During the first half of the 15th century, the achieving of the effect of realistic space in a painting by the employment of linear perspective was a major preoccupation of many painters, as well as the architects Brunelleschi and Alberti who both theorised about the subject. Brunelleschi is known to have done a number of careful studies of the piazza and octagonal baptistery outside Florence Cathedral and it is thought he aided Masaccio in the creation of his famous "trompe-l'œil" niche around the "Holy Trinity" he painted at Santa Maria Novella.