1. A method for generating an image, the method comprising: defining a reference surface for an object, the reference surface having at least a first attribute associated therewith, the first attribute having a value that is variable as a function of position on the reference surface; defining an auxiliary surface for the object, the auxiliary surface being positioned in relation to a specified portion of the reference surface, the auxiliary surface having at least a second attribute associated therewith, the second attribute having a value that is variable as a function of position on the auxiliary surface; tracing a ray for a pixel of an image raster, the ray intersecting a point on the reference surface and a point on the auxiliary surface; determining a value of the first attribute based at least in part on the intersection point of the ray with the reference surface; determining a value of the second attribute based at least in part on the intersection point of the ray with the auxiliary surface; and using the values of the first attribute and the second attribute to determine a pixel color for the pixel.