Shedding Light on the Weather

Virtually all methods in image processing and computer vision, for removing weather effects from images, assume single scattering of light by particles in the atmosphere. In reality, multiple scattering effects are significant. A common manifestation of multiple scattering is the appearance of glows around light sources in bad weather. Modeling multiple scattering is critical to understanding the complex effects of weather on images, and hence essential for improving the performance of outdoor vision systems. We develop a new physics-based model for the multiple scattering of light rays as they travel from a source to an observer. This model is valid for various weather conditions including fog, haze, mist and rain. Our model enables us to recover from a single image the shapes and depths of sources in the scene. In addition, the weather condition and the visibility of the atmosphere can be estimated. These quantities can, in turn, be used to remove the glows of sources to obtain a clear picture of the scene. Based on these results, we demonstrate that a camera observing a distant source can serve as a “visual weather meter”. The model and techniques described in this paper can also be used to analyze scattering in other media, such as fluids and tissues. Therefore, in addition to vision in bad weather, our work has implications for medical and underwater imaging. 1 Scattering from Light Sources Recently, there has been growing interest in the vision and image processing communities regarding image understanding in bad weather. Algorithms have been developed to recover 3D scene structure [14] and to restore clear day scene contrasts [15] and colors [14; 17] from bad weather images. Most of this work, however, assumes that the irradiance at any pixel is solely due to scattering within a small column of the atmosphere along the pixel’s line of sight. Figure 1 shows glows around light sources on a misty night. Light from the sources gets scattered multiple times and reaches the observer from different directions (figure 2). Clearly, multiple scattering of light must be modeled to better understand and exploit weather effects for improving the performance of outdoor vision systems. ∗This work was supported by a DARPA HumanID Contract (N000-14-00-1-0916) and an NSF Award (IIS-99-87979). Figure 1: Night image of light sources in mist. The glows around sources can reveal the weather condition as well as the relative depths and shapes of sources. Light Source Glow in Image Plane Pinhole Atmosphere Multiple Scattered Light