A spectral-particle hybrid method for rendering falling snow

Falling snow has the visual property that it is simultaneously a set of discrete moving particles as well as a dynamic texture. To capture the dynamic texture properties of falling snow using particle systems can, however, require so many particles that it severely impacts rendering rates. Here we address this limitation by rendering the texture properties directly. We use a standard particle system to generate a relatively sparse set of falling snow flakes, and we then composite in a dynamic texture to fill in between the particles. The texture is generated using a novel image-based spectral synthesis method. The spectrum of the falling snow texture is defined by a dispersion relation in the image plane, derived from linear perspective. The dispersion relation relates image speed, image size, and particle depth. In the frequency domain, it relates the wavelength and speed of moving 2D image sinusoids. The parameters of this spectral snow can be varied both across the image and over time. This provides the flexibility to match the direction and speed parameters of the spectral snow to those of the falling particles. Camera motion can also be matched. Our method produces visually pleasing results at interactive rendering rates. We demonstrate our approach by adding snow effects to static and dynamic scenes. An extension for creating rain effects is also presented. 1