Drawing is based upon perspective, which is nothing else than a thorough knowledge of the function of the eye.
Leonardo da VinciThe first object of the painter is to make a flat plane appear as a body in relief and projecting from that plane
Leonardo da VinciTo speak of this subject you must... explain the nature of the resistance of the air, in the second the anatomy of the bird and its wings, in the third the method of working the wings in their various movements, in the fourth the power of the wings and the tail when the wings are not being moved and when the wind is favourable to serve as guide in various movements.
Leonardo da Vinci