1. A multi-pass method of processing graphics data, comprising: receiving vertex data at a geometry processor; receiving first vertex perturbation data and the vertex data at a fragment processor; combining the vertex data received and the first vertex perturbation data received using the fragment processor to produce first perturbed vertex data without operating on fragment data; providing the first perturbed vertex data back to the geometry processor in a format initially compatible with the geometry processor; converting the first perturbed vertex data to fragment data; receiving second vertex perturbation data at the fragment processor; and combining the fragment data and the second vertex perturbation data received using the fragment processor to produce second perturbed vertex data.