Projective Comparisons, and One to One Correspondence — 

The two photographs below are of the same object —
a Cartesian graph, on paper.
There is onetoone correspondence of the
features in photograph 2 Thus, if linepairs converge in one photograph 
From the foregoing, we must have that,
So, either parallels meet,
Essentially, we (and the camera)
always see in
Perspective, which compares our plotted ordinates (intervals) to the
graph's axesor there are no parallels. in Perspectivities, centered on X and Y. Drag the green ‘dashdot’ lines on the graph below, to bring these “meetings of parallels”, X and Y, into view. 