A graphics processing system generates interpolated vertex shaded attribute data for plural sampling points of plural fragments of a quad fragment that is being used to sample a primitive. The interpolated vertex shaded attribute data for the plural sampling points is generated using a reference position for the quad fragment that is defined with respect to a first coordinate system, together with rotated sampling point delta values for the primitive that are defined with respect to a second coordinate system. The rotated sampling point delta values allow the interpolated vertex shaded attribute data to be generated more efficiently for the plural sampling points.