Here are some hints from the Lux-FAQ:
My scene has rendered for hours, but it's still very noisy
First, please note that even on powerful machines, certain scenes might take over 12 hours to get satisfactory results (and much more with high resolutions). However, there are several things one can try in order to get satisfactory results quicker.
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- For interiors, use portals where you have windows or other openings where light can enter.
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- If your scene contains a lot of glass but you don't need the refractive effect of the glass (windows or similar), turn on the "Architectural" option in the glass material.
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- Scenes containing very bright (white) walls can lead to a noisy image. Try making them slightly darker. You can try adjusting the tone mapping parameters to compensate.
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- Try using the "Final MLT/Bidir path tracer (interior) (recommended)" rendering preset in LuxBlend. While it may sound like it is primarily meant for interiors, it can often be superior for exterior (archviz) scenes.
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- As a last resort, once you've got a very smooth sampled image with still a bit of grain in it, you can try some denoising filter in image-editing software like Photoshop or Gimp. LuxRender itself already ships with 2 such options under the Noise Reduction tab: Greyscale Restoration and Chiu.
