1. A method for calculating pixel values, the method comprising: clipping geometric primitives that are partially within a predetermined viewport; transforming clipped geometric primitives from a first coordinate space to a second coordinate space; and for each transformed primitive: shifting the transformed primitive in the second coordinate space by a respective first sub-pixel offset, rendering the first shifted transformed primitive to generate at least one first sample value for a pixel, shifting the second transformed primitive in the second coordinate space by a respective second sub-pixel offset, rendering the shifted transformed primitive to generate at least one second sample value for the pixel, writing the first sample value to a first buffer and writing the second sample value to a second buffer, and combining the first and second sample values to generate a pixel value for the pixel.