More information about sampling and removing noise can be found here.
Supersampling control over the number of rays per pixel that will be traced from the camera. The higher the number of samples, the better the anti-aliasing quality, and the longer the render times. The exact number of rays per pixel is the square of this value. For example, a Camera samples value of 3 means 3x3 = 9 pixel samples. In practice, you may consider using a value of 4 for medium quality, 8 for high quality, and (rarely) 16 for super-high quality. This control acts as a global multiplier of all the different rays, multiplying the number of Diffuse and Glossy rays. Motion blur and depth of field quality can only be improved by increasing Camera samples.
Camera samples multiplies Diffuse, Glossy, and light samples after being squared.
For example, 6 Camera samples and 6 Glossy samples = 62 x 62 = 1296 rays per pixel for the Diffuse, and another 1296 rays per pixel for the indirect glossy specular. Therefore when you increase the Camera samples to get better geometric anti-aliasing, you should decrease the others to compensate.
Controls the number of rays fired when computing the reflected indirect-radiance integrated over the hemisphere. The exact number of hemispherical rays is the square of this value. Increase this number to reduce the indirect diffuse noise. Remember that the diffuse sampling is done for each AA sample, so high values for both AA Samples and Diffuse samples will tend to result in slow renders.
When Diffuse samples are more than zero, camera rays intersecting with diffuse surfaces fire indirect diffuse rays. The rays are fired in random directions within a hemispherical spread. Noise is introduced when there are insufficient rays to resolve the range of values from the environment.
Increasing the number of Diffuse samples will increase the number of diffuse rays fired from a point:
Indirect Diffuse Ray sampling & Indirect Diffuse Noise
The table below shows the effect of increasing the number of Diffuse samples (GI_diffuse_samples) to resolve indirect diffuse noise:
Diffuse samples (GI_diffuse_samples): 1 2 4 6. Render time: 131 156 271 427
This shows the performance impact when increasing the number of Diffuse samples (GI_diffuse_samples). Because indirect diffuse rays are so prevalent, this can get expensive. In this example, the performance hit from 1 to 6 samples was over 320%.
Indirect diffuse noise
This is one of the most common causes of noise, and can have a number of different sources. It manifests as granularity in the scene, usually in shadowed areas.
There are a couple of different methods to determine indirect diffuse noise. If you've rendered AOV's you can check the indirect diffuse AOV; if noise is present in this AOV only, you can be quite certain this raytype is responsible. You can check if an area of noise is created by indirect diffuse noise by turning Indirect Diffuse samples to zero; this will effectively turn off indirect diffuse. If this raytype is responsible then the noise will disappear. If the image darkens with the indirect diffuse gone, but the noise is still present, indirect diffuse rays are not responsible for the noise.
In the example below a directional light is pointing into an enclosed space. With Diffuse samples set to 0, no light is able to bounce off of the surfaces and therefore there is no indirect light in the scene. Increasing the Diffuse samples to 1 allows diffuse rays to bounce around the scene. However it produces a noisy result, especially in the corners of the scene. Increasing the Diffuse samples to 3 gives an improved result. It is good practise to use this value sparingly. Increase it incrementally and see if you notice any difference in the quality of the indirect diffuse component.
Remember that the Diffuse sampling is done for each AA sample, so high values for both AA Samples and Diffuse Samples will tend to result in slow renders.
Controls the number of rays fired when computing the reflected indirect-radiance integrated over the hemisphere weighted by a specular BRDF. The exact number of rays is the square of this value. Increase this number to reduce the indirect specular noise (soft/blurry reflections). Remember that the glossy sampling is done for each Camera (AA) sample, so high values for both Camera samples and Glossy samples will tend to result in slow renders.
Diagram showing how glossy reflection rays are propagated in an Arnold render
In the example below the floor surface has high specular and roughness values. In the image on the left you can see that there are not enough Glossy samples and therefore there is noise in the floor. Increasing the Glossy samples gives a better result.
If you reduce the number of Glossy samples to zero and the Glossy ray depth to zero and the noise disappears, then the noise is due to specular reflections.
Controls the number of samples used to simulate the microfacet-based glossy refraction evaluations. Increase this value to resolve any noise in the refraction.
If you switch this parameter to zero, the GI_refraction_depth to zero and the noise disappears, you will know that the noise is due to glossy refractions.
Diagram showing how glossy refraction rays are propagated in an Arnold render
Glossy Ray Sampling and Reflection/Refraction Noise
While it is normally a straightforward matter to determine whether it is a glossy reflection or refraction causing the noise, we need to confirm that glossy rays are responsible for a given type of noise: If you've rendered AOV's you can check the indirect diffuse AOV; if noise is present in this AOV only, you can be quite certain this raytype is responsible. Also, you can switch the GI_glossy_samples and GI_refraction_samples and glossy depth types to zero in the Arnold Render Settings. Again, this essentially turns off glossy rays. If the glossy rays are responsible, the noise will disappear with this test. If the glossy component disappears but the noise is still there, glossy rays are not responsible.
This value controls the number of lighting samples (direct and indirect) that will be taken to estimate lighting within a radius of the point being shaded to compute sub-surface scattering. Higher values produce a cleaner solution, but will take longer to render.
Some additional noise in indirect glossy and Diffuse GI originating from an object with SSS is expected, in particular if the Diffuse samples setting is set lower than the SSS samples setting. To combat this type of noise you can try using higher Camera settings and lower Glossy, Diffuse and SSS sampling rates or increase the Diffuse/ Glossy samples. Increasing the SSS samples will only make the subsurface effect have less noise in camera, reflection and refraction rays.
In the images below you can see that there is noise in the darker areas of the eye socket. Increasing the Diffuse samples will reduce this type of noise.
Note that to have SSS values spread across multiple objects, for example from a face to an eyeball, you will need to use 'SSS Set Name'.
Controls the number of sample rays that get fired to compute indirect lighting of the volume. Like the other sampling rate controls (Camera, light samples, Diffuse samples, etc), the number of actual samples is squared, so a setting of 3 fires 3x3=9 rays. Setting it to 0 turns off indirect lighting of the volume. Note that indirect volume lighting is tied to the 'Volume' ray depth render option and therefore there must be at least 1 Volume bounce for indirect lighting to be computed.
'Volume Indirect' samples and per-light 'Volume Samples' do not apply to 'Volume Scattering'. To improve the quality of the 'Volume Scattering' you must increase the 'Volume Scattering' samples.
Locks the AA_seed so that the sampling noise won't change with the frame number (avoiding the film grain look).
Enabling this option will take into account the effect that Displacement Autobump has on the ray traced BSSRDF's result. This helps capture the high frequency details of the surface more accurately when using Autobump.
Beware that enabling this option will triple shader evaluations with SSS, resulting in much longer render times.