Geometry Nodes: Allow attribute nodes to use different domains

Currently every attribute node assumes that the attribute exists on the
"points" domain, so it generally isn't possible to work with attributes
on other domains like edges, polygons, and corners.

This commit adds a heuristic to each attribute node to determine the
correct domain for the result attribute. In general, it works like this:
 - If the output attribute already exists, use that domain.
 - Otherwise, use the highest priority domain of the input attributes.
 - If none of the inputs are attributes, use the default domain (points).

For the implementation I abstracted the check a bit, but in each
node has a slightly different situation, so we end up with slightly
different `get_result_domain` functions in each node. I think this makes
sense, it keeps the code flexible and more easily understandable.

Note that we might eventually want to expose a domain drop-down to some
of the nodes. But that will be a separate discussion; this commit focuses
on making a more useful choice automatically.

Differential Revision: https://developer.blender.org/D10389

1 files changed, 46 insertions, 0 deletions

diff --git a/source/blender/nodes/geometry/node_geometry_util.cc b/source/blender/nodes/geometry/node_geometry_util.cc
index 95c25795356..3e30658e056 100644
--- a/source/blender/nodes/geometry/node_geometry_util.cc
+++ b/source/blender/nodes/geometry/node_geometry_util.cc
@@ -338,6 +338,52 @@ CustomDataType attribute_data_type_highest_complexity(Span<CustomDataType> data_
   return most_complex_type;
 }
 
+/**
+ * \note Generally the order should mirror the order of the domains
+ * established in each component's ComponentAttributeProviders.
+ */
+static int attribute_domain_priority(const AttributeDomain domain)
+{
+  switch (domain) {
+#if 0
+    case ATTR_DOMAIN_CURVE:
+      return 0;
+#endif
+    case ATTR_DOMAIN_POLYGON:
+      return 1;
+    case ATTR_DOMAIN_EDGE:
+      return 2;
+    case ATTR_DOMAIN_POINT:
+      return 3;
+    case ATTR_DOMAIN_CORNER:
+      return 4;
+    default:
+      /* Domain not supported in nodes yet. */
+      BLI_assert(false);
+      return 0;
+  }
+}
+
+/**
+ * Domains with a higher "information density" have a higher priority, in order
+ * to choose a domain that will not lose data through domain conversion.
+ */
+AttributeDomain attribute_domain_highest_priority(Span<AttributeDomain> domains)
+{
+  int highest_priority = INT_MIN;
+  AttributeDomain highest_priority_domain = ATTR_DOMAIN_CORNER;
+
+  for (const AttributeDomain domain : domains) {
+    const int priority = attribute_domain_priority(domain);
+    if (priority > highest_priority) {
+      highest_priority = priority;
+      highest_priority_domain = domain;
+    }
+  }
+
+  return highest_priority_domain;
+}
+
 }  // namespace blender::nodes
 
 bool geo_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)