Denoiser

A denoiser is a filter that removes the grainy speckle produced by sampling-based rendering methods such as path tracing. Because light is traced along random paths for every pixel, an image stopped after a small number of samples still shows noise. The denoiser looks at neighboring pixels, surface normals and color data to tell real detail from random error, so a frame that would need hours can be finished much sooner.

How a denoiser works

Most modern denoisers do not judge the final image alone. They also read auxiliary passes from the render engine, usually albedo (the plain surface color) and normals (the direction each surface faces). With that extra information they can keep a wood grain or a tile joint while smoothing out the noise around it. Some are trained with machine learning and run almost instantly on the GPU; others rely on more traditional filtering.

Using it well

A denoiser is not a magic fix. If the sample count is far too low, fabric weave, timber grain and fine grout lines get smeared into a waxy, plastic look. The usual approach is to give the scene a sensible number of samples first and let the denoiser handle the last stretch. In architectural interiors, where indirect light piles up in corners, it saves a great deal of time, but the final frame still deserves a close look at material detail. For other causes of grain and how to fix them, see our guide on how to reduce render noise.