Eurographics Symposium on Rendering (2007) Jan Kautz and Sumanta Pattanaik (Editors) Material Based Splashing of Water Drops Kshitiz Garg, Gurunandan Krishnan, and Shree K. Nayar Department of Computer Science, Columbia University, New York, NY, USA Email:{kshitiz,gkguru,nayar}@cs.columbia.edu Abstract The splashing of a water drop is a fascinating phenomenon that results from a variety of complex interactions between the drop and the material it impacts. In general, the distribution of droplets of a splash depends on the drop size and velocity; the surface roughness, rigidity, and wetness; and the angle of impact. Given the number of factors involved, it is difficult to develop an analytical model for the splash distribution. Instead, we take an em- pirical approach. We have measured the splashing behaviors of 22 different materials that are commonly found in the real world. These materials can be broadly classified as rough (e.g., wood and brick), smooth (e.g., marble and glass), flexible (e.g., silk and paper), and miscellaneous (e.g., water and moss). We have developed a stochastic model for splash distribution that builds upon empirical models previously developed in fluid dynamics and me- teorology. Our model is simple and only requires 7 coefficients for generating splashes for head-on impact for a material. A more general model for generating splashes for arbitrary impact angles (due to surface inclination or wind) requires 54 coefficients. The models of different materials may be combined to generate physically plausible splashes for novel materials that have not been measured. Our model is applicable for rendering splashes due to rain as well as water drops falling from large heights such as windowsills, trees, and rooftops. 1. Introduction Capturing the interaction of water drops with scene ele- ments is important in achieving realism. This is particu- larly crucial when rendering rain, where lack of interaction of falling drops with scene elements makes the scene ap- pear disconnected from the rain. Other common scenarios in- clude drops falling from large heights such as windowsills, trees and rooftops. Over the last few years numerous algo- rithms have been developed to realistically simulate some of these interactions, such as the flow of water drops on sur- faces [KKY93, KZYN96, WMT05] and the merging and de- formation of water drops [FHP98, WMT05]. However, the splashing of water drops, which is a visually prominent ef- fect, has not received much attention. The splashing of a drop is a fascinating phenomenon that involves complex interactions between the fluid and the sur- face it impacts. As a result, the dynamics of a splash de- pends on many factors – the material properties of the surface (roughness, rigidity, etc.), the properties of the falling drop (size, velocity, etc.), and the angle of impact. Figure 1 I(a,b) shows 3D visualizations of measured splash distributions for rusted iron and plastic samples with 0 ◦ inclination (head-on impact). The differences in the number of splash droplets and their trajectories arising from the differing material proper- ties are significant. Figure 1 I(c-d) shows measured splashes for the same samples inclined at 30 ◦ to the direction of the falling drop. The inclination causes strong biases in the ve- locities and the orientations of splash droplets. These exam- ples illustrate that when a splash is rendered, it is crucial to take into account the material properties and the inclination of the surface. Although splashing of drops has been extensively studied in various fields (fluid mechanics, meteorology, agricultural engineering, and mechanical engineering) there are very few works that focus on the dynamic aspect of splashes † . Some numerical and analytical methods have begun to emerge in the above fields and in graphics. However, these methods can- not account for the materials properties and hence cannot be used to render material dependent splashes. In the absence of a comprehensive framework for render- ing drop splashes, several approximate methods have been implemented. For example, in the ATI demo [Tat06], a single high-speed video of a splashing milk drop was used to add splashes in a rain scene. Using a single splash event for dif- † There has been significant work on the simulation of the dynam- ics of large bodies of fluids (see [Igl04] for a recent survey). These studies are primarily focused on the effects of external forces and boundary conditions on the dynamics of a body of fluid. In the case of water drop splashes, the dynamics (and hence the appearance) is mainly driven by the physical properties of the impacted material. c  The Eurographics Association 2007.