diff --git a/src/actor.h b/src/actor.h
index e5fb4384e..333059ce5 100644
--- a/src/actor.h
+++ b/src/actor.h
@@ -410,10 +410,11 @@ enum ActorRenderFlag
 	RF_FLATSPRITE		= 0x2000,	// Flat sprite
 	RF_VOXELSPRITE		= 0x3000,	// Voxel object
 	RF_INVISIBLE		= 0x8000,	// Don't bother drawing this actor
+	RF_ROLLSPRITE		= 0x40000,	//[marrub]roll the sprite billboard
+	RF_DONTFLIP			= 0x80000,	// Don't flip it when viewed from behind.
 
 	RF_FORCEYBILLBOARD		= 0x10000,	// [BB] OpenGL only: draw with y axis billboard, i.e. anchored to the floor (overrides gl_billboard_mode setting)
 	RF_FORCEXYBILLBOARD		= 0x20000,	// [BB] OpenGL only: draw with xy axis billboard, i.e. unanchored (overrides gl_billboard_mode setting)
-	RF_ROLLSPRITE		= 0x40000,	//[marrub]roll the sprite billboard
 };
 
 // This translucency value produces the closest match to Heretic's TINTTAB.
diff --git a/src/thingdef/thingdef_data.cpp b/src/thingdef/thingdef_data.cpp
index 49b6b0dd8..b9e6414ff 100644
--- a/src/thingdef/thingdef_data.cpp
+++ b/src/thingdef/thingdef_data.cpp
@@ -269,6 +269,7 @@ static FFlagDef ActorFlagDefs[]=
 			// [fgsfds] Flat sprites
 	DEFINE_FLAG(RF, FLATSPRITE, AActor, renderflags),
 	DEFINE_FLAG(RF, WALLSPRITE, AActor, renderflags),
+	DEFINE_FLAG(RF, DONTFLIP, AActor, renderflags),
 
 	// Bounce flags
 	DEFINE_FLAG2(BOUNCE_Walls, BOUNCEONWALLS, AActor, BounceFlags),