To add any lights to a scene an Arnold Light node needs to be created. This node is used for all lights in Arnold for Houdini. It can be used as a point, distant, spot, disk, cylinder, quad, meshskydome or photometric.

 

 

An Arnold Light node can be created by tab-typing Arnold Light in the Network Editor at object level (Tab > Arnold > Arnold Light).

The native Houdini lights won't be translated. Arnold lights must be used.

Common Parameters

These properties are available to all types of Arnold Lights.

Light Type

Controls the shape of the light source. Options are pointdistantspotdisccylinderquadmesh or skydome.

Enable Light

Sets whether a light contributed to the illumination of the current scene.

Although the light will not contribute in the render, it will still be visible in the viewport.

Color

Controls the color of the emitted light.

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 exactly 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).

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 glossy 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 glossy). This is easy to achieve by turning off global illumination; set the GI_diffuse_depth, GI_glossy_depth and GI_reflection_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.

Volume 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.

'Volume Samples' does not apply to the 'Volume Scattering' atmosphere shader in the Environment section. To improve the quality of the 'Volume Scattering' atmosphere shader, you must increase the 'Atmospheric Scattering' samples.

Decay Type

Decay type is not available for Directional, Distant or Skydome lights. Arnold supports two decay types: Constant will disable any decay, and is equivalent to an exponent of 0.  Quadratic will use an inverse-square decay following the formula 1/distance 2, which is actually the only physically meaningful decay in real-world lights.

It is strongly suggested to use quadratic decay.

The half distance attenuation that is available on Houdini lights is not supported in Arnold. To tweak the attenuation use the Light Decay filter.

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.


Shadows

These settings are common to all lights.

Shadow Color

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


Shadow 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. 


Cast Shadows

Enables the computation of shadows cast from the light. 


Cast Atmospheric Shadows

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

Enabled
Disabled


Filters

Arnold light filters are arbitrary shaders that can modify a light's output based on distance, position or other factors. This makes it easy to extend the built-in light sources with additional effects.

 

Light Filters are created within Arnold Shader Networks and must be connected to a light filter output node. You can connect several filters. The vopnet is then connected to Arnold Lights under the Light Filter tab. For convenience, a default shader network containing a light filter output for is created within the light node.

  • The Light Decay filter specifies the attenuation ranges of where the light starts and ends.
  • The Light Blocker filter will block out an arbitrary geometrically defined area of a connected light.
  • The Gobo filter is used to break up the light beam into an irregular, more natural pattern such as tree leaves etc.
  • Barndoors are opaque moving panels attached to the sides of the light's opening used for additional control over the shape of the light beam. 

More information can be found in the Light Filters section.


Contribution

Per-light scaling for Diffuse / Specular / SSS / Indirect and Volume. Weights scaling the light contribution to each of those components independently. Should be left at 1 to produce physically accurate results. These settings are common to all lights. Indirect was previously Bounce Factor.

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.


AOV Light Group

Per-light AOVs are available via a string parameter (currently only supported for volumes). Each light has an AOV parameter which writes out the light contribution to a separate AOV with a corresponding name.

Information about creating per-light AOVs can be found here.

Affect Viewport

Turns on/off the preview of the effect of the light in the Houdini viewport.

Although the light will not be visible in the viewport, it will still contribute to the render.

Affect Atmospherics

Allow the light to affect atmospheric scattering and fogDistant lights don't support atmospheric volumetrics.

 


Specific Light Parameters

See the pages below for more information on specific light types.

 

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