Here is a simple instancer procedural developed based on code by Gael Honorez.

This should be a good starting point for learning how to write an Arnold procedural since it covers reading user-declared procedural parameters, how to create a new node, set its parameters and setup motion blur matrices.

#include <ai.h>
#include <sstream>
#include <vector>

struct Instancer
   std::string name;
   AtArray *path_indices;
   AtArray *path_start_indices;
   AtArray *instance_matrix;
   std::vector<AtNode*> objects;
   int num_instances;


   AiParameterArray("path_indices"       , AiArray(0, 1, AI_TYPE_INT));
   AiParameterArray("path_start_indices" , AiArray(0, 1, AI_TYPE_INT));
   AiParameterArray("objects"            , AiArray(0, 1, AI_TYPE_STRING));
   AiParameterArray("instance_matrix"    , AiArray(0, 1, AI_TYPE_MATRIX));

   Instancer *instancer = new Instancer();
   *user_ptr = instancer;

   // get procedural name so it's the prefix of instanced nodes' name
   instancer->name = AiNodeGetStr(node, "name");
   instancer->path_indices = AiNodeGetArray(node, "path_indices");
   instancer->num_instances = AiArrayGetNumElements(instancer->path_indices);

   // this is the start index of each instance
   instancer->path_start_indices = AiNodeGetArray(node, "path_start_indices");
   AiMsgInfo("[instancer] number of instances: %d", AiArrayGetNumElements(instancer->path_indices));

   AtArray *object_names = AiNodeGetArray(node, "objects"); // to get the AtNode pointers for all the master nodes
   // allocate and create AtNode pointer array
   AiMsgInfo("[instancer] getting master node list ...");

   for (unsigned int i = 0; i < AiArrayGetNumElements(object_names); i++)
      const char *master_name = AiArrayGetStr(object_names, i);
      AiMsgInfo("[instancer] adding node %s to master list", master_name);

      AtNode *node = AiNodeLookUpByName(master_name);
      instancer->objects[i] = node;
      if (node == NULL)
         AiMsgWarning("[instancer] node %s does not exist!", master_name);

   // get matrices pointer and number of motion keys
   AtArray *instance_matrix = AiNodeGetArray(node, "instance_matrix");
   instancer->instance_matrix = instance_matrix;
   AiMsgInfo("[instancer] instance_matrix elements: %d, motion keys: %d", AiArrayGetNumElements(instance_matrix), AiArrayGetNumKeys(instance_matrix));

   return true;

   Instancer *instancer = (Instancer*)user_ptr;
   delete instancer;
   return true;

   // return how many nodes to generate
   Instancer *instancer = (Instancer*)user_ptr;
   return AiArrayGetNumElements(instancer->instance_matrix);

   Instancer *instancer = (Instancer*)user_ptr;

   int instance_index = AiArrayGetInt(instancer->path_start_indices, i); // get particle index for this instance
   int path_index = AiArrayGetInt(instancer->path_indices, instance_index);
   AtNode *object = instancer->objects[path_index];

   AtNode *currentInstance = AiNode("ginstance", "instancer", node); // initialize node as child of procedural node

   std::stringstream numStr; // setup node name by concatenate procedural name with instance node number
   numStr << "_" << instance_index;
   std::string currentName(instancer->name + numStr.str());

   AiNodeSetStr(currentInstance, "name", currentName.c_str());
   AiNodeSetPtr(currentInstance, "node", (void *)object); // setup ginstance node parameter
   AiNodeSetBool(currentInstance, "inherit_xform", false); // usually instancer doesn't move around, but maybe this should be set to true?

   int num_motion_keys = AiArrayGetNumKeys(instancer->instance_matrix);
   if (num_motion_keys > 1)
      // allocate and assign matrices
      AtArray *matrices = AiArrayAllocate(1, num_motion_keys, AI_TYPE_MATRIX);
      for (int j = 0; j < num_motion_keys; j++)
         AtMatrix matrix = AiArrayGetMtx(instancer->instance_matrix, instance_index + instancer->num_instances * j);
         AiArraySetMtx(matrices, j, matrix);
      AiNodeSetArray(currentInstance, "matrix", matrices);
      AtMatrix matrix = AiArrayGetMtx(instancer->instance_matrix, instance_index);
      AiNodeSetMatrix(currentInstance, "matrix", matrix);

   return currentInstance;

   if (i>0)
      return false;
   node->methods      = InstancerMtd;
   node->output_type  = AI_TYPE_NONE;
   node->name         = "instancer";
   node->node_type    = AI_NODE_SHAPE_PROCEDURAL;
   strcpy(node->version, AI_VERSION);
   return true;



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