X 
     
  
   
   
  
  
  
  
  
   
  
  
  
  
  
  
  
  
  
  
  
  
   
^». 
mx 
Fig. l. Two planes in perspective relation. The coordinate systems are chosen 
arbitrarily. 
  
0 is the perspective center, through which all the rays pass, which 
  
project the image. The two planes are assumed to be exactly parallel 
in the initial position. The position of the perspective center 0 with 
respect to the planes is given by 
1. the footpoints H' and H of the normals from 0 to the planes and 
2. the distances c and h from 0 to the planes or the lengths of the 
normals. 
H' and H are termed the principal points, c and h are termed the 
principal distances. 
The principal points are the origins of coordinate systems z', y' and 
x, y which in the original position are exaetly parallel to each other. 
The two planes can be rotated mutually through angles o, « and x 
around axes which intersect in the perspective center 0. In the initial 
position the ¢- and w-axes are parallel to the coordinate axes of the 
planes as indicated in fig. 2, and the x-axis coincides with the direction 
HOH’. The sequence of the rotations has to be defined. In fig. 2 
the sequence is indicated by the roman figures. The g-rotation is 
primary, the w-rotation secondary and the x-rotation tertiary. Later 
another sequence of the rotations will be treated, see fig. 3. In both