An appropriate register control method for a roll-to-roll (R2R) printing system must achieve not only high register accuracy but also the smoothness of the control signal and simple implementation. This paper compares recent register control approaches, i.e., model-based feed-forward proportional differential (MFPD), direct-decoupling proportional differential (DDPD) control, and fully decoupled proportional differential (FDPD) control, with three main performance indexes including register accuracy, implementation difficulty and smoothness of control signal. The mutual conversion between the different defined register errors in the three control methods is derived, so that the different defined register errors can be reciprocally transformed. According to the proposed mutual conversion analysis, the most appropriate conditions for using the three decoupling control methods are provided, which is very useful for the control method selection in industrial applications. Experiments and comparison are carried out to demonstrate the effectiveness of the proposed mutual conversion, which verify the derived most appropriate conditions of the three decoupling control methods.