Before revision: Complex 3D curves can be created by directly drawing mid-air in immersive environments ( AR/VR ). Drawing mid-air strokes precisely on the surface of a 3D virtual object  however, is difficult;
After revision: Complex 3D curves can be created by directly drawing mid-air in immersive environments ( AR/VR ). Drawing mid-air strokes precisely on the surface of a 3D virtual object , however, is difficult;