Snowfall and Snowcover Models for Simulating Snowy Landscape

利用计算机图形学（CG）进行景观模拟是进行城市规划、观光开发等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有待开发，但表达严冬的方法的开发是困难的。在本研究中，开发了以下模拟雪景的CG技术。(1)雪渲染方法：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积雪绘制方法，利用1/f噪声实现积雪密度的波动。(2)降雪模拟方法：降雪模拟产生一个气流场作为涡场，根据气流场和重力所给予的力来追踪雪花的运动。涡流场考虑了涡流与障碍物之间的相互作用。(3)积雪模拟方法的改进：我们改进了积雪模拟方法，使其能够考虑气流场。因此，这种方法可以生成受气流（如雪堆）影响而产生的积雪的形状。(4)定义积雪形状的融雪模拟方法：该方法可以模拟太阳和物体的辐射热对积雪形状的影响，以及雪中的热传播。此外，由于该方法考虑了融雪的再冻结，因此可以产生冰柱。(5)根据外力改变积雪形状的方法：开发了雪面上留下脚印或车辙的表示方法。这种方法通过考虑物体（例如脚或轮胎）的动能和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.