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, 11 insertions, 4 deletions

diff --git a/source/blender/functions/FN_field.hh b/source/blender/functions/FN_field.hh
index 78a49e342a5..ed5064fdf25 100644
--- a/source/blender/functions/FN_field.hh
+++ b/source/blender/functions/FN_field.hh
@@ -418,6 +418,8 @@ class IndexFieldInput final : public FieldInput {
  public:
   IndexFieldInput();
 
+  static GVArray *get_index_varray(IndexMask mask, ResourceScope &scope);
+
   const GVArray *get_varray_for_context(const FieldContext &context,
                                         IndexMask mask,
                                         ResourceScope &scope) const final;
diff --git a/source/blender/functions/intern/field.cc b/source/blender/functions/intern/field.cc
index 1f7bad134a8..c6125b65c5e 100644
--- a/source/blender/functions/intern/field.cc
+++ b/source/blender/functions/intern/field.cc
@@ -519,17 +519,22 @@ IndexFieldInput::IndexFieldInput() : FieldInput(CPPType::get<int>(), "Index")
 {
 }
 
-const GVArray *IndexFieldInput::get_varray_for_context(const fn::FieldContext &UNUSED(context),
-                                                       IndexMask mask,
-                                                       ResourceScope &scope) const
+GVArray *IndexFieldInput::get_index_varray(IndexMask mask, ResourceScope &scope)
 {
-  /* TODO: Investigate a similar method to IndexRange::as_span() */
   auto index_func = [](int i) { return i; };
   return &scope.construct<
       fn::GVArray_For_EmbeddedVArray<int, VArray_For_Func<int, decltype(index_func)>>>(
       mask.min_array_size(), mask.min_array_size(), index_func);
 }
 
+const GVArray *IndexFieldInput::get_varray_for_context(const fn::FieldContext &UNUSED(context),
+                                                       IndexMask mask,
+                                                       ResourceScope &scope) const
+{
+  /* TODO: Investigate a similar method to IndexRange::as_span() */
+  return get_index_varray(mask, scope);
+}
+
 uint64_t IndexFieldInput::hash() const
 {
   /* Some random constant hash. */