1. A processor-implemented method of shading primitives in a 3-D scene in a computer graphics system, comprising: identifying, by a processor, an intersection point between a primary ray and a planar primitive defining an object located in the 3-D scene; defining, by a processor, a secondary ray to have an origin determined from the intersection point of the ray with the primitive, the origin being offset from a surface of the primitive by an amount dependent on indicia of curvature calculated for the primitive and on a relative position of the intersection point to vertexes defining the primitive; tracing, by a processor, the secondary ray to identify an intersection involving the secondary ray; and using the results of the tracing of the secondary ray by a processor in shading of the primitive: wherein the indicia of curvature comprise coefficients associated with respective vertices of the primitive and the offset is determined based at least in part on using the coefficients in a polynomial that weights the coefficients using barycentric coordinates of the intersection point.