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Realistic rendering of scenes with anisotropic media

机译：使用各向异性媒体逼真的场景渲染

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摘要

We describe a method of tracing a backward (from camera) ray in a scene that contains birefrigent (uniaxial) media. The physics of scattering of an electromagnetic wave by a boundary between two media is well known and is a base for ray tracing methods; but processing of a backward ray differs from scattering of a "natural" forward ray. Say, when a backward ray refracts by a boundary, besides the energy transfer coefficient like for a forward ray, one must account for the radiance change due to beam divergence. We calculate this factor and prove it must be evaluated only for the first and the last media along the ray path while the contributions from the intermediate media mutually cancel. We present a closed numerical method that allows one to perform transformation of a backward ray on a boundary between two media either of which can be birefrigent. We hope it is more convenient and ready for usage in ray tracing engines than known publications. Calculation utilizes Helmholtz reciprocity to calculate directions of scattered rays and their polarization (i.e., Mueller matrices), which is advantageous over a straightforward "reverse" of forward ray transformation. The algorithm was integrated in the lighting simulation system Lumicept and allowed for an efficient calculation of images of scenes with crystal elements.
机译：我们描述了一种在包含双折射（单轴）媒体的场景中跟踪向后（来自相机）光线的方法。电磁波通过两种介质之间的边界散射的物理原理是众所周知的，并且是射线追踪方法的基础。但是后向射线的处理不同于“自然”前向射线的散射。可以说，当后向光线通过边界折射时，除了像前向光线一样的能量传递系数外，还必须考虑到由于光束发散而引起的辐射率变化。我们计算了该因子，并证明必须仅对沿射线路径的第一个和最后一个介质进行评估，而中间介质的贡献会相互抵消。我们提出了一种封闭的数值方法，该方法允许人们在两种介质之间的边界上执行后向射线的转换，而两种介质都可以是双折射的。我们希望它比已知的出版物更方便，更易于在光线跟踪引擎中使用。计算利用亥姆霍兹互易性来计算散射射线的方向及其偏振（即Mueller矩阵），这比前向射线变换的直接“逆向”更为有利。该算法已集成到照明仿真系统Lumicept中，可以有效地计算带有晶体元素的场景图像。

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