1. A method for generating a bitstream representing a 3D model, comprising: accessing information related to instances and respective patterns of a structure, the information including a pattern identifier and transformation information, indicative of a position of an instance within the structure, associated with the respective pattern and generating a bitstream including a header portion having a first indicator indicative of a mode for packing the information in the bitstream, and a data portion including a pattern data and an instance data, the instance data comprising the pattern identifiers and the transformation information, disposed in the bitstream, wherein the first indicator indicates that the information is packed in the bitstream in one of a first packing format and a second packing format, wherein the first packing format includes grouping together each of the pattern identifiers and their respective transformation information, and the second packing format includes grouping the pattern identifiers together, and the transformation information together.