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Even though this image is using the exact same GI sampling setting as the previous one, you can see that it has a lot more noise. This is because the environment map has a lot of high contrast areas with bright spots (windows, ceiling lights) and dark shadows, which is difficult to sample.

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Reducing Noise Using The Ray Switch Node

Warning

This section is mostly obsolete, you should be using the Skydome light instead.

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Using a very high contrast HDR image for lighting a scene can produce noise which can be hard to get rid of simply by increasing the Camera or Hemi/Glossy samples. An HDR image contains a tiny sun disc whose pixel values are in the hundreds of thousands or even millions, which is going to need a lot of samples for Arnold's brute-force hemispherical sampling to find. On the other hand, a an HDRI of a thick overcast day with no sun at sight is very easy to sample.

One way to fix the excessive sampling noise would be simply to blur the environment map so that it's easier for the GI and glossy rays to hit bright areas. However, that would cause the render to lose all the nice highlights seen in the reflections from the environment map. Instead, we will be using different environment maps for each ray type in the scene.

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  • One high-res and sharp map for reflection and refraction rays.
  • One mid-res and slightly blurred map for glossy rays.
  • One low-res and very blurred map for Indirect Diffuse rays.

For this purpose, you can use "Ray_Switch".

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Depending on the dynamic range of the HDRI you may need to blur more or less to achieve optimal results. Also, how blurry the map used for glossy rays should be depends on the material; for very sharp materials (almost mirror like) you don't need as much blurring, but if your glossy reflections are very soft you should use a bigger blur. Here's the resulting image from this setup:

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Note how with this setup the noise is now gone and the illumination is very similar to the version using a single high-res map for all ray types.

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Different amounts of blur for the diffuse environment map

Note

This section is mostly obsolete, you should be using the Skydome light instead.

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Ray Depth

The scene below is lit soley using a Sky shader with the color set to white. The number of Diffuse rays has been increased to 4 in order to reduce noise from the Sky around the floor.

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