1. A method for calculating an attribute value, comprising: receiving an attribute interpolation opcode; obtaining pixel offsets dx and dy that specify a sub-pixel sample position (x+dx, y+dy) that is offset from a reference position (x,y) within a pixel; calculating an interpolated attribute for the sub-pixel sample position by evaluating the function A*(x+dx)+B*(y+dy)+C, wherein A, B, and C are attribute-specific plane equation coefficients; and storing the interpolated attribute for the sub-pixel sample position in a destination register.