Particles: initial particle birth action

A particle action is some function that is triggered by some event.
Right now, users cannot control this. There is just a
randomize-velocity on-birth action. So the direction of spawned
particles is slightly randomized now.

This also adds a new integer attribute called "Hash" which is
useful for a number of things. Mainly for generating random numbers
for a specific particle. The ID of a particle is not necessarily a good source
of randomness.

1 files changed, 9 insertions, 0 deletions

diff --git a/source/blender/simulation/intern/particle_allocator.cc b/source/blender/simulation/intern/particle_allocator.cc
index 5ff5909f506..e47a6354d81 100644
--- a/source/blender/simulation/intern/particle_allocator.cc
+++ b/source/blender/simulation/intern/particle_allocator.cc
@@ -16,6 +16,8 @@
 
 #include "particle_allocator.hh"
 
+#include "BLI_rand.hh"
+
 namespace blender::sim {
 
 AttributesAllocator::~AttributesAllocator()
@@ -67,6 +69,13 @@ fn::MutableAttributesRef ParticleAllocator::allocate(int size)
         ids[pindex] = start_id + pindex;
       }
     }
+    else if (name == "Hash") {
+      MutableSpan<int> hashes = attributes.get<int>("Hash");
+      RandomNumberGenerator rng(hash_seed_ ^ (uint32_t)next_id_);
+      for (int pindex : IndexRange(size)) {
+        hashes[pindex] = (int)rng.get_uint32();
+      }
+    }
     else {
       type.fill_uninitialized(info.default_of(i), attributes.get(i).data(), size);
     }