Improved MLT

I recently revisited my bidirectional path tracer and found a small bug in my Metropolis implementation. For choosing the location on the image plane I was accidentally using new random numbers instead of the special random numbers from my random number stream. This meant that the eye subpaths were not properly correlated, which was causing a major reduction in image quality. After fixing this bug, the benefits of MLT became much more apparent.

Below are some approximately equal-time renders for comparison, as well as a more converged reference image. The area most improved by the bug fix is probably the reflection on the glass ball. MLT now outperforms bidirectional path tracing everywhere except the dark areas of the image. You can use the lightbox to switch between images.

MLT after bug fix.

MLT before bug fix.

Bidirectional path tracing.

Pure unidirectional path tracing from the camera (s = 0, t ≥ 2).

High quality bidirectional path tracing reference.