A graphics processing pipeline includes three architectural features that allow a fragment shader to efficiently calculate per-sample attribute values using barycentric coordinates and per-vertex attributes. The first feature is barycentric coordinate injection to provide barycentric coordinates to the fragment shader. The second feature is an attribute qualifier that allows an attribute of a graphics primitive to be processed without conventional fixed-function interpolation. The third feature is a direct access path from the fragment shader to triangle data storage hardware resources where vertex attribute data and/or plane equation coefficients are stored. Allowing the fragment shader to calculate per-sample attribute values in this way advantageously increases system flexibility while reducing workload associated with triangle plane equation setup.