Geometry Nodes: Draw curve data in the viewport

This patch adds relatively small changes to the curve draw
cache implementation in order to draw the curve data in the
viewport. The dependency graph iterator is also modified
so that it iterates over the curve geometry component, which
is presented to users as `Curve` data with a pointer to the
`CurveEval`

The idea with the spline data type in geometry nodes is that
curve data itself is only the control points, and any evaluated
data with faces is a mesh. That is mostly expected elsewhere in
Blender anyway. This means it's only necessary to implement
wire edge drawing of `CurveEval` data.

Adding a `CurveEval` pointer to `Curve` is in line with changes
I'd like to make in the future like using `CurveEval` in more places
such as edit mode.

An alternate solution involves converting the curve wire data
to a mesh, however, that requires copying all of the data, and
since avoiding it is rather simple and is in-line with future plans
anyway, I think doing it this way is better.

Differential Revision: https://developer.blender.org/D11351

1 files changed, 23 insertions, 0 deletions

diff --git a/source/blender/depsgraph/intern/depsgraph_query_iter.cc b/source/blender/depsgraph/intern/depsgraph_query_iter.cc
index ed002321729..df1cf8cc771 100644
--- a/source/blender/depsgraph/intern/depsgraph_query_iter.cc
+++ b/source/blender/depsgraph/intern/depsgraph_query_iter.cc
@@ -219,6 +219,29 @@ bool deg_iterator_components_step(BLI_Iterator *iter)
     }
   }
 
+  /* The curve component. */
+  if (data->geometry_component_id == 3) {
+    data->geometry_component_id++;
+
+    const CurveComponent *component = geometry_set->get_component_for_read<CurveComponent>();
+    if (component != nullptr) {
+      const Curve *curve = component->get_curve_for_render();
+
+      if (curve != nullptr) {
+        Object *temp_object = &data->temp_geometry_component_object;
+        *temp_object = *data->geometry_component_owner;
+        temp_object->type = OB_CURVE;
+        temp_object->data = (void *)curve;
+        /* Assign data_eval here too, because curve rendering code tries
+         * to use a mesh if it can find one in this pointer. */
+        temp_object->runtime.data_eval = (ID *)curve;
+        temp_object->runtime.select_id = data->geometry_component_owner->runtime.select_id;
+        iter->current = temp_object;
+        return true;
+      }
+    }
+  }
+
   data->geometry_component_owner = nullptr;
   return false;
 }