Matrices and Linear Transformations - 3D Math Primer for Graphics and Game Development - page 149

Figure 5.8. Projecting a 3D object onto a cardinal plane

5.3.1 Projecting onto a Cardinal Axis or Plane

The simplest type of projection occurs when we project onto a cardinal axis
(in 2D) or plane (in 3D). This is illustrated in Figure 5.8.
Projection onto a cardinal axis or plane most frequently occurs not
by actual transformation, but by simply discarding one of the coordinates
while assigning the data into a variable of lesser dimension. For example,
we may turn a 3D object into a 2D object by discarding the z components
of the points and copying only x and y.
However, we can also project onto a cardinal axis or plane by using a
scale value of zero on the perpendicular axis. For completeness, we present
the matrices for these transformations:

1 0

Projecting onto a
cardinal axis

P x = S 0 1 ,0 = ,

0 0

0 0

P y = S 1 0 ,0 = ,

0 1