Geometry Nodes: Make Random ID a builtin attribute, remove sockets

In order to address feedback that the "Stable ID" was not easy enough
to use, remove the "Stable ID" output from the distribution node and
the input from the instance on points node. Instead, the nodes write
or read a builtin named attribute called `id`. In the future we may
add more attributes like `edge_id` and `face_id`.

The downside is that more behavior is invisible, which is les
expected now that most attributes are passed around with node links.
This behavior will have to be explained in the manual.

The random value node's "ID" input that had an implicit index input
is converted to a special implicit input that uses the `id` attribute
if possible, but otherwise defaults to the index. There is no way to
tell in the UI which it uses, except by knowing that rule and checking
in the spreadsheet for the id attribute.

Because it isn't always possible to create stable randomness, this
attribute does not always exist, and it will be possible to remove it
when we have the attribute remove node back, to improve performance.

Differential Revision: https://developer.blender.org/D12903

2 files changed, 13 insertions, 30 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_distribute_points_on_faces.cc b/source/blender/nodes/geometry/nodes/node_geo_distribute_points_on_faces.cc
index 44beead86ad..0d481011f00 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_distribute_points_on_faces.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_distribute_points_on_faces.cc
@@ -58,7 +58,6 @@ static void geo_node_point_distribute_points_on_faces_declare(NodeDeclarationBui
   b.add_output<decl::Geometry>("Points");
   b.add_output<decl::Vector>("Normal").field_source();
   b.add_output<decl::Vector>("Rotation").subtype(PROP_EULER).field_source();
-  b.add_output<decl::Int>("Stable ID").field_source();
 }
 
 static void geo_node_point_distribute_points_on_faces_layout(uiLayout *layout,
@@ -329,7 +328,6 @@ namespace {
 struct AttributeOutputs {
   StrongAnonymousAttributeID normal_id;
   StrongAnonymousAttributeID rotation_id;
-  StrongAnonymousAttributeID stable_id_id;
 };
 }  // namespace
 
@@ -339,19 +337,16 @@ BLI_NOINLINE static void compute_attribute_outputs(const MeshComponent &mesh_com
                                                    const Span<int> looptri_indices,
                                                    const AttributeOutputs &attribute_outputs)
 {
-  OutputAttribute_Typed<int> id_attribute;
+  OutputAttribute_Typed<int> id_attribute = point_component.attribute_try_get_for_output_only<int>(
+      "id", ATTR_DOMAIN_POINT);
+  MutableSpan<int> ids = id_attribute.as_span();
+
   OutputAttribute_Typed<float3> normal_attribute;
   OutputAttribute_Typed<float3> rotation_attribute;
 
-  MutableSpan<int> ids;
   MutableSpan<float3> normals;
   MutableSpan<float3> rotations;
 
-  if (attribute_outputs.stable_id_id) {
-    id_attribute = point_component.attribute_try_get_for_output_only<int>(
-        attribute_outputs.stable_id_id.get(), ATTR_DOMAIN_POINT);
-    ids = id_attribute.as_span();
-  }
   if (attribute_outputs.normal_id) {
     normal_attribute = point_component.attribute_try_get_for_output_only<float3>(
         attribute_outputs.normal_id.get(), ATTR_DOMAIN_POINT);
@@ -379,9 +374,8 @@ BLI_NOINLINE static void compute_attribute_outputs(const MeshComponent &mesh_com
     const float3 v1_pos = float3(mesh.mvert[v1_index].co);
     const float3 v2_pos = float3(mesh.mvert[v2_index].co);
 
-    if (!ids.is_empty()) {
-      ids[i] = noise::hash(noise::hash_float(bary_coord), looptri_index);
-    }
+    ids[i] = noise::hash(noise::hash_float(bary_coord), looptri_index);
+
     float3 normal;
     if (!normals.is_empty() || !rotations.is_empty()) {
       normal_tri_v3(normal, v0_pos, v1_pos, v2_pos);
@@ -394,9 +388,8 @@ BLI_NOINLINE static void compute_attribute_outputs(const MeshComponent &mesh_com
     }
   }
 
-  if (id_attribute) {
-    id_attribute.save();
-  }
+  id_attribute.save();
+
   if (normal_attribute) {
     normal_attribute.save();
   }
@@ -512,6 +505,10 @@ static void point_distribution_calculate(GeometrySet &geometry_set,
     }
   }
 
+  if (positions.is_empty()) {
+    return;
+  }
+
   PointCloud *pointcloud = BKE_pointcloud_new_nomain(positions.size());
   memcpy(pointcloud->co, positions.data(), sizeof(float3) * positions.size());
   uninitialized_fill_n(pointcloud->radius, pointcloud->totpoint, 0.05f);
@@ -551,9 +548,6 @@ static void geo_node_point_distribute_points_on_faces_exec(GeoNodeExecParams par
   if (params.output_is_required("Rotation")) {
     attribute_outputs.rotation_id = StrongAnonymousAttributeID("rotation");
   }
-  if (params.output_is_required("Stable ID")) {
-    attribute_outputs.stable_id_id = StrongAnonymousAttributeID("stable id");
-  }
 
   geometry_set.modify_geometry_sets([&](GeometrySet &geometry_set) {
     point_distribution_calculate(
@@ -575,11 +569,6 @@ static void geo_node_point_distribute_points_on_faces_exec(GeoNodeExecParams par
         "Rotation",
         AnonymousAttributeFieldInput::Create<float3>(std::move(attribute_outputs.rotation_id)));
   }
-  if (attribute_outputs.stable_id_id) {
-    params.set_output(
-        "Stable ID",
-        AnonymousAttributeFieldInput::Create<int>(std::move(attribute_outputs.stable_id_id)));
-  }
 }
 
 }  // namespace blender::nodes
diff --git a/source/blender/nodes/geometry/nodes/node_geo_instance_on_points.cc b/source/blender/nodes/geometry/nodes/node_geo_instance_on_points.cc
index c7235fb2c29..78399fad2c0 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_instance_on_points.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_instance_on_points.cc
@@ -48,11 +48,6 @@ static void geo_node_instance_on_points_declare(NodeDeclarationBuilder &b)
       .subtype(PROP_XYZ)
       .supports_field()
       .description("Scale of the instances");
-  b.add_input<decl::Int>("Stable ID")
-      .supports_field()
-      .description(
-          "ID for every instance that is used to identify it over time even when the number of "
-          "instances changes. Used for example for motion blur");
 
   b.add_output<decl::Geometry>("Instances");
 }
@@ -91,12 +86,11 @@ static void add_instances_from_component(InstancesComponent &dst_component,
   const VArray<float3> *scales = nullptr;
   /* The evaluator could use the component's stable IDs as a destination directly, but only the
    * selected indices should be copied. */
-  const VArray<int> *stable_ids = nullptr;
+  GVArray_Typed<int> stable_ids = src_component.attribute_get_for_read("id", ATTR_DOMAIN_POINT, 0);
   field_evaluator.add(params.get_input<Field<bool>>("Pick Instance"), &pick_instance);
   field_evaluator.add(params.get_input<Field<int>>("Instance Index"), &indices);
   field_evaluator.add(params.get_input<Field<float3>>("Rotation"), &rotations);
   field_evaluator.add(params.get_input<Field<float3>>("Scale"), &scales);
-  field_evaluator.add(params.get_input<Field<int>>("Stable ID"), &stable_ids);
   field_evaluator.evaluate();
 
   GVArray_Typed<float3> positions = src_component.attribute_get_for_read<float3>(