AOVs (Arbitrary Output Variables) provide a way to render any arbitrary shading network component into different images. For 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, and motion vectors.
There are some limitations:
To the right of the Arnold Renderer tab in the Render Settings dialog, you should see the AOVs tab. The available AOVs for each group are shown in the left 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.
This allows you to enable AOVs, disable them, or specify that AOVs are only to be used in batch renders.
This allows you to select an AOV channel to preview in the render view. Remember to switch back to 'beauty' afterward.
Only shaders with AOV-writing capabilities should be added to 'AOV Shaders' in AOVs.
Use the fast, GPU-powered Nvidia OptiX AI denoiser. See the System tab for the GPU device selection options. When enabled a new AOV with the _denoise postfix is available in the AOV list.
Automatically outputs the optional AOVs (diffuse_albedo, Z, N). This should be used for denoising using Noice (Arnold denoiser), (multilayer EXR) or Kick.
A current limitation means that you must render in EXR and Merge AOVs need to be enabled.
This should also be enabled when using batch rendering with Noice.
The AOV Browser allows you to select the AOVs you want to be active for your render. You can also choose to add a custom AOV.
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.
The other AOV groups correspond to the shader nodes being used (assuming those shader nodes support AOV). For example, Shadow Matte provides:
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).
A tutorial about compositing AOVs can be found here.
Beneath the AOV browser, the active AOVs that you have selected for output are listed in 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 checkbox 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.
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:
AOV Light Group in Arnold attributes of light
2. Create an AOV and select it to show it in the attribute editor (in this case we have used a diffuse AOV).
3. Render the scene. The AOVs should now be visible in the RenderView by selecting them in the Light Groups List.
Rollover image for blue Light Group
The <LightGroup> token can be used in the output filename so that Light Group AOVs tokens can be inserted in custom ways.
A scene file that demonstrates light group AOVs (with denoising) can be downloaded here.