Perspective Projection - Transformations and Projections in Computer Graphics - page 78

Picture plane

Horizon

c

Viewer to picture

Transversals

Front view

Side view

Grid on ground

Figure 3.14: Alberti's Method of Traversals in One-Point Perspective.

closer and closer in the picture. The last step is to draw a diagonal line in the front
view to check for the accuracy of this geometric construction.

The canvas is an open window through which I see what I want to paint.
—Leon Battista Alberti
In his topic, Alberti also shows how such a floor, accurately drawn in perspective,
can serve to determine the correct dimensions (both horizontal and vertical) of objects
positioned on the floor and elsewhere in the picture. Figure 3.15 illustrates how a grid
on a floor is used to determine the height of a large, box-like object placed on the floor.
Alberti used the braccio (plural braccia), a length unit that equals approximately 58 cm
(or 23 in, roughly the length of a man's arm), and a length of four braccia, measured
on the floor, is employed to determine the heights of the box at its front and back.

Four braccia

Four braccia

Figure 3.15: Determining Vertical Dimensions from the Floor.