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Multiexcerpt
MultiExcerptNamesamples

Controls the quality of the noise in the soft shadows and direct specular highlight. The higher the number of samples, the lower the noise, and the longer it takes to render. The exact number of shadow rays sent to the light is the square of this value multiplied by the AA samples.

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A schematic of how light noise occurs in Arnold

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Note that setting light samples to 0 disables the light.

Noise from lights can sometimes be difficult to diagnose, particularly if the light source is broad in comparison to the scene and the shadows have an extremely wide penumbra. In these cases, it can sometimes be mistaken for indirect diffuse noise. It highlights the necessity for testing noise ray type. This diagram shows how light is traced in Arnold.

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The example below shows specular highlights from area light sources. There are four spherical mesh light sources of varying sizes and color temperatures. Underneath are four cubes with Standard shaders assigned to them with varying degrees of specular roughness. Note that more noise is apparent with smaller light sources. Increasing the number of light samples resolves the noise. 

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Specular roughness from top to bottom: 0, 0.1, 0.2, 0.3

 


If the issue is noise in a specular highlight, you will need to confirm that the source is the direct light and not a secondary ray type (such as specular). This is easy to achieve by turning off global illumination; set the Diffuse Depth and Specular Depth to zero (this essentially turns off all global illumination). If the noise is still there, we know it is the specular component of the illumination model. If the issue is shadow noise, then we can simply toggle ignore shadows in the Arnold render settings, and the noise will completely resolve.

The key, again, is to modify the sampling and observe the changes. Increasing the number of light samples should have an immediate and quantitive effect on the smoothness of the specular highlight and the shadow. If there is no change, light samples are not responsible for the noise.

Multiple importance sampling (MIS) is enabled by default in Arnold. The images below show the difference when rendering with and without multiple importance sampling.

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Multiexcerpt
MultiExcerptNamenormalize

If enabled, you will be able to tweak the shadow softness by changing the size (i.e., radius) of the light, without affecting the amount of emitted light. This is very handy for artistic control. Otherwise, if not enabled, the amount of emitted light is proportional to the light's surface area.

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camera/diffuse/specular/sss/volume

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Multiexcerpt
MultiExcerptNamespread

Emits light focused in the direction along the normal. The default spread value of 1 gives diffuse emission, while lower values focus the light more until it becomes almost a laser-like beam at value 0. Currently fully focused laser beams at value 0 are not supported, there is always a small minimum spread. Low spread values can be noisier than the default high spread, so be careful when using them.

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The animation below shows the effect when lowering the Spread spread value.

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Spread spread value lowered from 1 (Quad quad_light)

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MultiExcerptNamemotion_start

The time at which the first motion key of the shape is sampled. Other motion keys must be uniformly spaced within this range. By convention, the times are frame relative. For example, start and end times -0.5 to 0.5 indicate that the motion keys were sampled midway between the previous and current frame, and the current frame and next frame. This is applied to cameras, lights, and shapes.

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Multiexcerpt
MultiExcerptNamemotion_end

The time at which the last motion key of the shape is sampled. Other motion keys must be uniformly spaced within this range. By convention, the times are frame relative. For example, start and end times -0.5 to 0.5 indicate that the motion keys were sampled midway between the previous and current frame, and the current frame and next frame. This is applied to cameras, lights, and shapes.

 

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