1. A computer-implemented of determining data characterising a set of images of an object, the method being implemented in a computer comprising processor and a memory in communication with the processor, the method comprising: inputting, to the processor, data from a first set of images of said object captured from a plurality of different positions; determining, by the processor, frontier point data from said data for said first set of images and data defining said image capture positions, said frontier point data defining a plurality of frontier points on said object, and for each said frontier point, a direction of a normal to the surface of the object at the frontier point; inputting, to the processor, data from a second set of captured images of said object, said second set of images having substantially the same viewpoint and different illumination conditions; and determining, by the processor, data characterising said second set of images using said data for said second set of images and said frontier point data, said image set characterising data comprising object reflectance parameter data (β) and, for each image of said second set, illumination data (L) comprising data defining an illumination direction and illumination intensity for the image; wherein said determining of frontier point data uses pair of images of said first set of images, the two images of said pair being captured by first and second respective cameras; and wherein each frontier point is a point in three-dimensions on the surface of the object wherein the plane defined by the frontier point and centres of said first and second cameras viewing the frontier point is tangent to the object.