Geometry Nodes: add empty material slot to new meshes

This fixes T88455 by adding an empty material slot to newly
generated meshes. This allows the object to overwrite the
"default" material without any extra nodes. Technically,
all polygons reference the material index 0 already, so it
makes sense to add a material slot for this material index.

Differential Revision: https://developer.blender.org/D11439

1 files changed, 16 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/material.c b/source/blender/blenkernel/intern/material.c
index 73b64e6efb3..557fe5f7e75 100644
--- a/source/blender/blenkernel/intern/material.c
+++ b/source/blender/blenkernel/intern/material.c
@@ -770,6 +770,7 @@ int BKE_object_material_count_eval(Object *ob)
 
 void BKE_id_material_eval_assign(ID *id, int slot, Material *material)
 {
+  BLI_assert(slot >= 1);
   Material ***materials_ptr = BKE_id_material_array_p(id);
   short *len_ptr = BKE_id_material_len_p(id);
   if (ELEM(NULL, materials_ptr, len_ptr)) {
@@ -793,6 +794,21 @@ void BKE_id_material_eval_assign(ID *id, int slot, Material *material)
   (*materials_ptr)[slot_index] = material;
 }
 
+/**
+ * Add an empty material slot if the id has no material slots. This material slot allows the
+ * material to be overwritten by object-linked materials.
+ */
+void BKE_id_material_eval_ensure_default_slot(ID *id)
+{
+  short *len_ptr = BKE_id_material_len_p(id);
+  if (len_ptr == NULL) {
+    return;
+  }
+  if (*len_ptr == 0) {
+    BKE_id_material_eval_assign(id, 1, NULL);
+  }
+}
+
 Material *BKE_gpencil_material(Object *ob, short act)
 {
   Material *ma = BKE_object_material_get(ob, act);