Snowfall and Snowcover Models for Simulating Snowy Landscape

利用计算机图形学进行景观模拟是对城市规划、旅游开发等进行景观评价的有力工具。正因为如此，虽然表达不同季节的方法还有待开发，但尤其是严冬的表达方法的开发是困难的。在本研究中，我们开发了以下用于模拟雪景的CG技术。(1)雪的渲染方法：我们开发了两种方法。一种是基于三维纹理的雪花绘制方法，另一种是基于体绘制的雪盖绘制方法，它使用1/f噪声来实现积雪密度的波动。(2)降雪模拟方法：降雪模拟将气流场创建为涡旋场，并根据气流场和重力的作用跟踪雪花的运动。考虑了涡旋与障碍物之间的相互作用。(3)雪盖模拟方法的改进：改进了雪盖模拟方法，使之能够考虑空气流场。因此，该方法可以生成受气流影响产生的雪盖形状。(4)确定雪盖形状的融雪模拟方法：该方法使我们能够模拟由于太阳和物体的辐射热而产生的雪盖形状的变化，以及雪中的热传播。此外，由于该方法考虑了融雪的重新冻结，因此可以生成冰柱。(5)根据外力修改积雪形状的方法：提出了表示积雪表面上留下的足迹或车辙的方法。这种方法通过考虑物体(如脚或轮胎)的动能以及雪的密度来改变积雪的形状。

The landscape simulation using computer graphics (CG) is a powerful tool for the landscape assessment to city planning, development of sightseeing and so on. Because of this purpose, although the methods for expressing various seasons are desired to be developed, especially the development of the method for representing the severe winter is difficult.In this research, the following CG techniques for simulating snowy landscapes are developed.(1) The snow rendering methods: We developed two methods. One is the snowflake rendering method based on the three dimensional texturing, and other is the snow cover rendering method based on the volume rendering which uses 1/f noise for realizing the fluctuation of snowy density.(2) The snowfall simulation method: The snowfall simulation creates a field of air currents as a vortex field and trace the motion of snowflakes according to the forces given by the air current field and gravitation. The vortex field takes into account the interaction between vortices and obstacles.(3) Improvement of the snow cover simulation method: We improved the snow cover simulation method so as to be able to take an air current field into consideration. As a result, this method can generate the shape of the snow cover produced by the influences of an air current, such as snowdrift.(4) The snowmelt simulation method for defining shape of snow cover: This method enables us to simulate the transformation of the form of snow cover due to the radiant heat from the sun and objects, and the heat propagation in snow. Moreover, since the method considers the re-freeze of snowmelt, it can generate the icicle.(5) The method for modifying the shape of snow cover according to external forces : The method for representing a footprint or a rut left on a snow surface is developed. This method changes snow cover shape by taking into account kinetic energy of objects, e.g. feet or tires, and density of snow.