Geometry Nodes: Make random attribute node stable

Currently, the random attribute node doesn't work well for most
workflows because for any change in the input data it outputs
completely different results.

This patch adds an implicit seed attribute input to the node, referred
to by "id". The attribute is hashed for each element using the CPPType
system's hash method, meaning the attribute can have any data type.
Supporting any data type is also important so any attribute can be
copied into the "id" attribute and used as a seed.

The "id" attribute is an example of a "reserved name" attribute,
meaning attributes with this name can be used implicitly by nodes like
the random attribute node. Although it makes it a bit more difficult
to dig deeper, using the name implicitly rather than exposing it as an
input should make the system more accessible and predictable.

Differential Revision: https://developer.blender.org/D9832

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_geometry_set.hh b/source/blender/blenkernel/BKE_geometry_set.hh
index 90d444aa270..e4232a84a00 100644
--- a/source/blender/blenkernel/BKE_geometry_set.hh
+++ b/source/blender/blenkernel/BKE_geometry_set.hh
@@ -136,6 +136,11 @@ class GeometryComponent {
       const AttributeDomain domain,
       const CustomDataType data_type) const;
 
+  /* Get a read-only attribute interpolated to the input domain, leaving the data type unchanged.
+   * Returns null when the attribute does not exist. */
+  blender::bke::ReadAttributePtr attribute_try_get_for_read(
+      const blender::StringRef attribute_name, const AttributeDomain domain) const;
+
   /* Get a read-only attribute for the given domain and data type.
    * Returns a constant attribute based on the default value if the attribute does not exist.
    * Never returns null. */