Fix T63563: Grease pencil not visible when contained in a collection instanced in a foreign scene.

The check of DUPLI object was used, but as the original object was not in the scene, the VBO data was not available and the draw was empty.

Now, the function checks if the original data is in the scene, and set the object as not Dupli to generate the VBO data.

source/blender/draw/engines/gpencil/gpencil_cache_utils.c

@@ -31,6 +31,7 @@
 
 #include "BKE_library.h"
 #include "BKE_gpencil.h"
+#include "BKE_object.h"
 
 #include "gpencil_engine.h"
 
@@ -38,7 +39,33 @@
 
 #include "DEG_depsgraph.h"
 
- /* add a gpencil object to cache to defer drawing */
+/* verify if exist a non instanced version of the object */
+static bool gpencil_has_noninstanced_object(Object *ob_instance)
+{
+	const DRWContextState *draw_ctx = DRW_context_state_get();
+	const ViewLayer *view_layer = draw_ctx->view_layer;
+	Object *ob = NULL;
+	for (Base *base = view_layer->object_bases.first; base; base = base->next) {
+		ob = base->object;
+		if (ob->type != OB_GPENCIL) {
+			continue;
+		}
+		/* object must be visible (invisible objects don't create VBO data) */
+		if (!(DRW_object_visibility_in_active_context(ob) & OB_VISIBLE_SELF)) {
+			continue;
+		}
+		/* is not duplicated and the name is equals */
+		if ((ob->base_flag & BASE_FROM_DUPLI) == 0) {
+			if (STREQ(ob->id.name, ob_instance->id.name)) {
+				return true;
+			}
+		}
+	}
+
+	return false;
+}
+
+/* add a gpencil object to cache to defer drawing */
 tGPencilObjectCache *gpencil_object_cache_add(
         tGPencilObjectCache *cache_array, Object *ob,
         int *gp_cache_size, int *gp_cache_used)
@@ -76,7 +103,18 @@ tGPencilObjectCache *gpencil_object_cache_add(
 	cache_elem->idx = *gp_cache_used;
 
 	/* object is duplicated (particle) */
-	cache_elem->is_dup_ob = ob->base_flag & BASE_FROM_DUPLI;
+	if (ob->base_flag & BASE_FROM_DUPLI) {
+		/* Check if the original object is not in the viewlayer
+		 * and cannot be managed as dupli. This is slower, but required to keep
+		 * the particle drawing FPS and display instanced objects in scene
+		 * without the original object */
+		bool has_original = gpencil_has_noninstanced_object(ob);
+		cache_elem->is_dup_ob = (has_original) ? ob->base_flag & BASE_FROM_DUPLI : false;
+	}
+	else {
+		cache_elem->is_dup_ob = false;
+	}
+
 	cache_elem->scale = mat4_to_scale(ob->obmat);
 
 	/* save FXs */

source/blender/draw/engines/gpencil/gpencil_engine.c

@@ -620,7 +620,8 @@ void GPENCIL_cache_populate(void *vedata, Object *ob)
 
 				/* enable instance loop */
 				if (!stl->g_data->do_instances) {
-					stl->g_data->do_instances = ob->base_flag & BASE_FROM_DUPLI;
+					tGPencilObjectCache *cache_ob = &stl->g_data->gp_object_cache[stl->g_data->gp_cache_used - 1];
+					stl->g_data->do_instances = cache_ob->is_dup_ob;
 				}
 
 				/* load drawing data */