Real-Time Global Illumination Rendering with Dynamic Materials

DOI：

 作者 单位 孙 鑫 浙江大学 CAD/CG国家重点实验室,浙江 杭州 310027 周 昆 微软亚洲研究院 网络图形组,北京 100080 石教英 浙江大学 CAD/CG国家重点实验室,浙江 杭州 310027

现有的基于预计算的全局光照明绘制算法都假设场景中物体的材质固定不变,这样,从入射光照到出射的辐射亮度之间的传输变换就是线性变换.通过对这种线性变换的预计算,可以在动态光源下实现全局光照明的实时绘制.但是,当材质可以改变时,这种线性变换不再成立,因此,现有算法无法直接用于动态材质的场景.提出了一种方法:在修改场景中的物体材质时,可以实时得到场景在直接光照和间接光照下的绘制效果.将最终到达视点的辐射亮度根据其之前经过的反射次数及相应的反射材质分为多个部分,每个部分和先后反射的材质的乘积成正比,从而把该非线性问题转化为线性问题.又将所有可选的材质都表示为一组基的线性组合.将这组基作为材质赋予场景中的物体,就有各种不同的组合方式,预计算每种组合下所有部分的出射辐射亮度.在绘制时,根据各物体材质投影到基上的系数线性组合预计算的数据就能实时得到最终的全局光照明的绘制结果.该方法适用于几何场景、光照和视点都不发生变化的场景.使用双向反射分布函数来表示物体的材质,不考虑折射或者半透明的情况.该实现最多包含两次反射,并可以实时绘制得到一些很有趣的全局光照明效果,比如渗色、焦散等等.

All previous algorithms of real-time global illumination rendering based on precomputation assume that the materials are invariant, which makes the transfer from lighting to outgoing radiance a linear transformation. By precomputing the linear transformation, real-time global illumination rendering can be achieved under dynamic lighting. This linearity does not hold when materials change. So far, there are no algorithms applicable to scenes with dynamic materials. This paper introduces a real-time rendering method for scenes with editable materials under direct and indirect illumination. The outgoing radiance is separated into different parts based on the times and corresponding materials of reflections. Each part of radiance is linear dependent on the product of the materials along its path of reflections. So the nonlinear problem is converted to a linear one. All materials available are represented as linear combinations of a basis. The basis is applied to the scene to get numbers of different material distributions. For each material distribution, all parts of radiance are precomputed. This paper simply linear combines the precomputed data by coefficients of materials on basis to achieve the effects of global illumination in real-time. This method is applied to scenes with fixed geometry, fixed lighting and fixed view direction. And the materials are represented with bidirectional reflectance distribution functions (BRDFs), which do not take refraction or translucency into account. The authors can simulate in real-time frame rates up to two bounces of light in the implementation, and some interesting phenomena of global illumination, such as color bleeding and caustics, can be achieved.
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