基于概率模型的树木间接光照效果的实时模拟

太阳光照射到树冠上之后，会在叶片间进行相互的反射、折射，形成复杂的间接光照效果。在进行树木真实感绘制时，对这种间接光照的快速模拟是非常困难的。传统的光线跟踪、辐射度等算法均比较耗时，很难满足实时计算的要求。采用一种概率模型来快速估算树木叶片间的间接光照效果，实现了带间接光照效果的树木实时绘制。为树冠构建一个包围体，并假定该包围体中叶子的位置和法向基本符合均匀分布；为叶子的正反面分别定义反射系数和透射系数，并定义一个衰减函数来描述光线穿越叶子层后的衰减量；基于这种均匀概率分布的包围体模型，并利用叶片材质属性，就可以为每片叶子计算出叶子表面的出射光。对于中心在P]处的叶子来说，可以快速计算出太阳光经过其邻近叶子到达该叶子表面的间接光照效果。为了进一步提高绘制效率，并未采用传统的包含大量面片的树木模型，而是采用了一种利用Billboards结构进行树木枝叶表达的简洁的树木三维模型。将上述概率模型与这种表达方式进行了有机结合，完成了带间接光照效果的树木实时真实感绘制。

Real-time simulation of indirect illumination of trees based on proba-bilistic model

After the sunlight shines on tree crowns, it will be reflecting and refracting among leaves and develops into indirect illumination effect. It is quite difficult to make fast simulation of such indirect illumination, when doing realistic rendering of trees. Traditional way of calculation, including light trace and radiancy, costs more time, which is hard to meet the demand of timing. This paper adopts a model of probability to quickly assess indirect illumination effect among leaves which enables to realistic rendering of indirect illumination effect. It establishes a bounding volume and assumes that the location and normal of the leaves in the bounding volume are generally in line with even distribution. It sets up reflection coefficient and transmission coefficient for the front and back of the leaf, as well as the attenuation function to describe the attenuation function of sunlight that travels through the leaf. Based on the bounding volume model with even distribution and the material properties of leaves, emergent light from the surface of each leaf will be calculated. As for the leaves with P center, indirect illumination effect caused by sunlight passing through neighboring leaves is able to be quickly calculated. In order to further improve rendering efficiency, it does not take traditional tree model with large amount of patches, but adopts a concise 3D tree model with trees and branches based on Billboards structure. It tactfully combines above-mentioned probability model with such presentation to complete the realistic rendering with indirect illumination effect.