Geometry Nodes: simplify using selection when evaluating fields

We often had to use two `FieldEvaluator` instances to first evaluate
the selection and then the remaining fields. Now both can be done
with a single `FieldEvaluator`. This results in less boilerplate code in
many cases.

Performance is not affected by this change. In a separate patch we
could improve performance by reusing evaluated sub-fields that are
used by the selection and the other fields.

Differential Revision: https://developer.blender.org/D13571

1 files changed, 17 insertions, 0 deletions

diff --git a/source/blender/functions/FN_field.hh b/source/blender/functions/FN_field.hh
index cf96eff62bd..a591aaed34a 100644
--- a/source/blender/functions/FN_field.hh
+++ b/source/blender/functions/FN_field.hh
@@ -313,6 +313,9 @@ class FieldEvaluator : NonMovable, NonCopyable {
   Vector<OutputPointerInfo> output_pointer_infos_;
   bool is_evaluated_ = false;
 
+  Field<bool> selection_field_;
+  IndexMask selection_mask_;
+
  public:
   /** Takes #mask by pointer because the mask has to live longer than the evaluator. */
   FieldEvaluator(const FieldContext &context, const IndexMask *mask)
@@ -333,6 +336,18 @@ class FieldEvaluator : NonMovable, NonCopyable {
   }
 
   /**
+   * The selection field is evaluated first to determine which indices of the other fields should
+   * be evaluated. Calling this method multiple times will just replace the previously set
+   * selection field. Only the elements selected by both this selection and the selection provided
+   * in the constructor are calculated. If no selection field is set, it is assumed that all
+   * indices passed to the constructor are selected.
+   */
+  void set_selection(Field<bool> selection)
+  {
+    selection_field_ = std::move(selection);
+  }
+
+  /**
    * \param field: Field to add to the evaluator.
    * \param dst: Mutable virtual array that the evaluated result for this field is be written into.
    */
@@ -403,6 +418,8 @@ class FieldEvaluator : NonMovable, NonCopyable {
     return this->get_evaluated(field_index).typed<T>();
   }
 
+  IndexMask get_evaluated_selection_as_mask();
+
   /**
    * Retrieve the output of an evaluated boolean field and convert it to a mask, which can be used
    * to avoid calculations for unnecessary elements later on. The evaluator will own the indices in