Of course, the shields carried by armies such as the Romans, the Gauls or the Carthaginians were not decorated by machines - these all had to be painted by hand. In the earliest times, when armies were comprised primarily of citizens and not professional soldiers, I would argue that each shield was painted by the owner using either a solid color, simple geometric patterns or a very rudimentary symbol that perhaps had religious meaning. Shields would have been a very personal piece of equipment and the armies certainly would have appeared very diverse not only in the number of different shield decorations used but in the quality of that painting. Lines would have been crooked, alignment and spacing would be inconsistent, paint quality would have lead to variations in color saturation and circles and curves would have been far from perfect. Even when individuals decided together to paint the same decorations on their shields they would have varied greatly from shield to shield in the execution of the agreed upon design. Some people would simply have been better at it than others.
Not even the most talented artisan would have
been able to replicate this design perfectly
across hundreds of shields.
I very much enjoy the appearance of a beautifully painted army carrying shields decorated with design transfers. Aesthetically, it is a marvelous sight and while we are gaming the appearance of the armies on the table is a very, very important matter to everybody involved. My thinking here is not to dissuade anybody from using such things, but rather an intellectual exercise to dig a little deeper into the history of the things we play with.