Walls with low light appear to be cloudy on rendering with cycles

Hello,

This is my first time posting in this forum, can someone help me understand why the part of my walls with low light appear to have a cloudy texture? (image below)

Thank you in advance


This is the result of noise reduction performed on a very noisy image. Increase the number of samples to get a clearer image.

You’re basically lighting using indirect lights only, it’s bound to get noisy. The optimizations I tend to go for are:

  • More samples - not exactly optimizing for time, but sometimes it’s absolutely essential.

  • More diffuse samples - takes more time but allows more light to get bounced around to bring up the noise floor.

  • Use scene/simplify/ao bounces with as low multiplier as possible to match what high diffuse bounces gets you. Great speed improvement.

  • Possibly placebo, but I tend to turn off MIS for materials (especially dark ones) that doesn’t bounce light as much. Leave it on for floor, walls, and ceiling.

  • Make sure not to rely on caustics - turn them off unless absolutely required (certain materials). If you notice a significant drop in energy from turning it off, do a “if seen by diffuse, reflect as diffuse” light path trick. It won’t look the same, but it will keep the energy levels up.

  • Use fake architectural glass using transparent and glossy via facing (safe) or fresnel (unsafe, may need an IOR trick if you don’t watch the normals).

  • Try to replace HDR with a simple area lamp outside, maybe color it with a sky’ish gradient.

  • Similarly, use sun lamp to do sun, not HDRs (most you’ll find online a clipped, not reflecting the true sun levels anyway). If your background has a sun, it appears to be clipped in your case (shadow less).

  • The two points above will significantly reduce “HDR induced” noise, especially if memory limitations prevent you from using high world map resolutions (could be for diffuse only, making HDR visible to all the rest of them). Rendering with a sky gradient only (no direct sun) always cleans up much faster, but you’d have to expose higher.

Keep that noisy splotchy thing and use it as a reference for tweaking optimizations.

If relying on brute force alone with no optimizations, then yeah, more samples is the only way. A lot more samples. Too much samples.