A method and computer system are described for generating compressed image data, and for rendering pixel data using the compressed image data. A plurality of hulls are defined in the image, each hull being defined as a set of polygons in a desired coordinate system. For at least some of said hulls, a portal is defined, each portal being defined as a set of point in the designated coordinate system for that hull. A viewing transform allows the portal to be mapped to another, target hull. A data structure is stored which holds the defined hulls and portals in a manner describing their physical relationship in the image. The data structure can be transmitted via a number of different mediums and represents the compressed image data which can be used to generate pixel data to generate an image at a remote location.