Fix T84899: instance ids are not unique in common cases

Ids stored in the `id` attribute cannot be assumed to be unique. While they
might be unique in some cases, this is not something that can be guaranteed
in general. For some use cases (e.g. generating "stable randomness" on points)
uniqueness is not important. To support features like motion blur, unique ids
are important though.

This patch implements a simple algorithm that turns non-unique ids into
unique ones. It might fail to do so under very unlikely circumstances, in
which it returns non-unique ids instead of possibly going into an endless
loop.

Here are some requirements I set for the algorithm:
* Ids that are unique already, must not be changed.
* The same input should generate the same output.
* Handle cases when all ids are different and when all ids are the same
  equally well (in expected linear time).
* Small changes in the input id array should ideally only have a small
  impact on the output id array.

The reported bug happened because cycles found multiple objects with
the same id and thought that it was a single object that moved on every
check.

Differential Revision: https://developer.blender.org/D10402

1 files changed, 66 insertions, 3 deletions

diff --git a/source/blender/blenkernel/intern/geometry_set.cc b/source/blender/blenkernel/intern/geometry_set.cc
index a47a3dbc872..6db15c9393d 100644
--- a/source/blender/blenkernel/intern/geometry_set.cc
+++ b/source/blender/blenkernel/intern/geometry_set.cc
@@ -22,6 +22,8 @@
 
 #include "DNA_object_types.h"
 
+#include "BLI_rand.hh"
+
 #include "MEM_guardedalloc.h"
 
 using blender::float3;
@@ -542,6 +544,68 @@ bool InstancesComponent::is_empty() const
   return transforms_.size() == 0;
 }
 
+static blender::Array<int> generate_unique_instance_ids(Span<int> original_ids)
+{
+  using namespace blender;
+  Array<int> unique_ids(original_ids.size());
+
+  Set<int> used_unique_ids;
+  used_unique_ids.reserve(original_ids.size());
+  Vector<int> instances_with_id_collision;
+  for (const int instance_index : original_ids.index_range()) {
+    const int original_id = original_ids[instance_index];
+    if (used_unique_ids.add(original_id)) {
+      /* The original id has not been used by another instance yet. */
+      unique_ids[instance_index] = original_id;
+    }
+    else {
+      /* The original id of this instance collided with a previous instance, it needs to be looked
+       * at again in a second pass. Don't generate a new random id here, because this might collide
+       * with other existing ids. */
+      instances_with_id_collision.append(instance_index);
+    }
+  }
+
+  Map<int, RandomNumberGenerator> generator_by_original_id;
+  for (const int instance_index : instances_with_id_collision) {
+    const int original_id = original_ids[instance_index];
+    RandomNumberGenerator &rng = generator_by_original_id.lookup_or_add_cb(original_id, [&]() {
+      RandomNumberGenerator rng;
+      rng.seed_random(original_id);
+      return rng;
+    });
+
+    const int max_iteration = 100;
+    for (int iteration = 0;; iteration++) {
+      /* Try generating random numbers until an unused one has been found. */
+      const int random_id = rng.get_int32();
+      if (used_unique_ids.add(random_id)) {
+        /* This random id is not used by another instance. */
+        unique_ids[instance_index] = random_id;
+        break;
+      }
+      if (iteration == max_iteration) {
+        /* It seems to be very unlikely that we ever run into this case (assuming there are less
+         * than 2^30 instances). However, if that happens, it's better to use an id that is not
+         * unique than to be stuck in an infinite loop. */
+        unique_ids[instance_index] = original_id;
+        break;
+      }
+    }
+  }
+
+  return unique_ids;
+}
+
+blender::Span<int> InstancesComponent::almost_unique_ids() const
+{
+  std::lock_guard lock(almost_unique_ids_mutex_);
+  if (almost_unique_ids_.size() != ids_.size()) {
+    almost_unique_ids_ = generate_unique_instance_ids(ids_);
+  }
+  return almost_unique_ids_;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */
@@ -560,7 +624,7 @@ bool BKE_geometry_set_has_instances(const GeometrySet *geometry_set)
 
 int BKE_geometry_set_instances(const GeometrySet *geometry_set,
                                float (**r_transforms)[4][4],
-                               int **r_ids,
+                               const int **r_almost_unique_ids,
                                InstancedData **r_instanced_data)
 {
   const InstancesComponent *component = geometry_set->get_component_for_read<InstancesComponent>();
@@ -568,9 +632,8 @@ int BKE_geometry_set_instances(const GeometrySet *geometry_set,
     return 0;
   }
   *r_transforms = (float(*)[4][4])component->transforms().data();
-  *r_ids = (int *)component->ids().data();
-  *r_instanced_data = (InstancedData *)component->instanced_data().data();
   *r_instanced_data = (InstancedData *)component->instanced_data().data();
+  *r_almost_unique_ids = (const int *)component->almost_unique_ids().data();
   return component->instances_amount();
 }