1. A method for producing an image of a computer-simulated mannequin wearing a garment as defined by selected mannequin and garment parameter values, comprising: generating objects corresponding to a representative mannequin and a garment placed in a simulation scene within a three-dimensional modeling environment; simulating draping and collision of the garment with the representative mannequin within the simulation scene to generate a three-dimensional rendering frame of the representative mannequin wearing the garment; constraining portions of the garment to reside within or outside of one or more shells defined around the representative mannequin in the rendering frame during the draping and collision simulation, wherein each shell is a three-dimensional construct designed to mimic the physical interaction of the garment with another garment; and, rendering a two-dimensional image of the garment from the rendering frame and layering the rendered garment image upon a two-dimensional image of a selected mannequin.