AOVs

 

AOVs (Arbitrary Output Variables) provide a way to render any arbitrary shading network component into different imagesFor example, an artist might find it convenient to separate direct and indirect lighting contributions and later recombine them during compositing. Arnold provides built-in AOVs for outputting depth, position, motion vectors. 

Multichannel EXRs are possible using Merge AOVs.

MtoA supports Cryptomatte AOV shaders. A tutorial that shows how to set them up can be found here.

 

To the right of the Arnold Renderer tab of the Render Settings dialog, you should see the AOVs tab. This is what the tab will look like initially:

 

The available AOVs for each group are shown in the central list. When you select one, it will move across to the 'Active AOVs' list, which is the set of AOVs you want to use for your render.

Above the AOV Browser, two drop downs are provided, 'Mode' and 'Renderview AOV':

An introduction to AOV's' video can be found here.

Mode

This allows you to enable AOVs, disable them, or specify that AOVs are only to be used in batch renders.

Render View AOV

This allows you to select an AOV channel to preview in the render view. Remember to switch back to 'RGBA' afterward.

Default Drivers

Set to Maya by default, it allows you to use an external renderview like the experimental Houdini MPlay driver. To use mplay, you need to define a system environment variable called __HOUDINI_BINARY_FOLDER that points to the bin folder of your houdini installation.

Then, setup the mplay as the default driver and start a render, either from the renderview or from the external renderview experimental menu.

AOV Shaders  

A list of shaders can be defined that will be evaluated after the regular surface shader. With this, it's possible to add shaders to set specific AOVs without modifying the original shader tree. Shaders intended for this purpose should add a boolean metadata named aov_shader on the node itself, as a user-interface hint. If options.atmosphere or options.background are set, these global AOV shaders will also be run for atmosphere and background contexts.

 

Only shaders with AOV-writing capabilities should be added to 'AOV Shaders' in AOVs.

AOV shaders such as Cryptomatte can also be added to 'AOV Shaders' in AOVs.

Example 

If you want to see the result of a Utility shader in a custom AOV, you should assign a Write Color shader instead, and connect your Utility shader to the Write Color input of the Write Color shader. Then you must add the AOV name to the Write Color that you wish to use. If it's a custom AOV name, then you should also manually create it in the AOV list.

Utility shader connected to input of Write Color shader, which has been added to 'AOV Shaders'

AOV Browser

The middle section, AOV Browser, allows you to select the AOVs you want to be active for your render:

Either double click or select and use the arrow buttons to move AOVs from 'Available' to 'Active'.

 Arnold provides the following 'built in' system AOVs. These AOVs are always available, no matter what shader(s) you are using.

  • ID: Random number value derived from the name of the shape . You can also add specific ID numbers via the user options string field for an object. ie 'id 1'.
  • N: Smooth normal at the shading point (in world space).
  • P: Position of the shading point (in world space).
  • Pref:  Reference position of the shading point.
  • RGBA: Beauty AOV, containing the full rendered image.
  • Z: Depth of the shading points as seen from the camera.
  • albedo: Reflectivity, the surface or volume color without lighting or shadowing.
  • background: Emission from the background and skydome lights visible to the camera.
  • coat: Coat reflection.
  • coat_albedo: Coat color without lighting or shadowing.
  • coat_direct: Coat direct lighting.
  • coat_indirect: Coat indirect lighting.
  • cputime: This layer contains the CPU time (measured in “ticks”) to evaluate the samples in the pixel.
  • diffuse: Diffuse reflection.
  • diffuse_albedo: Diffuse color without lighting or shadowing.
  • diffuse_direct: Diffuse direct lighting.
  • diffuse_indirect: Diffuse indirect light.
  • direct: Direct lighting from all surfaces and volumes.
  • emission: Lights and emissive objects directly visible from the camera.
  • indirect: Indirect light from all surfaces and volumes.
  • motionvector: 2D vector representing the motion in screen space of the shading point during the given time interval (the shutter start and shutter end of the camera). If output to an RGB format, the vector is contained in the R and G channels.
  • opacity: RGB AOV with full three channel opacity (as opposed to single channel alpha).
  • raycount: Total number of rays traced for samples in the pixel.
  • shadow_matteShadows in the scene, computed as the the ratio of occluded direct lighting over unoccluded direct lighting.
  • specular: Specular reflection.
  • specular_albedo: Specular color without lighting or shadowing.
  • specular_direct: Diffuse direct lighting.
  • specular_indirect: Diffuse indirect lighting.
  • sss: Subsurface scattering and diffuse transmission.
  • sss_albedo: SSS and diffuse transmission color without lighting or shadowing.
  • sss_direct: SSS and diffuse transmission direct lighting.
  • sss_indirect: SSS and diffuse transmission indirect lighting.
  • transmission: Specular transmission (refraction).
  • transmission_albedo: Specular transmission color without lighting or shadowing.
  • transmission_direct: Specular transmission direct lighting.
  • transmission_indirect: Specular transmission indirect lighting.
  • volume: Volume scattering.
  • volume_albedo: Volume color without lighting or shadowing.
  • volume direct: Volume scatter direct lighting.
  • volume indirect: Volume scattering indirect lighting.
  • volume opacity: RGB AOV with the full three channel opacity for volumes only.

 

The other AOV groups correspond to the shader nodes being used (assuming those shader nodes support AOV). For example, Shadow Matte provides:

  • indirect_diffuse: Indirect diffuse.
  • indirect_specular: Indirect specular.
  • shadow: Direct light shadow.
  • shadow diff: A difference AOV which can be used to eliminate the shadow from the direct component.
  • shadow mask: This AOV can be used in comp to localize and tweak the shadow.

 

Other shaders used in your scene will support various other AOVs. Multiple shaders can contribute to the same AOV (for example a Standard Surface and a Lambert shader both write to the diffuse_direct AOV). 

Beneath the AOV browser the active AOVs you have selected for output are listed with more detail:

Each AOV is actually represented by three nodes - the AOV node itself, plus an associated driver node and filter node.

This dialog shows you the type, driver and filter of the chosen AOVs in drop down menus, and the check box to the left allows you to control which are active. If you click on the triangle at the far right a context menu is displayed which in addition to providing another way to remove or make an AOV active/inactive, also allows you to add an alternative output driver for each AOV (and to select the driver and filter).

 

Because the driver and filter nodes are separate to the AOV node, you can add multiple outputs for each AOV by adding extra driver nodes to that AOV node, for example allowing both exr and jpg to be written out by the same AOV. This extra flexibility in AOV output can be really useful in some situations (maybe you want to output your beauty pass in multiple formats or using different filters – or you might have a custom output driver that you want to output to at the same time as the normal render view).

 

Note that the driver drop-downs will show the driver which is currently the selected file output type in the Common tab of render settings in brackets, e.g., <exr> as in the above screenshot. So, you can choose a specific driver for each AOV, or if you choose the top one from each drop-down menu, shown in angle brackets, then you are choosing to use the current default output driver. So, in the above illustration where the driver is shown as <exr>, if you went to the common tab and altered the file output to be png, when you return to the AOV tab you'll see that all the drivers are now shown as <png>. If you don't want them to change to reflect the current default driver, select a driver name without angle brackets. Similarly, the filter selection is also shown with angle brackets if it is set to use the current default filter.

Each AOV has a node associated with it. You can use the attribute editor to examine these nodes in more detail.

A tutorial about using custom AOVs can be found here. 

You can define custom Light Path Expressions to write lighting components into separate AOVs using the following method:

  • Create a custom AOV.
  • Select the arrow to the right of the Custom AOV and choose Select AOV Node.
  • You can then set the LPE in the attribute editor.


Per-light AOVs

Each light object has an 'AOV Light Group' attribute which can be used to write out the light contribution to a separate AOV with a corresponding name. To create a per-light AOV you must do the following:

  1. Enter a name for the per-light AOV in the 'AOV Light Group' of the light. 
  2. Add a custom AOV with a "volume_" or "RGBA_" prefix in the Render Settings AOV tab. For example, if the light group name is “red”, then the AOV name should be “volume_red”.

 

Light Group Example

In this example a volume is lit by three lights, a blue, a red and a green light. The blue and the red one have different AOV light groups defined while the green light does not belong to any groups.

"RGBA_default" is a beauty AOV that contains volumes if they are lit by lights not in any AOV light group, whereas "volume_default" should have volume illumination from lights not in an AOV group.

"Volume_default" + "volume_red" + "volume_blue" give the total volume pass, whereas "RGBA_default" + "RGBA_red" + "RGBA_blue" give the total RGBA beauty pass.

The green light is not in an AOV group

A maximum of 16 different light AOVs are supported, although a given AOV can contain a bundle of any number of lights.

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