1. A tree model generating method comprising: separating and extracting a skeleton of a tree from an image including the tree to generate a matte image; applying a graph representation scheme of a two-dimensional (2D) object to the matte image to generate an object skeleton graph; generating a depth information value in accordance with a predetermined rule and allocating the generated depth information value to each node on the object skeleton graph to generate a three-dimensional (3D) primary tree model; adding branches to the primary tree model based on a branch library constructed in advance and the predetermined rule to generate a secondary tree model; and performing self-growth representation with respect to the secondary tree model based on a distance between each node on the object skeleton graph to generate a tertiary tree model.