A sphere tree is first generated. Accessibility values and occlusion angles corresponding to each vertex of a 3D object are calculated according to the sphere tree. A 2D texture and a 3D texture are then generated to store the accessibility values and occlusion angles. When a self-shadow effect of the 3D object is required to be generated, the relation of the spheres in the sphere tree is updated, and a 3D image is formed by retrieving the values in the 2D texture and the 3D texture according to the updated sphere tree. Since the 2D texture and the 3D texture are pre-stored, the real time calculation load is hugely reduced while the self-shadow effect is processed.