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 transparency attribute associated therewith, the transparency attribute having a value that is variable as a function of position on the auxiliary surface, wherein the first attribute and the transparency attribute are different attributes, and wherein the auxiliary surface has a different geometry from the reference 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 and independently of the intersection point of the ray with the auxiliary surface; determining a value of the transparency attribute based at least in part on the intersection point of the ray with the auxiliary surface and independently of the intersection point of the ray with the reference surface; and using the values of the first attribute and the transparency attribute to determine a pixel color for the pixel.