The accuracy of flexible forming processes with mobile tools is generally insufficient for industrial needs. Modern stereovision cameras should allow geometric feedback to improve accuracy, but to date it has been assumed that on-line control is in practice impossible because non-linear process models are too slow. This paper demonstrates for the first time that an appropriate process model can be formed from a set of spatial impulse responses found by linearisation around a pre-planned tool path. This allows on-line feedback control of part geometry. A general formulation is presented, and is evaluated through application to an axisymmetric incremental sheet forming process.