1. A computer-implemented method of generating an object representation from a bitmap image, said method comprising the steps of: selecting a set of regions from said bitmap image, said set of regions including a background region of said bitmap image; estimating colour and partial transparency parameters of a first graphical object with partial transparency that overlaps a second graphical object according to colours of the set of regions, said estimated colour and partial transparency being consistent with a transparency compositing model, the transparency compositing model defining a colour of a region of overlap of two graphical objects in terms of colour and partial transparency parameters; constructing geometric models of said first and second graphical objects from said set of regions and the estimated colour and transparency parameters of the first graphical object; and generating dependent upon said geometric models said object representation.