US Patent 7098910 Hair rendering method and apparatus

A method for rendering hair particles includes determining a grid with vertices and voxels bounding the hair particles, determining hair densities for the vertices, smoothing the hair densities, solving a distance function to form a distance field in response to the smooth hair densities, wherein a distance function returns zero at a pre-determined hair particle density, determining a surface normal direction for a hair particle in response to the distance field values, determining a hair illumination value in response to a first illumination source, and determining a shading value for the hair particle using the hair illumination value and the surface normal direction for the hair particle.