Geometry Nodesのハマりどころから、Blenderのソースコードを垣間見る

static void node_declare(NodeDeclarationBuilder &b)
{
  b.add_input<decl::Float>("Size X")
      .default_value(1.0f)
      .min(0.0f)
      .subtype(PROP_DISTANCE)
      .description("Side length of the plane in the X direction");
  b.add_input<decl::Float>("Size Y")
      .default_value(1.0f)
      .min(0.0f)
      .subtype(PROP_DISTANCE)
      .description("Side length of the plane in the Y direction");
  b.add_input<decl::Int>("Vertices X")
      .default_value(3)
      .min(2)
      .max(1000)
      .description("Number of vertices in the X direction");
  b.add_input<decl::Int>("Vertices Y")
      .default_value(3)
      .min(2)
      .max(1000)
      .description("Number of vertices in the Y direction");
  b.add_output<decl::Geometry>("Mesh");
  b.add_output<decl::Vector>("UV Map").field_on_all();
}

BaseSocketDeclarationBuilder &BaseSocketDeclarationBuilder::field_on_all()
{
  if (this->is_input()) {
    this->supports_field();
  }
  if (this->is_output()) {
    this->field_source(); // <- 今回は出力なのでこちら
  }
  field_on_all_ = true;
  this->structure_type(StructureType::Field);
  return *this;
}

// field_source() の定義
BaseSocketDeclarationBuilder &BaseSocketDeclarationBuilder::field_source()
{
  BLI_assert(this->is_output());
  decl_base_->output_field_dependency = OutputFieldDependency::ForFieldSource();
  this->structure_type(StructureType::Field);
  return *this;
}

/** \file
 * \ingroup fn
 *
 * A #Field represents a function that outputs a value based on an arbitrary number of inputs. The
 * inputs for a specific field evaluation are provided by a #FieldContext.
 *
 * A typical example is a field that computes a displacement vector for every vertex on a mesh
 * based on its position.
 *
 * Fields can be built, composed and evaluated at run-time. They are stored in a directed tree
 * graph data structure, whereby each node is a #FieldNode and edges are dependencies. A #FieldNode
 * has an arbitrary number of inputs and at least one output and a #Field references a specific
 * output of a #FieldNode. The inputs of a #FieldNode are other fields.
 *
 * There are two different types of field nodes:
 *  - #FieldInput: Has no input and exactly one output. It represents an input to the entire field
 *    when it is evaluated. During evaluation, the value of this input is based on a #FieldContext.
 *  - #FieldOperation: Has an arbitrary number of field inputs and at least one output. Its main
 *    use is to compose multiple existing fields into new fields.
 *
 * When fields are evaluated, they are converted into a multi-function procedure which allows
 * efficient computation. In the future, we might support different field evaluation mechanisms for
 * e.g. the following scenarios:
 *  - Latency of a single evaluation is more important than throughput.
 *  - Evaluation should happen on other hardware like GPUs.
 *
 * Whenever possible, multiple fields should be evaluated together to avoid duplicate work when
 * they share common sub-fields and a common context.
 */

static void node_geo_exec(GeoNodeExecParams params)
{
  Field<float3> position_field{AttributeFieldInput::Create<float3>("position")};
  params.set_output("Position", std::move(position_field));
}

GVArray IDAttributeFieldInput::get_varray_for_context(const GeometryFieldContext &context,
                                                      const IndexMask &mask) const
{

  const StringRef name = get_random_id_attribute_name(context.domain());
  if (auto attributes = context.attributes()) {
    if (GVArray attribute = *attributes->lookup(name, context.domain(), CD_PROP_INT32)) {
      return attribute;
    }
  }

  /* Use the index as the fallback if no random ID attribute exists. */
  return fn::IndexFieldInput::get_index_varray(mask);
}

enum class AttrDomain : int8_t {
  /* Used to choose automatically based on other data. */
  Auto = -1,
  /* Mesh, Curve or Point Cloud Point. */
  Point = 0,
  /* Mesh Edge. */
  Edge = 1,
  /* Mesh Face. */
  Face = 2,
  /* Mesh Corner. */
  Corner = 3,
  /* A single curve in a larger curve data-block. */
  Curve = 4,
  /* Instance. */
  Instance = 5,
  /* A layer in a grease pencil data-block. */
  Layer = 6,
};

static void node_register()
{
  static blender::bke::bNodeType ntype;

  geo_node_type_base(&ntype, "GeometryNodeMeshGrid", GEO_NODE_MESH_PRIMITIVE_GRID);
  ntype.ui_name = "Grid";
  ntype.ui_description = "Generate a planar mesh on the XY plane";
  ntype.enum_name_legacy = "MESH_PRIMITIVE_GRID";
  ntype.nclass = NODE_CLASS_GEOMETRY;
  ntype.declare = node_declare;
  ntype.geometry_node_execute = node_geo_exec; // <- この行
  blender::bke::node_register_type(ntype);
}

/** \file
 * \ingroup nodes
 *
 * This file mainly converts a #bNodeTree into a lazy-function graph, that can then be evaluated to
 * execute geometry nodes. This generally works by creating a lazy-function for every node, which
 * is then put into the lazy-function graph. Then the nodes in the new graph are linked based on
 * links in the original #bNodeTree. Some additional nodes are inserted for things like type
 * conversions and multi-input sockets.
 *
 * If the #bNodeTree contains zones, those are turned into separate lazy-functions first.
 * Essentially, a separate lazy-function graph is created for every zone that is than called by the
 * parent zone or by the root graph.
 *
 * Currently, lazy-functions are even created for nodes that don't strictly require it, like
 * reroutes or muted nodes. In the future we could avoid that at the cost of additional code
 * complexity. So far, this does not seem to be a performance issue.
 */

/**
 * This file implements the evaluation of a lazy-function graph. It's main objectives are:
 * - Only compute values that are actually used.
 * - Stay single threaded when nodes are executed quickly.
 * - Allow spreading the work over an arbitrary number of threads efficiently.
 *
 * This executor makes use of `FN_lazy_threading.hh` to enable multi-threading only when it seems
 * beneficial. It operates in two modes: single- and multi-threaded. The use of a task pool and
 * locks is avoided in single-threaded mode. Once multi-threading is enabled the executor starts
 * using both. It is not possible to switch back from multi-threaded to single-threaded mode.
 *
 * The multi-threading design implemented in this executor requires *no* main thread that
 * coordinates everything. Instead, one thread will trigger some initial work and then many threads
 * coordinate themselves in a distributed fashion. In an ideal situation, every thread ends up
 * processing a separate part of the graph which results in less communication overhead. The way
 * TBB schedules tasks helps with that: a thread will next process the task that it added to a task
 * pool just before.

 （=== 後略 ===）

*/

    // (=== 前略 ===)
    
    const bool use_individual_instance = pick_instance[i]; 
    // メモ: ↑ ここでPick Instanceの値が使われている
    
    if (use_individual_instance) {
      if (src_instances != nullptr) {
        const int src_instances_num = src_instances->instances_num();
        const int original_index = indices[i];
        /* Use #mod_i instead of `%` to get the desirable wrap around behavior where -1
         * refers to the last element. */
        const int index = mod_i(original_index, std::max(src_instances_num, 1));
        if (index < src_instances_num) {
          /* Get the reference to the source instance. */
          const int src_handle = src_instances->reference_handles()[index];
          dst_handle = handle_mapping[src_handle];

          /* Take transforms of the source instance into account. */
          mul_m4_m4_post(dst_transform.ptr(), src_instances->transforms()[index].ptr());
        }
      }
    }

    // (=== 後略 ===)