Final answer:
Leonardo da Vinci advocated for artists to design their paintings as illusionary windows, using techniques like linear perspective and atmospheric perspective.
Step-by-step explanation:
The artist who advocated in his writing that artists should design their paintings as illusionary windows is Leonardo da Vinci. Leonardo was known for his innovative techniques and his exploration of perception and space in his artwork. He believed in creating the illusion of depth and realism in his paintings, often using techniques like linear perspective and atmospheric perspective to achieve this effect. One of his famous works that exemplifies this approach is the Mona Lisa, where he used subtle shading and perspective to create a lifelike portrayal of the subject.