diff --git a/src/viewport_func.h b/src/viewport_func.h
--- a/src/viewport_func.h
+++ b/src/viewport_func.h
@@ -51,7 +51,7 @@ void OffsetGroundSprite(int x, int y);
 void DrawGroundSprite(SpriteID image, PaletteID pal, const SubSprite *sub = nullptr, int extra_offs_x = 0, int extra_offs_y = 0);
 void DrawGroundSpriteAt(SpriteID image, PaletteID pal, int32 x, int32 y, int z, const SubSprite *sub = nullptr, int extra_offs_x = 0, int extra_offs_y = 0);
 void AddSortableSpriteToDraw(SpriteID image, PaletteID pal, int x, int y, int w, int h, int dz, int z, bool transparent = false, int bb_offset_x = 0, int bb_offset_y = 0, int bb_offset_z = 0, const SubSprite *sub = nullptr);
-void AddChildSpriteScreen(SpriteID image, PaletteID pal, int x, int y, bool transparent = false, const SubSprite *sub = nullptr, bool scale = true);
+void AddChildSpriteScreen(SpriteID image, PaletteID pal, int x, int y, bool transparent = false, const SubSprite *sub = nullptr, bool scale = true, bool relative = true);
 void ViewportAddString(const DrawPixelInfo *dpi, ZoomLevel small_from, const ViewportSign *sign, StringID string_normal, StringID string_small, StringID string_small_shadow, uint64 params_1, uint64 params_2 = 0, Colours colour = INVALID_COLOUR);