1. Introduction
Terrain generation is an indispensable visual content when creating realistic representations of virtual worlds. In computer games field, the size of terrain does not need to be fixed at the beginning, which saves memory overhead and the designer's workload. In order to enhance the user's interaction with the virtual worlds, users are even allowed to create their own terrain. Existing works often completely regenerate the terrain when expanding its size, instead of only generating the extension part while the existing part unchanged.