1. A computer-implemented method of generating an object representation from a bitmap image, the computer comprising a memory storing a computer program and a processor unit coupled to the memory, said method comprising the steps of: selecting, using the processor and the memory, a set of regions from said bitmap image, said set of regions including a first region assumed to be a region of overlap of a first graphical object and a second graphical object, a second region assumed to be part of a first graphical object which overlaps a second graphical object, a third region assumed to be part of the second graphical object, and a background region, the first region being adjacent to the second and third regions; determining, using the processor and the memory, a colour difference between a colour of the first region of said bitmap image and a colour defined by a transparency compositing model given an assumed transparency of the first graphical object and colours of the second and third regions and the background region; determining, using the processor and the memory, if the colour difference is smaller than a predetermined threshold; if the colour difference is smaller than the predetermined threshold, applying the assumed transparency and the colour of the second region, using the processor and the memory, to the first graphical object as parameters of a first graphical object; constructing, using the processor and the memory, a geometric model of said first object from said set of regions and the applied parameters of the first graphical object; and generating, using the processor and the memory, said object representation dependent upon said geometric model.