Photorealistic rendering of rain streaks with lighting and viewpoint effects is a challenging problem. Raindrops undergo rapid shape distortions as they fall, a phenomenon referred to as oscillations. Due to these oscillations, the reflection of light by, and the refraction of light through, a falling raindrop produce complex brightness patterns within a single motion-blurred rain streak captured by a camera or observed by a human. The brightness pattern of a rain streak typically includes speckles, multiple smeared highlights and curved brightness contours. In this work, we propose a new model for rain streak appearance that captures the complex interactions between the lighting direction, the viewing direction and the oscillating shape of the drop. Our model builds upon a raindrop oscillation model that has been developed in atmospheric sciences. We have measured rain streak appearances under a wide range of lighting and viewing conditions and empirically determined the oscillation parameters that are dominant in raindrops. Using these parameters, we have rendered thousands of rain streaks to create a database that captures the variations in streak appearance with respect to lighting and viewing directions. We have developed an efficient image-based rendering algorithm that uses our streak database to add rain to a single image or a captured video with moving objects and sources. The rendering algorithm is very simple to use as it only requires a coarse depth map of the scene and the locations and properties of the light sources. We have rendered rain in a wide range of scenarios and the results show that our physically-based rain streak model greatly enhances the visual realism of rendered rain.
