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Current Perturbation Methods

A fixed number of clamped-plate splines (CPS) are placed at a corresponding set of fixed positions. Their direction is chosen stochastically and their intensity (magnitude) is increased to achieve varying levels of perturbations. Linear relationship was assumed between the intensity and the amount of perturbation, i.e. the average number of pixels by which a point is displaced. (For more details on the actual implementation, ask Vlad. Carole says that it is aggregation of warps that increases the perturbation. So, added displacement is only known vaguely)

The expected pitfalls are as follows:

• The composition of warps was not taken into consideration. Increasing warp intensities do not necessarily increase the displacements (not linearly at least).
• Boundaries prevented warps from being applied properly. In the case of brain images, this may not be crucial.
• Images began to break some constraints when the perturbation increased excessively.