Perspective Projection - Transformations and Projections in Computer Graphics - page 76

Campanile

53 0

PIAZZA

Corso degl'Alimari

Via de Martelli

Tempio di S.
Gio. Batta

Colonna del Miracolo
de San Zanobi

Volta de'
Pecori

Canto alla
Paglia

Palazzo Arcivescovile

Figure 3.13: Plan of the Piazza del Duomo, Florence (After [Sgrilli 33]).

Brunelleschi then rotated the panel 180 ◦ and looked through the hole at the Bap-
tistery. He then inserted a mirror and held it at arm's length as shown in Figure 3.12bc
and looked at his painting reflected in the mirror. This became Brunelleschi's cele-
brated peepshow experiment, which proved the lifelike qualities of perspective. In his
biography, Manetti claims to have held this painting in his hands and to have repeated
the experiment. He was unable to tell the difference between the image reflected in the
mirror and the real scene (without the mirror). (However, modern travelers to Florence
recommend the use of a pair of heavy-duty tripods to hold the image and the mirror at
their precise locations.)

[Brunelleschi] had made a hole in the panel on which there was this painting; ... which
hole was as small as a lentil on the painting side of the panel, and on the back it
opened pyramidally, like a woman's straw hat, to the size of a ducat or a little more.
And he wished the eye to be placed at the back, where it was large, by whoever had
it to see, with the one hand bringing it close to the eye, and with the other holding
a mirror opposite, so that there the painting came to be reflected back ... which on
being seen, ... it seemed as if the real thing was seen: I have had the painting in my
hand and have seen it many times in these days, so I can give testimony.
—Antonio Manetti, The Life of Brunelleschi (1480s)

Manetti mentions another interesting fact. The painting was about 12 inches wide
and Brunelleschi recommended watching it from a distance of 6 inches, so the reflection
seen in the mirror appears to be at a distance of 12 inches from the viewer. We know
that tan 26 . 6 ◦ =0 . 5, which implies that the apex angle of an isosceles triangle whose
height equals its base is 2 53 ◦ (see also Exercise 3.29). This trigonometric fact

suggests that, as seen from the viewing point specified by Brunelleschi, the Baptistery

× 26 . 6 ≈