Reblend

Guide / Light

Bidirectional

For light that Path Tracing renders as noise or fireflies: a lamp behind a shade, a spotlight tucked into a fixture, a caustic on a table. Bidirectional renders that light with far less noise, at a higher cost per sample. With it selected a Bidirectional panel appears under Sampling.

  • Light Paths: how much work goes into light from the lamps each sample. Lower renders faster; higher gives cleaner caustics and cleaner light from small or hidden lamps. Default 0.25.
  • The bounce limit is the usual one, Light Paths → Max Bounces → Total.
  • The Photon Caustics and Path Guiding panels are hidden while Bidirectional is selected: they work with Path Tracing only.

It works on CPU, Metal, CUDA and OptiX, on one device or several at once. On OptiX the first Bidirectional render prepares its kernels once; see known issues.

×Custom OSL cameras

Supported custom OSL cameras work on CPU, NVIDIA OptiX and Apple Metal. On Metal, they work with Path Tracing, Bidirectional, Vertex Merging and Photon Caustics. This adds camera support, not OSL materials. CUDA does not support OSL cameras.

Caustics depend on the camera. Some custom camera scripts cannot show certain caustics with Bidirectional alone. For those cameras, enable Vertex Merging or use Photon Caustics. Reblend includes an example camera for script authors.

LensSim. Supported LensSim configurations work without script changes. On CPU and Metal, pure Bidirectional supports the tested perspective lens settings with depth of field. Its full caustics coverage requires chromatic aberration, halation, bloom, glare, sensor motion blur, radial blur, bokeh density, bokeh swirl, additional distortion and anamorphic bokeh aspect to be off. This automatic LensSim support for pure Bidirectional is not available on OptiX; use Vertex Merging or Photon Caustics there.

Limits. Metal supports a subset of OSL camera scripts; unsupported features produce an error. Camera scripts that read textures or trace rays are not supported on Metal. Bidirectional with motion blur on a custom camera falls back to Path Tracing.