Perspective collinear image of a straight line
The construction for determining the perspective collinear image of a given point is given on the figure 15. Determining the image of any other plane figure is based on this basic construction.
The perspective collineation (S,o,X_{1},X_{2}) and the straight line a_{1} are given on the figure 16. Construct the perspective collinear image of the straight line a_{1}, i.e. the straight line a_{2}.
Figure 16


1st step: An arbitrary point A_{1}∈a_{1} is chosen and its perspective collinear image A_{2} is constructed (following the construction algorithm in figure 15).
2nd step:
Since every straight line is uniquely determined with two of its points, the line a_{2} can be constructed as a straight line passing through the point A_{2} and the intersection point of the line a_{1} and the axis of perspective collineation.
 Move the point A_{1}∈a_{1}
and notice that the image of the straight line a_{1}, i.e. the line a_{2}, does not depend on the chosen position of the point A_{1}.
 The position of the points X_{1} and X_{2} has uniquely determined the perspective collineation. Move those points in various positions (on the same side or opposite sides of the axis or center) to change the given perspective collineation. Notice that the construction algorithm remains unchanged.
3rd step:
By moving the point Y bring the line a1 in position
parallel to the axis. Notice that then its image line a2 is also
parallel to the axis. By changing the position of the starting
points X1 and X2, observe that this statement applies to any
perspective collineation.

We already know that the perspective collineation
maps points to the points of the same plane. Thus, each
point may be treated as an element of the domain, as
well as an element of the codomain. In the previous
examples, domain elements were marked by index 1,
and codomain by index 2. For example, we have
observed the following point and lines transformations:
A1→A2, X1→X2, p1→p2, a1→a2 …
On the other hand, the perspective collineation is a
bijective transformation for which the inverse
transformation is also a perspective collineation with
the same provisional elements but with reversed
indices, the field with the index 2 becomes domain
filed, and the field with index 1 the codomain field of
the inverse mapping. Thus, for the inverse mapping we
have the following transformations: A2→A1, X2→X1,
p2→p1, a2→a1 …
In all the following examples, these two mapping will be
treated equally, i.e. we will map from the field 1 to two
field 2 and vice versa. In doing, so, one should always
pay attention to the proper denotation of the indices.
The perspective collinear images of two straight lines a_{2} and b_{2} are given in figure 17 and 18. Those lines intersect at point O_{2} in figure 17, and are parallel in figure 18 (intersection point is at infinity and denoted with O_{2}^{∞}).
The perspective collinear image of the pair of intersecting lines in figure 17 is constructed so that first the image of their intersection point O_{1} is determined. Then the property that a pair of corresponding straight lines obtained by perspective collineation intersects at the axis of the perspective collineation is used.
On the figure movable points are X_{1}, X_{2} (for changing the perspective collineation) and O_{2} (changing the position of lines a_{2}, b_{2}). Notice the following:
In general, the perspective collineation does NOT preserve the angle measure between two lines, i.e. it is not a conformal transformation.
In figure 18 the perspective collinear image of a pair of parallel lines is is determined by first
constructing the images of any points A_{2}∈a_{2} and B_{2}∈b_{2}. Then the property that a pair of
corresponding straight lines obtained by perspective
collineation intersects at the axis is used. The lines a_{1} and b_{1} intersect at the point O_{1} which is the image of the point at infinity O_{2}^{∞}. Notice the following property:
In general, theperspective collineation does NOT preserve parallelism.
Created by Sonja Gorjanc, translated by Helena Halas and Iva Kodrnja  3DGeomTeh  Developing project of the University of Zagreb, made with GeoGebra
