12 
■A 
first order approximation. The electronic versions enable more complex trans 
formations [ 12 ] . Thus, they can be made more flexible and efficient thamoptical 
printers. 
Printing may be continuous, by parallel narrow bands, or incremental, by areal 
' ' 2 
elements (s.c. patches). These elements can be very small (fractions of a mm ) 
or, at the other extreme, the whole photograph may be covered by a single patch. 
Optica] printers operate in continuous mode, whereas electronic versions may operate 
either continuously or incrementally. Continuous printing is accompanied by dynamic 
errors which may be the limiting factor in performance. 
In incremental operation, however, dynamic errors do not occur. By implementing 
a complex transformation, the size of patches can be substantially increased, and 
similarly the speed of printing. With given D. T. M. data, computation and printing 
may be carried out either on-line in a single stage, or off-line in two successive 
stages: preparation, and printing. 
Preparation primarly concerns generation of the control data for printing. These 
data can be derived from given D. T. M. data and the orientation data of the photographs 
concerned. The control data encompass two basic groups: locational data (needed for 
tracking devices) and image transformation data. < «• 
The generation of locational data is relatively simple. A coarse flow chart of the 
computation is shown in fig. 3. 
Fig. 3 
Flow chart of the locational computation