1. A robotic method for performing surgery on a patient through a minimally invasive aperture, the method comprising: positioning an elongate shaft of a tool through the aperture so that the shaft pivots at an actual pivotal center adjacent the aperture; acquiring an image of the tool within the patient; determining image-based pivotal center correction information using the image; determining a position and/or orientation of the tool by imposing the image-based pivotal center correction information onto joint-based data, the joint-based data being used by a robotic command module to calculate movements of a tool manipulator supporting the tool.