities exist between reflection and emission. Our Factorized Inverse
Path Tracing (FIPT) addresses these challenges by using a factored light
transport formulation and finds emitters driven by rendering errors. Our
algorithm enables accurate material and lighting optimization faster than
previous work, and is more effective at resolving ambiguities. The exhaustive
experiments on synthetic scenes show that our method (1) outperforms
state-of-the-art indoor inverse rendering and relighting methods particularly
in the presence of complex illumination effects; (2) speeds up inverse path
tracing optimization to less than an hour. We further demonstrate robustness to
noisy inputs through material and lighting estimates that allow plausible
relighting in a real scene. The source code is available at:
https://github.com/lwwu2/fipt
