AOVs - Arnold Developer Guide

Arbitrary output variables (AOVs) make it possible to output image besides the beauty render, for using in compositing software.

Light Path Expression AOVs

For Arnold light path expressions are used to output light into specific AOVs, with the beauty RGBA AOV being the most commonly used one. Rather than writing to specific AOVs from shaders, LPEs can be used to extract specific light paths using regular expressions. Builtin LPEs are available for the common cases.

See Light Path Expression AOVs for more details.

Builtin AOVs

In addition to builtin LPEs, these builtin AOVs are available.

Custom AOVs

Shaders can output custom AOVs for cases that are not covered by LPEs and builtin AOVs. For example to output mattes or masks, ambient occlusion, textures, etc. AOVs are only written for camera rays.

Here's an example shader, writing a noise texture to a float AOV:

write_aov.cpp

#include <ai.h>

AI_SHADER_NODE_EXPORT_METHODS(WriteAovMtd);

struct WriteAovData
{
   AtString aov_name;
};

enum WriteAovParams
{
   p_aov_name,
};

node_parameters
{
   AiParameterStr("aov_name", "");
   AiMetaDataSetBool(nentry, "aov_name", "linkable", false);
}

node_initialize
{
   AiNodeSetLocalData(node, new WriteAovData());
}

node_update
{
   // register AOV
   WriteAovData *data = (WriteAovData*)AiNodeGetLocalData(node);
   data->aov_name = AiNodeGetStr(node, AtString("aov_name"));
   AiAOVRegister(data->aov_name, AI_TYPE_FLOAT, AI_AOV_BLEND_OPACITY);
}

node_finish
{
   WriteAovData *data = (WriteAovData*)AiNodeGetLocalData(node);
   delete data;
}

shader_evaluate
{
   const WriteAovData *data = (WriteAovData*)AiNodeGetLocalData(node);
   const float noise = AiPerlin3(sg->P);

   // write AOV only if in use
   if ((sg->Rt & AI_RAY_CAMERA) && AiAOVEnabled(data->aov_name, AI_TYPE_FLOAT))
      AiAOVSetFlt(sg, data->aov_name, noise);

   sg->out.FLT() = noise;
}

node_loader
{
   if (i != 0)
      return false;
   node->methods     = WriteAovMtd;
   node->output_type = AI_TYPE_FLOAT;
   node->name        = "write_aov";
   node->node_type   = AI_NODE_SHADER;
   strcpy(node->version, AI_VERSION);
   return true;
} 

options
{
 AA_samples 9
 outputs "my_custom_aov FLOAT /out/arnold1:gaussian_filter /out/arnold1:exr"
 aov_shaders "customaov"
 camera "camera"
 GI_diffuse_depth 1
 GI_specular_depth 1
 GI_diffuse_samples 3
}

write_aov
{
 name customaov
 aov_name "my_custom_aov"
}

driver_openexr
{
 name /out/arnold1:exr
 filename "av-custom_aov.exr"
}

gaussian_filter
{
 name /out/arnold1:gaussian_filter
}

persp_camera
{
 name camera
 position 0 15 15
 look_at 0 0 0

}

sphere
{
 name ball
 radius 10
}