Cycles: Free Blender side data as soon as we don't need it

Currently this is possible after built-in images are loaded in memory.
Allows to save memory used by dependency graph and copy-on-write.

In practice this lowers peak system memory usage from 52GB to 42GB on
a production file of spring 03_035_A.lighting.

Note, that this only applies to F12 and command line renders.

Bigger note, that this optimization is currently only possible if
there are no grease pencil objects to be rendered.

2 files changed, 24 insertions, 2 deletions

diff --git a/source/blender/render/extern/include/RE_engine.h b/source/blender/render/extern/include/RE_engine.h
index 1ca889f2347..faa4c8b3184 100644
--- a/source/blender/render/extern/include/RE_engine.h
+++ b/source/blender/render/extern/include/RE_engine.h
@@ -193,4 +193,6 @@ struct RenderData *RE_engine_get_render_data(struct Render *re);
 void RE_bake_engine_set_engine_parameters(
         struct Render *re, struct Main *bmain, struct Scene *scene);
 
+void RE_engine_free_blender_memory(struct RenderEngine *engine);
+
 #endif /* __RE_ENGINE_H__ */
diff --git a/source/blender/render/intern/source/external_engine.c b/source/blender/render/intern/source/external_engine.c
index a06389bfcd4..758600e89aa 100644
--- a/source/blender/render/intern/source/external_engine.c
+++ b/source/blender/render/intern/source/external_engine.c
@@ -738,8 +738,13 @@ int RE_engine_render(Render *re, int do_all)
 
 			type->render(engine, engine->depsgraph);
 
-			/* grease pencil render over previous render result */
-			if (!RE_engine_test_break(engine)) {
+			/* Grease pencil render over previous render result.
+			 *
+			 * NOTE: External engine might have been requested to free its
+			 * dependency graph, which is only allowed if there is no grease
+			 * pencil (pipeline is taking care of that).
+			 */
+			if (!RE_engine_test_break(engine) && engine->depsgraph != NULL) {
 				DRW_render_gpencil(engine, engine->depsgraph);
 			}
 
@@ -813,3 +818,18 @@ void RE_engine_register_pass(struct RenderEngine *engine, struct Scene *scene, s
 		}
 	}
 }
+
+void RE_engine_free_blender_memory(RenderEngine *engine)
+{
+	/* Weak way to save memory, but not crash grease pencil.
+	 *
+	 * TODO(sergey): Find better solution for this.
+	 * TODO(sergey): Try to find solution which does not involve looping over
+	 * all the objects.
+	 */
+	if (DRW_render_check_grease_pencil(engine->depsgraph)) {
+		return;
+	}
+	DEG_graph_free(engine->depsgraph);
+	engine->depsgraph = NULL;
+}