Fig. 3:

Illustration on a simulation dataset. Fig. (a): data point X and its projection X_p onto the decision boundary. In the feature domain the model is a linear classifier. Fig. (b) Corresponding points x and x_p in the input domain, calculated as inversion of points from (a). Fig. (c) and (d): Points from two classes are sampled around two interleaving half circles. Decision boundary is a line in the feature domain (c), and is a curve in the input domain (d).