雨线的真实感绘制极具挑战性。雨滴在下落过程中会发生快速的震荡,由于光线在其中的反射和折射而具有复杂的光学效果,在人眼和摄像机观察时会产生运动模糊形成雨线。提出一种基于Monte Carlo光线跟踪的方法用于绘制真实感雨线。通过预先对雨滴震荡周期内每个离散时间点的雨滴三角形网格建立加速结构,并利用雨滴运动包围盒与采样光线的相交计算,大大加速了光线与雨滴的相交测试;使用自适应采样技术避免了对图像中每个像素点都使用大的采样数,提高了采样效率,加速了图像的绘制。将本文方法集成于离线绘制器(PBRT)中,生成了具有真实感的雨线图像和雨景动画。实验结果表明,本文方法能真实地模拟雨滴的震荡和光线在雨滴中的多次反射和折射,并能在包含任意类型光源的场景中有效地绘制真实感雨线。

Photorealistic rendering of rain streaks is a challenging task. A falling raindrop has complex optic effects due to its rapid oscillation and the reflections & refractions of a ray in itself. Furthermore, it is perceived as a motion-blurred rain streak observed by a human or a camera. In this paper, we present a Monte Carlo ray-tracing based approach to render photorealistic rain streaks. An acceleration structure is built in advance for the triangle mesh of a raindrop at every discrete time point in its oscillation period and an intersection computing is made between its motion bounding box and the sampled ray, which make a large acceleration to the ray-raindrop intersection test. An adaptive sampling technique, which avoids many samples for every pixel, is used to improve the sampling efficiency and to accelerate the image rendering. We integrate the approach into the off-line renderer(PBRT) and produce photorealistic images of rain streaks and rain scene animation. Experimental results show that our approach can be used to correctly simulate the reflections and refractions of a ray in a raindrop and efficiently render photorealistic rain streaks in the scene with any type of light source.