Lights

To add an Arnold light, select one from the Get->Primitive->Light menu, or from the main Arnold->Lights menu, listing all the different types of light sources supported in Arnold.

SItoA will make use of Softimage's built-in icons used for lights, for example, to display the area-light shape/size in the viewport. However, since each Arnold light has some parameters different from the Softimage lights, each Arnold light has its own custom light shader listing its extra options.

Arnold lights and light filters can be added from the Arnold top menu


In the following topics, we'll first describe the parameters that are common to all Arnold lights, then those specific to each type of light, and finally we'll describe the light filters, pluggable Arnold shaders that extend the lights with new features.

 


Common Light Parameters

The parameters described below are common to all lights in SItoA (except for distant and skydome which does not support decay).

Color and Decay

Color

The color of the light.

Sets the intensity of each RGB color channel. The slider ranges from 0 to 1, but you can enter higher values.

Intensity

Intensity controls the brightness of light emitted by the light source by multiplying the color.

Exposure

Exposure is an f-stop value which multiplies the intensity by 2 to the power of the f-stop. Increasing the exposure by 1 results in double the amount of light.

In Arnold, the total intensity of the light is computed with the following formula:

color * intensity * 2exposure

You can get the same output by modifying either the intensity or the exposure. For example, intensity=1, exposure=4 is the same as intensity=16, exposure=0.  Note: 20 = 1, not 0.

1 * 1 * 24 = 16

1 * 16 * 20 = 16

The reasoning behind this apparent redundancy is that, for some people, f-stops are a much more intuitive way of describing light brightness than raw intensity values, especially when you're directly matching values to a plate. You may be asked by the director of photography (who is used to working with camera f-stop values) to increase or decrease a certain light by 'one stop'. Other than that, this light parameter has nothing to do with a real camera's f-stop control. Also, working with exposure means you won't have to type in huge values like 10,000 in the intensity input if your lights have quadratic falloff (which they should).

If you are not used to working with exposure in the lights, you can simply leave the exposure parameter at its default value of 0 (since 20 = 1, the formula then simplifies to: color * intensity * 1).

Per-light scaling for Diffuse, Specular, SSS, and Volume. Weights scaling the light contribution to each of those components independently. Should be left at 1 to produce physically accurate results.

Indirect

The relative energy loss (or gain) at each bounce. This should be left at its default value of 1.0 for physically meaningful results. Values bigger than 1 make it impossible for GI algorithms to converge to a stable solution.

 

Max Bounces

The maximum number of times the energy from this light will be allowed to bounce in the scene. A Max Bounces value of 0 means that the light will only be part of the direct lighting computations, effectively disabling GI for this light. Note that this value works along with the global ray depth controls, so the default value of 999 bounces per light is just a theoretical maximum; in practice, the global ray depth limits are set much lower.


Area

This group has different controls based on the light type, but all the lights share Samples.

 

Sampling

Samples

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. A schematic of how light noise occurs in Arnold can be found here. 

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 raytype. This diagram shows how a light is traced in Arnold.

 

The example below shows specular highlights from area light sources. There are four spherical mesh light sources of varying size and color temperature. 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. 

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.

More information about light samples and removing noise can be found here.

 

Shadows


Enabled

Enables the computation of shadows cast from the light. 


Color

Sets the intensity of each color channel for shadows. Normally this would be black.


Density

Sets the shadow density, or strength. This controls how the shadow blends with the material on which the shadow is cast: a value of 1.0 produces an opaque, black shadow, and a value of 0.0 gives no shadow. Normally this would be 1.0. 


Volume

Cast Volumetric Shadows

Determines if volumetric shadows will be computed. This option is not in Distant or Skydome Lights. 

Enabled (default)
Disabled


Samples

The volume samples parameter handles the number of samples used to integrate the in-scattering from direct light. Like the "samples" parameter for surfaces, it is also a squared number.

This parameter does not apply to Atmosphere Volume. To improve the quality of the Atmosphere Volume, you must increase the Atmosphere Volume samples.

 

Normalize 

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.


AOV Light Group

Per-light AOVs are available via a string parameter. Each light has an AOV parameter which writes out the light contribution to a separate AOV with a corresponding name. See also Light Path Expression AOVs.

There are two render channels that you can enable to contain the lights contribution, called Arnold_Lights and Arnold_Volume_Lights. Their format must be set manually to EXR, their data type to RGBA.

  • Enter a name for the per-light AOV in the 'AOV Light Group' of the light. For example, if the light group name is “red”, then the AOV name should be “volume_red”.

 


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